LIBRARY STATE g^yAf^r, BOARD ^_^^^ United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plsint Quarantine DEVELOPMENT OF IKSECT RESISTANCE TO INSECTICIDES^ By Frfimk H. Babers, Dirision of Insects Affecting Han and Animals The use of the term "resistant insects'* to denote those tolerant to insecticides is probably open to criticism. However, for lack of a better term it is used herein to designate strains of Insects that for some reason are able to withstand a larger dose of an Insecticide than are other apparently normal insects of the same species. The resistant strains in nature often occur within a few miles of nonreslstant strains, and differences in susceptibility become very noticeable. The phenomenon came into prominence when Melander (^ , speaking of the San Jose scale, raised his now famous question "Can Insects become resistant to sprays?" In 1902 Piper (2A) ^^ obtained excellent control of the scale with lime-sulfur. Melander in the same eirea about 11 years later found 74> percent of the scales alive although the dose of insecti- cide had been increased 10 times. Since Melander* s observation the list of resistant insects described in the literature has grown to sizable proportions. Quayle (26) noted the increased resistance to hydrocyanic acid fumigation of the California red scale, Aonldiella aurantii (Mask.)^ ( Chrysomohalus aurantii Mask.), and the black scale, Salssetia oleae (Bem)T As usually practiced, fumigation was expected to effect such a high mortality of scale insects that re- treatments would be needed only about every 3 years. The results of fumi- gation with hydrocyeuilc acid had in 1916 become so unsatisfactory that often trees did not remain free from scales for a single year. Hough ( 31) found a strain of the codling moth, Carpocapsa laomonella (L.), that could enter apples sprayed with eirsenicals more readily than could the normal insect. The same year Boyce (^) actually developed strains of Drosophila melanogaster Meig. and Aphis gossvpii Glov, in the laboratpry that were resistant to hydrocyanic acid fumigation. Resistance to l^ydrocyanlc acid fumigation by the cltrlcola scale. Coccus pseudomagnollarup (Kuw.), was i/ This work was conducted under funds allotted by the National Military Establishment to the Bureau of Entomology and Plant Quarantine. 2/ The scientific names used in the literature sometimes differ from the name currently accepted by the Association of Economic Entomologists (68). The current names are used, fo3J.owed in parentheses by those em- ployed by the author of the citation. - 2 - described "by Qoayle (7d). The progeny of confused flour beetles, Trlbollun confusua Duv. , resistant to hydrocyanic acid fumigation were also resistant, according to Gough (2^* In South Africa an arsenic- resistant tick, P9?pb1,ili^p decoloratufl (Eoch) , was found by Du Tolt and others (22). Boyce and Perslng (J) found resistance to tartar emetic sprays among the citrus thrlps, Sclrtothrlps cltrl (Moult*), and Knipllng (M) found that larvae of the screw-worm, Callltroga amerlcana *. (Cochllomyl) thlazlne. Hosna ( 67) described a variety of the mosquito, Culex plplens C, and P. T Cochllomyla amerlcana) ■ could acquire resistance to pheno- autogenlcus. resistant to DDT, and Mlsslroll (62) told of the failure of DDT to control files In Italy In 19i;5-19ii6 due to their resistance. Llndqulst and Wilson (^ and Wilson and Gahan (^7) have developed a laboratory strain of the house fly, ttusca domestlca L., that Is resistant to DDT and other Insecticides. There seems little doubt therefore that both In the field and In the laboratory there have been developed Insects that differ In physiological response to Insecticides from other Insects that are, as far as can now be shown with one exception, the same morpho- logically. It is the purpose of this publication to review the development of resistance to Insecticides by the several Insects In an effort to determine whether sufficient evidence has been presented to warrant a theoxy as to the cause of this resistance. San Jose Scale In his original report on resistant San Jose scale, Ifelander ( 60) stated that he had observed failure of line-sulfur to kill all the insects first in 1903 and again in 1910. However, it was not until 1913 that he atteiq>ted extensive experiments. Where all the nonresist- ant scales were dead within 6 weeks after spraying, from 4 to 17 percent of the insects from the resistant area were normal* Trees sprayed with oil eaulsion, hovever, were free of scale in both areas* He noted that nany of the surviving resistant Insects were males* In the usual field procedure about evezy 10th generaticQ of scales is sprayed* An acquired iamninity therefofre seemed unliksly to Melander* Be did believe, however, that the resistance was Inherited. He gave additional evidence in 1915 ( 61) to show that the resistance was nob due to weather, faulty applica- tion, improperly mixed sprays, the condition of the trees, or apparently to any cosibination of extrinsic factors* In his third paper Helaader ( 62) abridged and sonmarized the data accumulated sinoe 1906* In attempts to start a labor at ozy colony of San Jose scale, he brought "literally millions** of scales fraa Infested areas* Scaly apples were tied on trees, scaly scions were grafted, and scaly limbs entwined in young trees. Every attempt to acclimatize the scale to the Pullman, Wash., area failed, and yet this was the Insect that was able to sur^ Vive very heavy doses of lime-sulfur. He found great individual variar- tion in the tolerance to lime-sulfur, soda-sulfur, and barium-sulfur sprays* The tolerance varied from locality to locality and from year to year, and winter mortality varied with the severity of the weather* - 3 - A. temperature of -30* F, destroyed more scales than the usual spraying, whereas in mild winters 93 percent of the young scales survived. There was no evidence that the San Jose scale was beccraing more and more resistant to polysulfide sprays. Flint (2^) supported Melander's stand that in certain areas the San Jose scale, Aspidiotus pemiciosus Comst., was resistant. He stated (concerning orchard owners)* "A number of these men are university graduates and are thoroughly familiar with the theoretical as well as practical side of spraying. Not one in 50 is satisfied with lime- sulfur for controlling the San Jose scale. "Whatever the entomologists may think, it is practically useless to advise these men to continue spraying with lime-sulfur." Apparently Melander's work had aroused great controversy, alt hou^ I find no rebuttal published. Flint, as did Melander, obtained control of the insect •with oil emulsions. California Red Scale Two years after Melander's original observation, Quayle ( 76 ) gave evidence to show that both the red and black scales in certain areas were much more difficult to kill with hydrocyanic acid than in former years. Additional data were given in i^uayle's next paper (77). He stated that he had some evidence that survivors of a fumigation are more resistant to a second fumigation than are individuals that have not been previously fumigated; also that the greatest resist^jace is shown by scales on trees that have been fumigated regularly once or even twice a year. He found considerable difference in susceptibility among insects froir: various parts of the same tree, and the susceptibility seemed to be related to the food supply of the scale. During the molting period resistance was at its highest. Results of field experiments varied because changes in the humidity of the air affected the dryness of the canvas tent used in fiani- gation and therefore its ability to retard diffusion of the ges. The dos- age of hydrocyanic ecid required for control o.f the resistant strains was so high that it was unsafe for the tree except under the most favorable conditions. Woglum (23) supported the observation of Quayle that both red and black scales were, in certain areas, resistant tc lumigation with hydro- cyanic acid. Gray and Kirkpatrick (2.6. 27. 