UNIVERSITY OF CALIFORNIA PUBLICATIONS 
 
 COLLEGE OF AGRICULTURE 
 AGRICULTURAL EXPERIMENT STATION 
 
 THE RED SPIDER OF CITRUS TREES 
 
 C. W. WOODWORTH 
 
 THE RED SPIDER 
 
 BULLETIN NO. 145 
 
 BERKELEY: THE UNIVERSITY PRESS 
 
 November, 1902 
 
Benjamin Ide Wheeler, Ph.D., L.L.D., President of the University. 
 
 EXPERIMENT STATION STAFF. 
 
 E. W. Hilgard, Ph.D., LL.D., Director and Chemist. 
 
 E. J. Wickson, M.A., Horticulturist, and Superintendent of Central Station 
 Grounds. 
 
 W. A. Setchell, Ph.D., Botanist. 
 
 R. H. Loughridge, Ph.D., Agricultural Geologist and Soil Physicist. (Soils and 
 Alkali.) 
 
 C. W. Woodworth, M.S., Entomologist. 
 
 *M. E. Jaffa, M.S., Assistant Chemist. (Eoods, Fertilizers.) 
 
 G. W. Shaw, M.A., Ph.D., Assistant Chemist. (Soils, Beet-Sugar.) 
 
 George E. Colby, M.S., Assistant Chemist. (Fruits, Waters, Insecticides.) 
 
 Leroy Anderson, M.S. A., Animal Industries, San Luis Obispo. 
 
 A. R. Ward, B.S.A.. D.V.M., Veterinarian, Bacteriologist. 
 
 E. H. Twight, B.Sc, Diplome E.A.M., Viticulturist. 
 
 E. W. Major, B.Agr., Dairy Husbandry. 
 
 A. V. Stubenrauch, M.S., Assistant Horticulturist and Superintendent of Sub- 
 stations. 
 
 *J. Burtt Davy, Assistant Botanist. 
 
 H. M. Hall, M.S., Assistant Botanist. 
 
 C. A. Triebel, Ph.G., Student Assistant in Agricultural Laboratory. 
 
 C. A. Colmore, B.S,, Clerk to the Director. 
 
 Emil Kellner, Foreman of Central Station Grounds. 
 
 John Touhy, Patron, ) _ 
 
 _. _ }- Tulare Substation, Tular< 
 
 Julius Forrer, Foreman,) 
 
 R. C. Rust, Patron, 
 
 , Foothill Substation, Jackson. 
 John H. Barber, Foreman, ' 
 
 S. D. Merk, Patron, ) n , D a , . .. ^ „ , , 
 
 T XT „ , TT , , > Coast Range Substation, Paso Robles. 
 
 J. H. Ooley, Workman m charge,) 
 
 S. N. Androus, Patron,) _ ,. _ _.. .«,,.. ( Pomona. 
 
 T ttt -nr -^ r Southern California Substation, -< _ i . 
 
 J. W. Mills, Foreman, j ( Ontario. 
 
 V. C. Richards, Patron, ) _ _,_ A . 
 
 m T _, . }■ Forestry Station, Chico. 
 
 T. L. Bohlender, in charge,] 
 
 Roy Jones, Patron, \ ^ . 
 
 TTT „ y Forestry Station, Santa Monica. 
 
 Wm. Shutt, Foreman,) J ' 
 
 * Absent on leav< 
 
 The Station publications (Reports and Bulletins) will be sent to any citizen 
 of the State on application, so long as available. 
 
THE RED SPIDER OF CITRUS TREES. 
 
 The investigation of the red-spider on the citrus trees of southern 
 California was undertaken in response to a very urgent request of the 
 Horticultural Commissioners of Los Angeles County, and has been 
 conducted in cooperation with them. A large part of the work was 
 under the immediate charge of Mr. W. H. Volck, a student in the 
 Entomological Department, who was appointed as inspector by the Los 
 Angeles County Board for this purpose. The present bulletin is only 
 a report of progress, since the investigation is still under way and some 
 of the most important facts remain yet to be determined. The season 
 is nearly here for the red spider to resume its injury, and this is 
 published in order to place in the hands of the orange growers the 
 information already accumulated, so that they may make use of it 
 during the present season. 
 
 Red spiders have been injurious in southern California for many 
 years, but little attention has been given to the matter, chiefly on 
 account of the extremely minute size of these creatures. During the past 
 shipping season the injury to the fruit caused by the improper use 
 of distillate sprays, awakened more than usual interest in the subject 
 of spotted fruit, including that produced by other agencies. The 
 theory was advanced, and generally believed by the growers, that the red 
 spider was accountable for one of the commoner forms of spotting very 
 prevalent during the past season. The results of the studies made 
 this summer on its habits have thrown great doubt, if not indeed dis- 
 proved the possibility of this mite producing anything like this kind 
 of injury. There is no doubt that its work, however, has resulted in 
 considerable losses to the growers of oranges and lemons that have not 
 been clearly recognized. 
 
 The injuries produced by the red spider are chiefly of two sorts, 
 both dependent upon a diminution of the general health of the tree. 
 The one most readily recognized is the unusual or excessive dropping 
 of the fruit from the trees badly affected. Usually this dropping is not 
 to be attributed to the work of the mite alone, but other things 
 contribute to the result. The other form of injury still more difficult 
 to estimate is the loss which results from the decrease in the size and 
 sugar content of the fruit, that cannot help but occur when the leaves 
 
4 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 are injured in the way that results from the feeding of this mite. To 
 these may be added the effects that may occur in a diminution of the 
 growth of the tree ; but it is possible that the dropping of the fruit may 
 relieve the tree and counterbalance, in part at least, the direct injury. 
 
 The appearance of the fruit from trees badly infested with the red 
 spider is very characteristic, and easily recognized when once known, 
 but has not, as far as we can learn, been taken into consideration in 
 the grading of fruit. Should the market come to recognize and begin 
 to discriminate against the paler fruit, upon which the mite has been 
 at work, the losses would become very large. 
 
 The Species Concerned. — The red spider that attacks citrus trees in 
 southern California is an entirely different creature from that, known 
 by the same name, that occurs, all over the State, upon deciduous trees, 
 particularly the stone fruits. The species also differs from the one 
 most prevalent on citrus trees in Florida, and from the greenhouse 
 pest which in this State often attacks ornamental plants quite seriously. 
 These latter species belong to the same genus as the mite on our orange 
 trees, and are therefore quite closely allied. Many other mites do 
 injury in California and some of them attack citrus trees. The best 
 known of these is the silver mite, so troublesome in San Diego County, 
 which belongs to an entirely different group than do the red spiders, and 
 is capable of producing a much greater direct loss to the grower than 
 any other mite which attacks these fruits in this State. 
 
 The red spider with which we have to deal was first described by 
 Hubbard in his report on the orange insects of Florida. Curiously 
 enough, Hubbard supposed it to be carnivorous, but Banks, in Bulletin 8 
 of the technical Series of the Division of Entomology U. S. Depart- 
 ment of Agriculture, in monographing the members of this group, 
 states that Hubbard must have been mistaken, since all the other 
 members of the genus are vegetable feeders. Our species, Tetranychus 
 mytilaspidis, differs in a number of respects from most of the other 
 members of the genus, particularly in the presence of conspicuous 
 tubercules upon which the large spines are borne. The only other 
 species possessing these tubercules and known to occur in the United 
 States, has not as yet been reported from this State. 
 
 Life History. 
 
 The Egg. — The eggs of the red spider are very peculiar in the 
 possession of a long slender stalk projecting from the middle of the 
 top side, extending upward nearly twice as high again as the egg itself. 
 Not including the stalk, the egg is perhaps a third wider than high, and 
 circular in outline as seen from above. The stalk is transparent and 
 
RED SPIDER — LIFE HISTORY. 
 
 surmounted on the top by a very slightly enlarged transverse section, 
 to which is attached, immediately after laying, a series of rather 
 regularly arranged delicate silken threads, about fourteen in number, 
 radiating from this point to the surface of the leaf, forming a series of 
 guys by which the egg is held firmly in position in spite of the fact 
 that otherwise it is very loosely attached to the leaf. These are shown 
 in Fig. 1. The egg- sucking Coniopterijx described in the latter part of 
 this bulletin, very commonly pushes the egg loose from its attachment 
 at the base, in its efforts to puncture it, but does not succeed in pushing 
 it off from the plant on account of the strength of the guys. 
 
 The egg when first laid is uniformly bright red in color. Very 
 soon one may observe a very curious series of changes taking place in 
 the substance of the egg, showing the progress of the development of 
 the young creature within. The length of time necessary to develop 
 and hatch varies more or less with the temperature, being distinctly 
 more rapid in summer than in winter, but also varies a great deal when 
 all the conditions are the same. Of two eggs laid the same day by the 
 same parent, and lying adjacent to each other on the same leaf, one will 
 sometimes require nearly twice the time as the other before the young 
 mite appears. In general, the time may be given for the majority of 
 the eggs as between one and two weeks. 
 
 The egg shell breaks open to allow the exit of the young mite in a 
 very regular manner. This break occurs all the way around the egg 
 at the equatorial line. The top half is held in position by the guys 
 previously described, so that when the shell is empty the two halves 
 may retain almost their same relative position. 
 
 Fig. 1. 
 the egg of the red spider showing the stalk and silken guys. 
 
6 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 The Young Mite. — Upon hatching, the creature which appears 
 resembles the adult mite very closely except in size and in the absence 
 of the hind pair of legs, making the number six, thus resembling 
 insects to this extent. Many other mites are six-legged when first 
 hatched from the egg. 
 