2S) also agreed that there were resistant and nonresistant strains. They showed that mortality among both strains was greatly reduced if the insects were first exposed to a low dose of hydrocyanic acid before extjosure to the killing dose. The resistant strains, however, could still withstand much the heavier doses. This protective mechanism was called protective stupefaction. These authors recomraended a fumigation technique that 7'ould obtain a high initial concentration and thus avoid the protective stupefaction. Pratt, Swain, &nd Eldied (7^) also studied tills protective stupefac- tion for red, black, and citrus scales, and confirmed Gray and Kirk- patrick' s findings on both resistant and nonresistcOit strains. Accoraing to Quayle (7£, p. 198), A. F. Swain in 1928 and 1931 conductea tests on . u- the comparative resistance of red scale to fumigation under form trees and in a fumatorium. He foiand significant differences between resistant end nonresistant strains. These data apparently have not been published. Also according to Quayle, A. F. Kirkpatrick carried on extensive tests in 1936 on comparative resistance of the scales from various localities and found significant differences. Quayle gave no literature reference to this work. Ebeling's (2^) data indicated that "in any given grove, the trees in the higher portions of the grove usually have the greatest infestation of scales." He stated: "It is a common observation that the red scale can be more easily controlled by the usual control measures in the less elevated portions of groves than on the higher elevations." In a grove where the highest elevation was UO feet above the lowest, the difference in temperature was about 4.. 73° F. Because the foothill districts were warmer, they were generally planted to lemons, and it is in these areas that the so-called resistant scales were foimd, Ebeling states: "It is not known to what extent * resistance' as applied to red scale in the foot- hill districts is due to the higher temperature prevailing in these districts. Nor is it known to what extent the apparent resistance in these districts may be accounted for by the fact that the lemon is the preferred host of the red scale." He also suggested that, when the population density was so great that the insects overlapped at their margins, mortality from fumigation might be reduced. Knight (^) pointed out that the tolerance of the red scale to fumi- gation with hydrocyanic acid was in inverse ratio to its activity. This was true whether the inactivity was induced by natural or artificial means. Insects in the pupal stage and during molting were least suscep- tible to hydrocyanic acid. These observations, of course, do not explain the differential resistance of the two strains. In 1932 Knight (^) found that oil sprays gave poorer kills on heavily infested fruit than on lightly infested fruit. The kill on lemons infested with 1-100 scales was 95.97 percent, with 100-200 was 83. 5^ percent, and with 200-300 only 72.90 per- cent. He suggested that a similar reduction might occur when heavily in- fested fruit was fumigated, Moore (6^ stated that "under favorable conditions, there is no sig- nificant difference between the kills of 'resistant' and 'nonresistant' red scale." Density of scale population had no effect on the kill ob- tained by fumigation with hydrocyanic acid, nor did the concentration, provided there was no faulty distribution. Using insects from the area where resistance was first found, he determined the time-concentration constant (Knight 4£, Brinley and Baker 2) • His results were irregular and indicated 'that there was some uncontrolled factor influencing the success of a fiimigation which overshadowed the time-concentration constant. In other words, if conditions are \mfavorable, increasing the dosage or the time will not insure a perfect kill." Exposure to a low concentration prior to the regular fumigation did not always reduce the kill. Several field fumigations were made to determine whether the number of late-second- molt and early-gray adults, commonly considered the least susceptible - 5 - stages, had any effect. He stated "clearly the stage of development is not the controlling factor," and in the same paragraph that "It irould appear as if conditions at the time of the fumigation decide its success or failure and the stage of development of the insect decides whether it will or will not be a survivor of an unsuccessful fumigation." Temperature and humidity had an effect on the kills obtained by fumiga- tion. Uoore ( 61) listed as "unfavorable" conditions high temperature, low relative humidity, pre-exposure to low concentrations, exposure to low rela- tive humidity preceding the fumigation or low temperature following the fumigation. Under these conditions he stated that "with the 'resistant* strain the VITl may fall very low" emd that "under the same conditions the nonresistant red scale reacts but slightly." Data are not given to support these statements. In experiments where resistant and nonresistant insects are directly compared, the effects of fumigating warm and cold lemons, of fumigating at varying temperatures and humidities under varying environmental conditions, and of varying the temperature before and after fumigation were determined. Many of the kills were higher than 90 percent and therefore not too acceptable from a statistical standpoint. In every case, however, the kill of resistant insects was considerably lower than that of the nonresistant. It is not dear, therefore, how Moore's conclusion that "under favorable conditions, there is no significant difference between the kills of 'resistant' and 'nonresistant' red scale" was reached. Hoore (^ concluded that the difference in kill between the two "strains" was due to climatic conditions, method of application, or variations in the concentration of hydrocyanic acid, since good and poor kills were obtained the same night. In 1936 he (66) studied the relationship of concentration (£) and exposure time (X) and concluded that lethal constants for resistant and nonresistant strains were CT9*5 and CT^'^. respectively, apparently reversing his stand that there was no difference in tolerance to hydrocyanic acid. He concluded that the main differences between the resistant and nonresistant red scales in California were their reactions to concentration, exposure, and the temperature at which the fuaigation is conducted, and these differences were not acquired from previous fumigations. Such conclusions of course do not e3q)laln the phenomenon. Cunningham (i^ stated that, "when one considers the complexity of the problems involved in toat fumigations, it becomes obviotis that many points must be investigated before the theory of 'resistant' scales can be accepted." Haas (22) attempted to relate resistance to hydrocyanic acid fumigation to the chemical content of several scale insects. Because Clayton (ig) increased the tolerance of tomato plants to hydrocyanic acid by the injection of glucose, and because Keeser ( 3?) found resistance of rabbits to orally administered sodium cyanide was increased irtien the iron content of the tissues was increased, Haas suspected that the glucose and iron content of the scale insects Bii^t also be related to resistance. He determined ash, iron, phosphorus, manganese, - 6- nitrogen, sulfur, copper, glucose, irax, alcohol-soluble material, and crude chltin content of the Insects* No relatlooi -was established betireen resistance and iron or phosphorus content, A reduced copper content and an increase in the amount of vaz and reducing substances present might be related. Quayle (28) reviewed the entire subject of resistance to faordrocyanic acid fumigation by the scale insects and presented additional data. He stated (p. 206) that Knight, several years before and then a colleague of his, had concluded that the resist- ant red scale was more resistant to oil sprays than the nonresistant strain. Qiamberlin, also a colleague, concluded that there was a difference between the two strains with respect to desiccation. Quayle did not consider these data sufficient to justify their claims, and their published paper (De Ong, Kni^, and ChaiBberlin 17) did not contain statements to that effect. Quayle concludes that the resistance spread frcm an original focus. He also reported that the resistance differential occurred when methyl bronide and ethylene oxide were used as fumigants. Carbon disulfide was tried but injured the fruit, and the results therefore were not reported. Dobzhansky (21, p. 190) concluded that the resistance was brought about through natural selection. It first appeared either through rnuta^ tions or because a mixture of the resistant and nonresistant strains was present in the original infestations. In his opinion the exact origin would not be determined. He stated (p. 191) » "However that nay be, the emergence of the resistant race of the red scale is clearly due to the differential survival of the two genotypes in fumigated orchards." Lindgren (^) confirmed the existaice of the two strains and also studied protective stupefaction. He used laboratory strains of the resistant and nonresistant insects. No difference in results was noted between these laboratory-reared insects and those collected frcm areas in whidi varying resistance was observed, Dickson (IB) concluded that the resistance is inherited. It depends on a single gene or group of closely linked genes in the X- chromosc»ne and is therefore sex-linked. Crosses between the resistant and nonresistant strains gave female offspring intermediate between the two. The Fj^ males inherited their mother's resistance. Lindgren ( ^9) studied the effect of time of exposure, high peak concentration, ten^ra^ ture, and other factors on the fumigation result. He used laboratory- reared insects of both resistant and nonresistant strains. Differences in susceptibility to fumigation occurred in insects of all stages. Lindgren and Dickson (^ found that resistant and nonresistant red scales showed no difference in susceptibility to oil sprays. Cressman ( 14) studied the relative susceptibility of the two strains to sprays containing oil, solvent, and cube resins. With the oil and solvent alone the mortality of the resistant strain was 25 •? percent and of the non- resistant strain 29«7 percent. This difference was not significant. With 0.006, 0.008, and 0.013 percent of cube resins, the respective mortalities were 77,4-81.6, 88.2-89.9, and 94-1-96.8 percent. (The first figure of each pair is for the resistant strain and the second for the nonresistant.) The difffcrences in mortality between the two strains were - 7 - Bot grMt bat Tore olalmed bj the authors to be highly slgnlf leant statistically* The method of statistical analysis used was not given* The two strains were found by Hardman and Craig (20) to be different in their physiological respoise to low doses of hydrocyanic add* In both strains the spiracles close 3 to 5 minutes after hydrocyanic add has been admitted to the fui^gatlon chaisber. In the ncnresistant strain th^ remain closed only for 1 minute, then open, and death follows* In the resistwrt strain the spiracles remain closed at least 30 minutes, and the scalLes can survive lethal concentrations for that time. Quayle (2fi) disag:r<;es with Hardman and Craig's conclusion that the ability to keep the spiracles closed explains the resistance. HLs reasoning is based on thtj behavior of the two strains toward protective stupefaction* This phenomenon occurs lAren exposure to cyanide is so short that spiracles do not dose* Lindgren aid Sinclair (^ farther ccn firmed the eTdstence of the resistant and nonresistant strains of red scale. They found that after funlgation with hydrocyanic acid gas more hydrocyanic add was recovered from the nonresistant than from the resistant insects* Tust, Nelson, and Busbey ( 106) found that the protective stupefaction of the red scale occurred with both the resistant and nonresistant insects* JLt 59* F» tlie protecticn for nonresistant females persisted less than 1 hour, but at 77* it lasted at least 2 hours* The amount of protection was not affected by repeated exposure to low dosages, but the duration of protection was* lust and Busbey ( 100 ) con^ared the susceptibility of the two strains to methyl bromide* The resistant mature females were more resistant than the nonresisteuit insects, but in the early gray adult stage, a stage in which the difference in susceptibility to hydrocyanic add is marl<»d, the nonresistant insects were somewhat more resistant to methyl bromide at 40-minute exposures* At longer exposures, 120 to 180 minutes, the difference was in the same direction but was not considered significants. In concentration-mortality tests the median lethal concen- tration for methyl bromide for early gray adults of the resistant strain was 68 mg* per liter and for the nonresistant insects 73 mg* per liter* Using the resistant strain only, lust, Busbey, and Howard ( 102) investigated the effect of mixtures of methyl bromide and hydrocyanic add* nth methyl bromide alone a concentration that gave only 16*4- percent kill at 50* F* gave 99 • 5 percent at 77*. "When a concentration of methyl bromide that alone gave no kill was used, a mixture of methyl bromide and hydrocyanic acid gave a better kill of insects in all stages than was obtfidned with the separate gases. Methyl bromide used alone seemed to protect the scales somewhat in the late second-molt and early gragp-adult stages from a later hydrocyanic add fumigation. Protection of the scales against hydrocyanic add by other gases was previously observed in their laboratory, but no references or data were given* In another experiment a mixture of methyl bromide and hydrocyanic acid was used in which the concentrations were vuch that approximately equal kills would be obtained if the gases were used separately* Under these conditions there was a synergistic action on the gray adult stage and an antagonistic action on the mature females* Tust, Busbey, and Nelson ( 103) detennined the reaction of laboratory- reared Insects of the two - 8 - strains to various factors, such as temperatures before, during, and after fumigation, exposure to sublethal dosages, and certain combina- tions of these factors. When the scales were preconditioned and post- conditioned for 4 hours at the treatment temperature, the kill of resistant second-molt and nature-female stages and nonresistant second- molt insects was higher with the lowering of fumigation temperature but the kill of nonresistant mature females was not influenced. The kill of resistant scales was influenced more by the temperature after fumiga- tion than was the kill of the nonresistant strain. With second-molt insects the kills of both strains were decreased by prefumigation with sublethal dosages, but the differences in kill were more marked with the resistant strain. Yust and Howard ( 105) studied the factors that influenced the results when red scale was fumigated in the laboratory with hydrocyanic acid. Using insects from the laboratory stock of the resistant strain, they determined the effects of condition of host, isolation of the insect on the fruit, difference of age within stages, and tsnperature variation. Better kills were obtained on lemons that were slightly soft, and scales that were fused together were slightly more difficult to kill than were isolated insects. Difference in age within stages also affected the kill, as did a rise in tenperature following fumigation. Still using laboratory- reared resistant insects, Yust and Busbey (101) determined the effect of fumigating wet and dusty fruit. Protective stupefaction occurred on both wet and diy fruit, but the kill was significantly better on wet fruit whether or not protective stupefaction occurred. The kill was slightly better on dean fruit than on dusty fruit. That the resistance to l^roqyanic acid was inherited was demon- strated by Yust, Nelson, and Busbey ( 107 ). Laboratory colonies were started in 1935 by collecting resistant and nonresistant insects in the field. Throughout a period of more than 6 years both the differential and degree of resistance were maintained. No differential in growth rates was observed, and the permeability of the wax covering of the insect was not a factor. As would be expected, the resistance of the female off- spring of crosses of the two strains was intermediate between the two. The Ti males resembled their mother in resistance. The resistance of the resistant strain, the nonresistant strain, and crosses of the two could be increased by exposure to repeated fumigations. The increased resistance was apparent after only two or three fumigations in the resist- ant stock and crosses but was apparent after only one fumigation of the nonresistant stock. The authors concluded, therefore, that there were some nonresistant insects in the resistant stock and some resistant insects in the nonresistant colony. Yust (22.) found the maximum number of progeny frcwi one female scale of the resistant strain in natural conditions to be 300. The influence of repeated fumigations was further investigated by Yust, Nelson, and Busbey ( 108 ). When laboratory-reared insects from the resistant strain were fumigated with such concentration that complete kill was not obtained, the resistance of the offspring of the survivors was greatly increased. When nonresistant insects were treated similarly. - 9- after the fourth fumigation the resistance was greater than that of the ordinary laboratory-resistcuit strain. The authors had previously been of the opinion that the laboratory colony of resistant insects contained a few nonresistant scales nhich would be eliadnated after a few fumiga- tions* However, they now concludej "In view of the high mortality of the non-resistant strain in these tests, it is difficult to attribute increased resistance entirely to the elimination of nonresistant individuals in the original stocks," In field studies on the influence of concentration on fonigatlon results with resistsnt scales, lust, Busbey, and Nelson ( 104. ) found that a high average concentration of gas was essential but that a high initial concentration was not necessaiy. lust. Nelson, and Busb^ ( 109 ) famigated scales collected from a nuBfcer of neighboring groves, six within a 2-Bile- square area and a seventh about 10 miles away* The survival ranged from 84 to 12 percent^ and all degrees of resistance were encountered* Protective stupefaction was further studied \3y Yust, Nelson, and Busbey ( IjLQ) ^ using the resistant strain only and making no coflq>arison with nonresistant insects* k higher concentration of prefumigation gas was required to prockice stupefaction at 77* F* than at 50** In some cases death occurred before the Insect was protectively stupefied* The stage of devaLopoient affected the length of time stupefaction persisted* In general, the duration was longer with mature females than with scales in the second molt* Yust, Nelson, Busbey, and Fulton ( 111) Investigated the Influence of various exposure- concentra- tion combinations on fumigation results. Here again, only the resistant strain was used and no ccraparlson was made with nonresistant insects* In general, the kills were proportional to the ccn cent rati on when the product of exposure multiplied by time was constant. The relationship did not hold with protectively stupefied insects* Lindgren and Dickson (^) , using strains that had been reared 65 to 70 generations in the labcratory without fumigation, also found the differential resistance to be maintained* This number of generations occurs in about 25 to 30 years in the natural state* From a genetics stsuidpoint th^ conclude t "The mere nearly the population becomes one of purely resistant scales, the greater is the likelihood that the genes for non-resistance present will be contained in semi-resistant females and hence the slower the population change." QLckson and Lindgren ( 19) determined the number of generations of red scale in the field to be from 2 to 3.2 per year. In summarizing their experiments over 10 years to determine whether there are strains resistant to hydrocyanic acid fumiga- tion, they concluded that the resistant strain then formed a large part of the scale popiilation. Dickson and Lindgren (,20) found that the number of generations of red scale varied from 2 per year along the coast to 3,5 in the interior. Insects belonging to both resistant and nonresistant strains were present in all areas examined, Lindgren and Gerhardt (^ found no difference between the strains in susceptibility to fumigation with ethylene dlbroolde. When they used 7*7 mg, of this fumigant per liter of air, 90 percent of the nonresistant and 92 percent of the resistant Insects were killed. When 0*1 mg, of hydrocyanic acid plus 7*7 mg* of ethylene Albromide was used, the kill - 10 - dropped to 70 percent of the nonresistant and 32 percent of the resist- ant strain. With 0,2 mg. of cyanide plus 23.1 mg. of ethiylene dibromide per liter the kill was 98 percent of nonresistant and A8 percent of resistant scales. When 0,2 mg, of l^drocyanic acid was used alone, the kills were 95 and 50 percent, respectively. Monger (^) found no difference in reproductive ability and normal mortality between the two strains at varying conditions of temperature. He also (personal communication lust to King 1948) found no difference in the developmental time of the two strains. Black Scale In his first paper on resistant scale insects, Quayle (76) included the black scale, Saissetia oleae (Bern.), His attention had been called in 1915 to the difficulty in killing this insect near a small town in California. His experiments showed considerable variation in susceptibil- ity to hydrocyanic acid fumigation between insects from this locality and the so-called normal or nonresistant strain. His 1922 paper (22) added con- firming evidence. Woglum (28) first noted resistant black scales in 1912 in the area described by Quayle (2§) • ^s findings were not published at that time. He agreed that resistance in that area had increased. Gray and Kirkpatrick (26, 22j 28), on the basis of laboratory experiments, decided that the resistant strain of black scale actually is resistant to fumigation with hydrocyanic acid but not immune. However, a dose high enough to give effective control is dangerous to the tree. As vrith red scale, protective stupefaction occurred, Pratt, Swain, and Eldred (25.) confirmed the presence of a resistant strain of black scale and also agreed that protective stupefaction occurred when exposure was first made to a low dose of hydrocyanic acid before the application of the killing dose, Swain end Buckner (22) found no difference in the susceptibility of eggs from resistant and nonresistant black scales to fumigation with hydrocyanic acid, Lindgren and Dickson (52) > using laboratory-reared offspring of collected insects from areas in which strains occurred, found the same variation in resistance, Lindgren and Sinclair (55), followjbag fumigation, recovered more hydrocyanic acid from nonresistant insects than from resistant insects. Codling Moth The codling moth, Caroocapsa pomonella (L.), first appeared in Colorado about I'W, and general spraying with arsenicals was begun 3 or 4 years later. Hough (^, 2l) found the ability of larvae from a Colorado strain of this moth to enter apples sprayed with lead arsenate to differ from that of larvae from the strain common in Virginia. Haseman and Burk (J2) fed measured doses of arsenic to codling moth larvae of both Colorado and Missouri strains. As they observed no difference in the killing dose, they concluded from limited experiments that no difference in resistance existed. Haseman and - 11 - Mefferb (21), using sodium arsenite and lead arsenate, could find no difference in susceptibility between Virginia, Ck)lorado, and Missouri strains when the toxicants were injected into the heraocoele, or through the mouth into the digestive tract. They agreed with Haseman and Burk that no resistance to arsaiic had been developed by the Colorado strain. They admitted, however, that in the field this strain was more difficult to control with arsenical sprays than was the Virginia strain. Webster (24) reported that both lead arsenate and oil-lead arsenate sprays were becoming less effective each year for the control of the codling moth. Hough (26) found the Colorado strain more able to enter apples sprayed with cryolite, barium fluosilicate, and rotenone, as well as lead arsenate, than was the Virginia strain. By rearing successive generations of larvae from both strains on fruit sprayed with lead arsenate, he found that both strains, now called Colorado-K and Virginia- K, had greatly increased their ability to enter sprayed fruit. Crosses between "normal" Colorado and "normal" Virginia strains produced larvae intermediate between the two in ability to enter sprayed fruit. This intermediate position was maintained throughout the 10 generations tested, Colorado larvae endured starvation better than did the Virginia strain. No difference in tolerance to potassium cyanide fumigation was noted between the eggs from the four strains when the eggs were less than 24. hours old. However, eggs containing embryos showed a similar variation to that exhibited by mature larvae. Hough (22) in his introduction states t "The history of codling moth control in commercial apple*- growing districts of Virginia during the past 25 years in- cludes the following pertinent items* the number of sprays has doubled, and in many orchards it has trebled; the volume of sprays applied per tree of bearing age and similar size has more than doubled; spraying equipment has greatly in- creased in mechanical efficienqy; the number of sprayers per unit of acreage has increased; the minimiim dosage of 2 pounds of lead arsenate in 100 gallons of water has increased to 3 pounds; effectiveness of lead arsenate has been improved by refinemaits in its manufacture; lead arsenate i&s been fortified by the use of oil as an ovicide and as a sticker; other materials have been added to the sprays to increase efficiency and orchard sanitation adopted to supplement chemical control; but the codling moth is still the most important apple insect and the percentage of injury is at least as high as it was 20 to 25 years ago," From larvae collected from a number of sources he was flble to de-^elov strains with increased ability to enter fruit sprayed with various insecti- cides. Steiner, Arnold, and Summerland (88), working in Vincennes, Ind., found that codling moths from two orchards in the area differed greatly in their ability to enter apples sprayed with lead arsenate. The two orchards had been sprayed in a similar manner until 6 years before the - 12 - experiments, when the spraying of the orchard containing the nonresistant strain was discontinued* The authors therefore assume that the differen- tial had developed since that time, although no experimental evidence is given to justify the assvunption. No difference in ability to enter un- sprayed fruit was observed. Drosophila and Cotton Aphids Boyce (^, by artificial selection, produced strains of the vinegar fly, Drosophila melanogaster Meig., and the cotton aphid. Aphis gossypii Glov., that showed a slightly increased resistance to fumigation after seven generations. Selection was made by exposing the population to a concentration of hydrocyanic acid that would effect a high percentage of kill. The eggs from the survivors were collected and allowed to hatch, and the process was repeated. Boyce 's report was described by him as a progress report only, but no subsequent publications have been noted. Boyce was also using the granary weevil, Sitophilus granarius (L.), the confused flour beetle, Tribolium confusum Duv,, and the saw-toothed grain beetle, Orysaephilus surinamensis (L.)^ but no report of his re- sults with these insects was made. Quayle (28) mentioned that Droso- phila was then being tested in his laboratory to determine the possibil- ity of using it in resistance studies. L'Heritier and Teissier (4i) reported that a strain (ebony) of Drosophila in their laboratory reacted in a very abnormal manner to carbon dioxide. Normally, Drosophila can be anesthesized readily with the gas and maintained in that state for a considerable time. The insect then recovers completely and without any apparent residual effect. The susceptible strain, however, under certain conditions of temperature and gas concentration, did not recover. In further work these workers (46) showed that, although the susceptible factor was inherited, it was transmitted independent of the diromosomes and could therefore not be considered as a Mendelian mechanism. Later they (4Z) postulated that the susceptibility to carbon dioxide is subordinated by the presence in the cytoplasm of a diffusible substance which they call "sigma." L'Heritier and de Scoeux (48) transplanted ovaries from wild resistant flies into the susceptible strain and then mated them with resistant males. After eggs were laid, the females with transplanted ovaries were still susceptible to carbon dioxide. From the eggs both ebony and phenotypically wild-type fHes were obtained. All the ebony strain were susceptible, and also some of the wild type. When the F^ flies were mated, the males did not transmit the susceptibility but all the females did. Sonnebom (82) considers the phenomenon to be similar to that occurring in Parame cium aurelia when certain strains develop a substance that is toxic to other "normal" strains. This trait is inherited and is apparently due to a cytoplasmic rather than to a genie difference. B, R» Bartlett (personal communication) collected 16 strains of Drosophila. representing diversified climatic and food-habit features, from laboratory and field sources. These strains he isolated as family lines and tested as to their susceptibility to DDT, tartar emetic, and hydrocyanic acid. Differential resistance between strains was demonstrated - 13 - and shewn to be an Inherited factor rather than nutritional. Successive selective insecticidal treatments resulted in increased resistance in some strains but not in others ^ and when it did occur the increase vas very slow* Resistance to DET and hydrocyanic acid was determined for adults only, but resistance to tartar emetic was demonstrated for both larvae and adults* k number of strains were resistant to tartar emetic, and the resistance was not specific* The most iQrdrocyanic acid-resistant strain was thought to have high specific resistance to this fumigant but the author also states t "Adult flies of this strain were shown, however, to be resistant to diethyl ether and to subfreezing cold closure as well as anesthesia induced from either of these two treatinents . ** Since only a summary of Bartlott's work is available to the reviewer, this seeming discrepancy as to specificity cannot be clarified* The strain showing greatest resistance to DDT was also resistant to the fluorine analog, moderately resistant to hydrocyanic add and tartar emetic, and slightly if at all resistant to isobom^rl thiocyanoacetate* Ca.trlcola Scale The story of resistance in the citrioola scale. Coccus pseudo- magnollarum (Kuw*), is scmewhat the same as for the other insects* Quayle (78) states that hds attentlai was called to unsatisfactory hydrocyanic add fumigation results against this insect in a very small area in California in 1925* Prior to that time satisfactory control had been obtained* The dosage recowsended was sbar{>ly Increased, but results were still poor* The area in which tolerance was manifested extended rapidly until 1933-1934-, when for some unknown reason the dtricola scale dis- appeared so «ompletely that, except in a vexy few cases, treatment for the pest was not renewed* The scale began to reappear in 1936* Test fumigations indicated that the resistance continued* In the meantime laboratory studies on the resistant strain's survival of hydrocyanic acid fumlgaticn agreed with the results observed in the field* Flour Beetles Using the confused flour beetle, Gough (g^^ developed a strain resistant to fumigation with hydropyanic acid by selecting the survivors from a series of fumigations in which the dose was so regulated that all Insects were not killed. This resistance was shown to be inherited. The female insects were much more susceptible than the males in one generation, but in others little difference was shown. Pupae were least susceptible, followed by the adult, larvae, and eggs, Gough was unable to detect any morphological difference between the two strains. He could find no correlation between length of life qy cle or bo(fy size and resistance* There did seem to be sOTie relation to oxygen uptake. The resistant strain used 2*803 cu, mm, of cocygen per hour per milligram of live weight, and the nonresistant only used 2*163 cu, mm, Gough noted that under certain conditions adult beetles emitted a substance that was toxic to the beetles* - u- Screw-lrVorm Knipling {l^ found that phenothiazine showed considerable varia- tion in toxicity to larvae of La cilia sericata (Meig.) and !• cuprina ^ied. He stated that R. C, Bushland had noted (unpubliehedj a similar fact concerning the screw-vform. Knipling developed a resistant strain of screw-Tsrorms bj rearing the larvae through successive generations exposed to a sublethal amount of phenothiaisine in the breeding medium. After 11 such generations the number of the resistant s crew-T;orms sur- viving toxic concentrations of phenothiazine was 18 times that of the norms.l stock. The fifth generation showed no differential resistance to diphenylamine or diphenylamine oxide. The susceptibility of later generations to these chemicals was not determined. Blue Tick Early in 1940, according to DuToit, Graf, and Bekker (22), reports were received that the single-host blue tick, Boophilus decoloratus (Koch), was not being controlled by the sodium arsenlte dips that formerly had proved effective. Only a small area (30 by 15 miles) in South Africa was then affected. Field studies ty the authors confirmed the reports. The usual procedure of checking the water used, the quality of chemicals, dipping methods, etc., was followed, but the ticks appeared to be def- initely resistant. Other species, Rhipicephalus evertsi. R, appendic- ulatus . R. simus, R. capensis and Amblyomma hebraeim, in the area did not display this resistance and were effectively controlled by arsenic, Omer-Cooper and -1/Vhitnall (21) collected both resistant and nonresistant strains of these ticks and dipped them in arsenical solutions in the laboratory. A marked variation in susceptibility between the two strains was noted. Concentrations of the toxicant were used that would be dangerous to use on cattle, but the ticks were still resistant, Ihitnall and Bradford (2^) found that the resistant ticks were 100 percent con- trolled vdth 0,0029 percent of gamma benzene hexachloride, while 1 percent of DDT only gave 60 percent control. Comparison was not made with non- resistant insects. Citrus Thrips A strain of citrus thrips, Scirtothrips citri (Moult,), that was tolerant to tartar emetic was described by Boyce and Persing- (6), Persing et_ al, (22.), and Boyce, Persing, and Bamhart (8), In 1939 tartar emetic-sucrose solution had been recommended for the control of citrus thrips (Boyce and Persing (6)), and in 19-^ resistance was noted in certain lemon groves in the San Fernando Valley, Calif, In lt.boratory tests 1,695 nonresistant insects were killed, but 19«3 percent of the resistant strain survived out of 1,365 sprayed. Four times the usual recommended dosage (12 pounds each of tartar emetic and sugar per acre) still gave unsatisfactory control. Smith (8^) suggests that resistant strains of this thrips may be present in certain areas in Transvaal, South Africa, - 15 - McGregor (^) collected in the field thrips that were resistant and nonresistant to tartar enetic. After seven generations in the laboratory, the differential in resistance was still apparent. No survivors of spray- ing or their progeny were used in the tests. The resistance was not due to insects avoiding feeding on the tartar emetic. Mosquitoes Mosna (62) found that Culex p ipiens autogenicus taken from the Pontine marshes near Rome irithstood the action of DDT for 32-4S hours whereas his laboratory strain exposed to similar doses died in 3-5 hours. The existence of resistant strains of mosquitoes was also acknowledged by Missiroli (63) • House Flies Missiroli (6^) also emphasized that the house fly in Naples, Italy, had become resistant to DDT, McGovran et al, (^) had described a temporary resistance in house flies that had received a knock-down dose of pyrethrins before being exposed to a higher "killing" dose. This phenomenon was apparently similar to the so-called "protective stupefac- tion" of Gi-ay and Kirkpatrick (26). Wiesmann (§6) tested flies collected from. Amas, about 1,000 kilo- meters north of Stockholm, Sweden, The control of house flies in this area in 194^6 with DDT had been much poorer than expected. Absorbed through the tarsi, the lethal dose of DDT for the Amas strain was 2,5-5 gajuma, whereas for his normal strain it was only 0,025 ganmia. The Amas strain was also more resistant to temperature than the Basel laboratoiy strain. In addition to physiological differences Wiesmann found certain morphological differences between the two strains. The extremities of the resistant, or Amas, strain were notably more pigmented. The tarsal hair tuft was much stiffer, the tarsal joints were larger, and the pulvilli and articular membranes of the joints were about one-third thicker. The two strains could therefore be considered as two races, Sacca (82) reported the presence of resistant house flies in certain areas in Italy, ]ji his opinion they are a different species and he suggested the name Musca domestica yar , tiberina , Lindquist and Wilson ( 56) developed a strain of house flies in the laboratoiy that was much more resistant to DDT than the so-called normal laboratory stock, A large population of flies from the laboratory colony was sprayed with such concentration of DDT that about 90 percent kill was obtained. Succeeding generations were each sprayed with DDT, As the exact quantity required could not be estimated in each cas^ the 90 percent mortality finally was obtained by exposing the sprayed insects to either heat to decrease mortality or cold as required (personal ccMmiunication from H, G, Wilson), After 3 generations of such treatment a difference in susceptibility to DDT was noted. After 14- generations it was marked, only 29 percent of the special stock being killed by a dosage of DDT that killed 68 percent - 16 - of the regular stock. The same year Wilson and Gahan (22) found that the resistant flies were also more resistant to chlordane, pyrethrins plus piperonyl cyclonene, chlorinated camphene, rotenone, and Thanite. By selective breeding, therefore, a race of resistant flies had been developed. The laboratory of Blickle, Capelle, and Morse (4) was accidentally contaminated with benzene hexa chloride. Their house fly colony became reduced to a few individuals and their present colony has been developed froDi those survivors. The flies have been exposed to benzene hexachloride from the time they emerged from the puparia until they were used in tests. This situation has continued for over 3 years. In the first generation 39.9 percent were killec oy exposure to 0.01 percent of the gamma isomer. In the 28th generation only 6 percent were killed following exposure to 0.03 percent. A differential resistance to DDI, pyrethrins, and Lethane 38-4 Special (aliphatic thiocyanate) also was present, but not to the extent of that with benzene hexachloride. From a resort hotel at Ellenville, N. Y,, Barber end Schmitt (l) collected flies in v/hich resistance to DTTT had apparently developed. From these flies a color^r was established and tested in the laboratory. A definite differential existed when the resistant flies were ccmpared with the ordinary laboratory strain in resistance to DDT (technical or pure), methoxychlor, and TDE. The authors also state that these flies "showed no resistance at all to residues of toxaphene, chlordajie, parathion, the gamma isomer of benzene hexachloride, and tetraeth^rl pyrophosphate. Seemingly, therefore, flies of the Ellenville line have acquired a specific resistajice to DDT in its various foniis but none to certain other compounds." To the reviewer, from the data included, this statement seems justified only for toxaphene. With chlordane 15 minutes' exposure to M./ mg, per square foot gave mortalities of 95 ^^ percent for the laboratory strain and 60 percent for the wild strain. With parathion the same exposure gave 96,2 and 77.8 percent mortalities, respectively. With the gamma isomer all flies were killed at all exposiires and concentrations tested. With tetraethyl pyrophosphate long exposure to a lov? concentration had no effect and short exp-osure to the high concentration used gave complete kill. With chlordane and parathion, from casual inspection of these figures, a dif- ferential resistance seems definite, but no conclusions can be drawn from the data for the gamma isomer or tetraethyl pyrophosphate. Therefore, unless considerable data other than those published in this paper are avail- able, the clsim that the resistance is specific for DDT and its analogs must be questioned. It is possible that the authors have in mind, instead of a physiological differential resiste^nce between the two strains, a resistance of such degree that economic control of the insects by insecticides other than DDT would be prevented. From their data a statement that satisfactory control could still probably be obtained with insecticides other than analogs of DDT would seem justified but not that a specific resistance was indicated. - 17 - Bettini and Barachini (2) report failure to control houfle files with 5,3 grams of DDT per square meter in an area irtiere excellent control had previously been obtained. Complete control \q> to 6 months was obtcdned following treatment with a 1.5-percent kerosene solution of Octa-CLor (chlordane) applied at the rate of 2.35 grams per square meter. Although the control was attributed to the 0cta-4CLor alone, it should be noted that owln^ to the scarcity of kerosene, in some cases the material had been added to kerosene already containing 5 percent of DDT; so a combined effect is possible. With Gammexane alone (percentage of gamma Isomer not given) dosages vcp to 3*57 grams per square meter gave unsatisfactory re- sults. Vhen the Gcum&exane was dissolved in the kerosene containing 5 percent of DDT, excell«it control of flies was obtained. Howefver, the workmen f^n ei supervisors complained of nausea and headache. The development of strains of house flies resistant to DDT, pyrethrizus, and an unnamed botanical extract was cledmed bj Barber, Stames, and Stames (2). Measured doses (apparently ^cq) of each insecticide in acetone were applied to the thorax of adult riles. Half the treated files were retained in petrl dishes and half in ovipositlon cages. Succeeding generations were treated in a like manner. After 1 generation and con- tinuing through 10 generations, fewer treated flies died among those held in petrl dishes than among untreated checks. The figures for the DDT- treated files held in the ovipositlon cages were Inconclusive, but those for flies treated with pyrethrum and the botanical extract were considered to be significantly different. The authors conclude: "The evidence appears to show that the flies of the treated lines had beccnie measurably resistant to each of the Insecticides by the second generation and maintained such resistance throughout the experiments." The average weight of pupeuria trcm the ID genera ticos tested was 16 mg« for the checks and 18 mg. for the treated flies. The authors suggest that this difference may have been due to killing off of the weaker flies, idilch may have been the smallest. They report the average weight of the p\:9)aria of their t«ro generation as 20 mg. and do not comment on the seeming discrepancy. Hore puparla fedled to emerge from treated lines than ftom the checks. The authors postulate that small amounts of the active principles of the insecticides might be trans- mitted from the treated flies through the eggs and larvae to the piqparla. To the reviewer this postulatlon seems unjustified and. Indeed, until the differences in survival between the "petrl-dish" and "oviposition-cage" flies is explained, the authors* conclusion as to resistance seems ques- tionable. Wild strains of house flies were collected by King and Gehan (^ from areas In Texas, Georgia, North Carolina, California, and Florida i^ere failure to obtain control with DDT had been reported. Such failure has probably been due in part to Inadequate sanitary practices and faulty ap- plication of the insecticide (personal cosBiunicatlon from E. F. Knipling). The resistance of several of these strains when tested under carefully controlled conditions was definitely higher than that of the laboratfrry strsLln with which they were compared, but was much lower than that exhibited by the resistant strain of Wilson and Gsihan. The resistance was apparently general, but the differential for other insecticides was not so great as for DDT. ^ yRAH^' STATE PLANT BOARD - 18 . Evidence "was presented that seme of the treated surfaces in bame were repellent to flies. It was not Indicated whether adequate control* were run to distinguish between the repellency due to the insecticide and the possible effect of the other ingredients in the ndrtures used# Chlordane wettable powders seemed somewhat attractive to the flies* C^psy Moth According to Melander (60), R. ¥. (Uaser had informed him that by feeding increased doses of lead arsenate to gypsy moths a strain had been reared that fed on heavy doses of the chemical apparently without toxic effects. On the other hand, Campbell (ll) states that "GQAser himself, however, was never convinced that he had proved or disproved the development of immunity or of tolerance to arsenic, and consequently the results of his experiments were never published." Discussion From the foregoing reports the use of either term "tolerant" or "resistant" insects seems applicable. Certainly the phenomenon is distinct from that connoted by the term "immune," Huff (28) lists many insects in which true immunity has been demonstrated, Newell (70). speaking of colonies of honey bees that are resistant to American foul- brood, states that the characteristic is resistance, not immunity. At least part of the resistance to foulbrood is due to the resistant bees* habit of cleaning out dead diseased larvae from the comb and not allowing the disease to reach the spore stage. The nonresistant strains do not do this* In other words, the resistant bees are the better housekeepers* The resistance described is also distinct from the resistance or weakening induced by diet (Phillips and Swingle J^,, Markos and Campbell 57) , and the susceptibility encountered due to the age of the insect. The variation in sueceptibility to a toxicant between various stages of insects is well known (Campbell ^Q^ Siaanton and Miller B^ , The same variation was found in many of the insects meatioBed above. Suscepti- bility also varies with respiratory rate (Cotton ^ , Ripper (82) believes that a ccmbination of chemical and biological control may be necessary. However, the delicate balance between concen- tration of insecticide, time of spraying, and number of parasites re- quired would seem to preclude the general use of this method of control* The theory underlying the variations in the number of individuals of two animal species living together where c«ie species is a parasite of the other is discussed by Volterra (93)* Reviews of the general subject of resistant insects have been made by Thorpe (21), Quayle (28, 22, 81,)»and Smith (86)* It is difficult to arrive at definite conclusions as to whether the -19- resistant insects are generally tougher insects or whether some of the resistance is specific for the chemical at hand, Tust (personal communicatian to W, V, King) considers the resistance of red scale specific for hydrocgranic acid. The results of Knight, Quayle, Ghamberlin, and Cressman, however, do not entirely support this conclusion. That the resistant strain of red scale and the codling moth give rise to resistant progeny seems conclusive; with other insects the evidence vajries. There are many points that reqiiire clarification. Both resistant and non- resistant strains of red scale have been reared for years under laboratory conditions without being subject to fumigation. The original stocks in each case were obtained from field collections. It seems veiy in5)robable that pure strains of the two species were collected in each oasej yet after 10 years and longer without fumigation the variation in resistauice was still pronounced and of the same order of magnitude. With one colony of resistant house flies (King ^ the resistance dropped rapidly when spraying was discontinued, and was nearly normal in about 12 generations, — an indication that it is an acquired resistance. Melander's (60) observation that both strains were effectively con- trolled by oil emulsions was based on field tests. No careful laboratory work w&s done to determine whether any differential resistance existed. Other results of field tests may have been misinterpreted owing to the influence of other factors, Quayle (22) > ^^r instance, obtained about 5 1/2 percent greater kill of scale insects en thinly foliated lemon trees than on those with large amounts of foliage. Insects were also much more difficult to kill on fruit than on twigs or leaves and more difficult to kill on vigorous shoots such as suckers than on less thrifty twigs and leaves, Thompson (22) showed that fewer scale insects were found on citrus trees deficient in magnesium than on "normal trees," Baseman ( 31) found that the mineral content of the soil on which host plants were grown affected to a considerable degree the nunber and hardihood of insect parasites feeding on them, Lindgren and Id.ckson (^ and Cressman ( 14.) in laboratory tests, however, found no difference between the strains of red scale's resistance to oil spray, Cressman found the resistant strain mere resistant to cube resins than the nonresistant insects, although the differential was not so large as that obtained with l^dro- cyanic acid. Indeed, in the three pairs of figures given, the differences were only 4,2, 1,7, and 2,7 percent. These differences, although highly significant statistically, without statistical analysis would usually have been considered normal biological variation, Lindgren and Gerhardt ( 54. ) found no difference in resistance to fumigation with ethylene di- bromide, HVhen mixtures of ethylene dibromide and faydzoqyanic acid were used, the usual differential occurred. However, the qviantity of ethylene bromide used apparently had little effect. Therefore, it would seem that further work must be done before a definite conclusion can be drawn. An arsenie-resistant cattle tick was easily controlled with gamma benzene hexachloride, but no figures were given to compare the relative resistance of normal ticks. The resistant house flies reported by Wilson €uid Gahan (22) > Blickle et al, (4), Barber, Stames, and Stames (2), and King and Gahan (Qj were resistant to all insecticides tested. No report of testing against other materials than DDT was made by Wiesmann, - 20 - but his resistant strain iias also more resistant to high ten^eratures than were his normal insects. His resistant insects, however, exhibited certain morphological differences froa the nonresistant strain. These differences were the only morphological differences reported in the papers covered by this review. Specific resistance to analogs of DDT for a strain of house flies was claimed by Barber and Schmitt (g), but their conclusions must be questioned. The general subject of tolerance to drugs is covered In a short discussion in Cusfany*s Pharmacology (16)* The author states that congenital tolerance to drugs is well knoim. The hedgehog apparentlj is unaffected by mar^ ^poisona," and the rabbit is tolerant to very large doses of atropine* The most familiar example of acquired tolerance is that of the user of tobacco to nicotine* In a shcrt tiae nicotine is a ncrmal ccaistituent of all tissues* Some tissues of an individual may acquire tolerance to a drug while others do not* The brain, for instance, may be tolerant to large doses of mor{dilne while the bowels are nob and the subject becoanes very constipated* A. chronic drunkard may become less sensitive to other poisons acting on the same cells. In some cases the tissues destroy more of the poison than pre- viously. Others excrete it more rapidly and still others absorb it less readily. Tolerance is soon lost if the drug is discontinued for a short time. The arsenio-eating habits of natives of Styria and Tyrol are often given as examples of arsenic tolerance. However, no tolerance to arsenic in solution has ever been demonstrated, and until thl« if done, the whole subject of arsenic tolerance must remain unsettled* In Reichenstein miners exposed to arsenio-containing ores are «hort llTed and exhibit all signs of arsanie poisoning* Caapbell {12J ^v*^ tinable to produce tolerance to arsenic in silkworms by feeding sublethal dofles to larvae. The tolerance of the rabbit to atropine mentioned above can probably be compared to the lack of 8usceptibili"ty of certain insects to various insecticides. DDT, for instance, readily controlled the Japanese beetle, codling moth, house fly, and mosquito (92) . Other insects, such as the boll weevil, red spider, and Mexican bean beetle, seon to be little af- fected. Since DDT acts as both a contact and stomach poison, these dif- ferences become all the store interesting. To summarize the present status of resistant insects, there is no question that several insects have developed resistance in the field to certain insecticides that formerly gave good kills. Similar resistance has been developed in laboratory strains through breeding successive generations of insects that are exposed to toxic but not lOO-percent lethal doses of insecticides. Many factors cure involved and no single type of resistance is apparent. The following conclusions may be drawn from the literature reviewed in this paper: 1. That the differential resistance of strains of the California red sceile and the codling moth, at least, are inherited through Mendelian laws, with genes playing their usually accepted role. - 21 - 2, nth Drosophlla sua cjeptibility to csarbon dioxide is inherited, but seems to be due to a cytoplasmic factor rather than to a gene. 3« In one strain of house flies morphological as well as phjsiological differences have been reported, 4, The resistance may be either natural and inheritable or, theoret- ically, acquired and not transmitted genetically. 5. No clear explanation of the phenomenon has been offered. 6* The evidence seems to be in favor of a generally Increased resistance rather than a specific resistance, although the evidence is not conclusive. 7* The resistance is not to be confused with imBonlty. d« Finally, sufficient evidence has not yet been accumulated to permit postulation of a theory as to the cause of the phenomenon. -22- Uterature Cited (1) Barber, George E», and Sdaaitt, John B« 1948. House flies resi9i«ant to DDT residual zptaya* N. J* Agr* £xpt. Sta. Bui* 742, B pp* (2) Stames, Ordway, and Stames, Eleanor B, 1948* Resistance of houseflies to insecticides* Soap and Sanit* Chem. 24(11) t 120, 121, 143. (3) Bettini, S., and Barachini, B* 1948. Primi resultatidella lotta can L'octa-Klor ed il Gamaesano contro le mosche domestiche resistenti al DDT* Rir* di Parassitol* 9(2)» 85-91* (4) Blidcle, Robert L*, Capelle, Asher, and Morse, W, J* 1948* Insecticide resistant houseflies* Soap and Sanit* Chem* 24(8) » 139, la, U9* (5) Boyce, k. M* 1928* Studies on the resistance of oerbain Insects to hordro- cyanic acid* Jour, Econ* Bit* 21t 715-720. (6) and Persing, C* 4* 1939 • Tartar emetic in control of citrus thrips on lemons* Jour* Econ* Ent* 32 j 153. (7) and Persing, C. A* 1942* Resistance of citrus thrips to tartar emetic in the San Fernando Valley and a tentative substitute control program for emergency conditions* Calif* Agr* E^qjt* St a. News Letter 21, 2 pp. (8) Persing, C* A,^ and Bamhart, C. S* 1942. The resistance of citrus thrips to tartar e«etic- sucrose treatment* Jour. Econ. Ent. 35» 790-791.. (9) Brinley, F. J., and Baker, R. H* 1927. Some factors affecting the toxicity of hydro 477-492* (30) Hardman, N. F., and Craig, Roderick. 1941. A pfaysiological basis for the differential resistance of the two races of red scale to HCN. Science 94« 187. (31) Haseman, Leonard 1946. Influence of soil minerals on insects. Jour. Econ. Ent. 39: 8-11. (32) , and Burk, Virgil F. 1929. A determination of the lethal dosage of arsenic for Missouri and Colorado strains of codling moth larvae. Jour. Econ. Ent. 22 t 655-656. (33) and Meffert, R. L. 1933. Are we developing strains of codling moths resistant to arsenic? Mo. Agr. Sept. Sta. Res. Bol. 202, pp. U* (34) Hough, Walter S. 1928. Relative resistance to arsenical poisoning of two codling moth strains. Jour. Econ. Ekit. 21: 325-329. (35) 1929* Studies of relative resistsn ce to arsenical poisoning of different strains of codling moth larvaa. Jour. Agr. Res. 38: 245-256. (36) (37) 1934. Colorado and Virginia strains of the codling moth in relation to their ability to enter sprayed and unsprayed apples. Jour. Agr. Res. 48: 533-553. 1943. Development and characteristics of vigorous or resistant strains of codling moth. Va. Agr. Ejcpt. Sta. Tech. Bui. 91, p.* 32. - 25 - (38) Huff, Clay G. 19^0, Inmunity in invertebrates, Physiol, Rev, 20: 68-88, (39) Keeser, F. 1930, Eisengehalt und Widerslandsfahigkeit des Organisnus gegen Blausaiure und Schirefel-Wasserstoff • Arch, f • Erpt, Path, u, Pharmakol, 156» 340-345, (40) King, W, V. 1948, Some results of recent work on newer insecticides, Amer. Jour. Trop. Med. 28j 487-497, (41) —— — — ®^^ Gahan, J. B, 1949. Failure of DDT to control houseflies. Jour. Econ. Ent. (In press.) (42) Knight, H, 1925, Factors affecting efficiency in fumigation with hydro— (granic acid, Hilgardia, 1« 35-56. (43) 1932, Some notes on scale resistance and population density. Jour, Bit, and Zool, 24: 1* (44) Khipling, E, F. 1942, Acquired resistance to phenothiazine by larvae of the primary screw worm. Jour, Econ, Ent, 35 » 63-64» (45) L»Heritier, Philippe, and Teissier, Georges 1937* Une anomalie physiologique hereditaire chez la Drosophile. [Paris] Acad, des Sci, Compt. Bend, 205 » 1099-1101, (46) _______ and Teissier, Georges 1938, Un mecanisme hereditaire aberrant chez la Drosonhile, [Paris] Acad, des Sci, Compt, Rend. 206t 1193-1195, (47) and Teissier, Georges 1938, Transmission hereditaire de la sensibilite au gaz carbonique chez la Drosophile , [Paris] Acad, des Sci; Compt. Rend. 206: 1683-1685, (48) and de Scoeux, F, Hugon 1946, Transmission of the carbon dioxide susceptibility of Drosophila by grafting. Nature [London] 157s 729. (49) Lindgren, D. L. 1938. The stupefaction of red scale Aonidiella aurantii by HCN. Hilgardia 11: 213-225. (50) 1941* Factors influencing the resiilts of fumigation of California red scale, Hilgardia 13: 491-511, - 26 - (51) Lindgren, D. L,, and Dickson, R. C. 1942. Spray fumi^tion «cperiments on California red scale* JoTir, Econ. Ent. 35* 827-^29. 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