 The young red spider at once begins to feed and is very soon ready 
 to prepare for the change of skin which results in the assumption of 
 the fourth pair of legs. About a day's feeding will furnish sufficient 
 growth for this change. The old skin is not changed at once, but the 
 mite goes into a resting condition which requires nearly as much time 
 as does the feeding period. Mites when in their resting condition 
 place themselves in a very characteristic attitude. The position of the 
 feet is similar to that assumed when feeding, which is shown in the 
 figures of the adult spider, but the mouth-parts are retracted. The 
 color of the whole anterior end of the body is changed, becoming much 
 paler or entirely white, the red coloring being withdrawn entirely from 
 this region. The legs are pulled partly back into the body, so that 
 one can see the feet, which are being remodeled, lying within the skin 
 about to be discarded and appearing as one leg within another. At 
 this time, in a specimen that has been cleared by soaking in alcohol and 
 especially if it is also stained, one can also very plainly see the newly 
 forming fourth pair of legs. These lie doubled up against the ventral 
 side of the abdomen just behind the third pair. 
 
 After about a day of rest, during which the new skin is formed, 
 the back of the old skin splits and the first eight-legged stage appears. 
 This eight-legged mite is scarcely larger than the newly hatched one, 
 and would not be easily distinguished but for the number of legs it 
 possesses. The feeding and rest periods in this stage cover three or 
 four days, and there is a very material growth to be noted. When the 
 mite enters upon its second rest-period, preparatory to a second moult, 
 it has become about half grown. The history of the third stage of the 
 mite is almost an exact repetition of that of the second, only that the 
 mite is much larger. 
 
 Feeding Habits. — The mouth parts of the red spider consist of an 
 oval plate projecting forward above and between the front legs, bearing 
 on the underside a sharp sword-shaped vertical blade, on each side of 
 which there is a rather fleshy palpus and a slender lancet. When the 
 mite has selected a location and is about to begin to feed, it bends the 
 mandibular plate down towards the leaf, pulling the whole organ at 
 the same time into the body and bringing the point of the median 
 blade against the surface of the leaf. The blade is guided and held 
 
RED SPIDER — LIFE HISTORY. 7 
 
 in place by the palpi. The mite now forces the mouth parts forward, 
 effectively slitting the tissue of the leaf with the blade. Before 
 performing this last operation it has carefully placed the two 
 anterior pairs of legs directly in front of the body in the position 
 shown in the figures so that they will hold to the best advantage as 
 an anchorage while the surface of the leaf is being broken. The strain 
 is so hard that the feet will often slip during the process. 
 
 After the slit is made the blade seems to be withdrawn and the 
 lancets inserted, the palpi serving to direct them; these lancets can be 
 thrust out and retracted by means of muscles attached directly to their 
 bases. The juices of the leaf are sucked up through a tube-like cavity 
 between the palpi; and where each slit is made and the contents 
 pumped out a paler spot remains. Each spot is almost microscopic 
 in size, but the combined work of many mites on a leaf produces an 
 injury that is very evident. On the leaf the color of the spot is a dull, 
 pale green, which as the leaf turns yellowish becomes quite undistin- 
 guishable from other parts of the leaf. On the fruit of the orange the 
 appearance while it remains green is the same as on the leaf. As the 
 fruit ripens the spots do not become invisible but are pale yellow, 
 contrasting even more clearly than before with the normal color of the 
 fruit. Spots made by the mite on the ripe orange are the same in color 
 as those made while the fruit was green. 
 
 The young red spider feeds almost continuously, except at the rest 
 periods preceding each change of skin. It does not stay at any one 
 spot any great length of time, but after pumping one place dry will 
 walk a short distance and then take another meal. The time when 
 the mite wanders most is just after the last moult, which usually 
 occurs on an older leaf. The male may remain on these leaves, but 
 the female generally proceeds to young foliage that has grown during 
 the period the mite was coming to maturity. In this way the newer 
 foliage will seem to have only adult females. 
 
 The Adult Male. — The male is very much smaller than the female, 
 though possessing legs of about equal length. It is extremely active, 
 wandering about, particularly on the older leaves where the young 
 insects are most abundant, feeding from time to time, but apparently 
 spending the major portion of its life in moving. They are not to be 
 found at all, apparently, on the more exposed leaves of the plant, but 
 are fairly abundant on the under sides of the older leaves. The total 
 numbers on the tree are probably about one-half as many as of the 
 females. The male examines and seems to work over the resting stages 
 of the young insect with its palpi very persistently, and will often 
 
8 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 remain for a long time perfectly quiescent beside one of these young 
 individuals. Indeed the most ready means of recognizing the male is 
 by searching for pairs of individuals standing side by side. 
 
 Fig. 2. 
 the male red spider. 
 
 The Adult Female. — The appearance of the adult female is well 
 shown in Figure 3 and in the frontispiece. It is on this insect that 
 the hairs will be seen to be best developed. The exact arrangement 
 of the hairs can be made out by the comparison of the illustration. The 
 hairs consist of two kinds — short ones upon the legs and a few at the 
 extremities of the body, and large, curved bristle-like hairs upon the 
 back. These latter are densely beset with fine projections, which have 
 been supposed to afford points for condensation of moisture, keeping 
 the body of the insect itself dry. The function of the hairs appears to 
 
RED SPIDER — LIFE HISTORY. 
 
 be largely that of protection against the creature's enemies. They are 
 so placed that it is very difficult for an insect to reach the body of the 
 mite with its jaws. The feeding habits of the adult are the same as 
 described for the young insect. 
 
 Fig. 3. 
 
 THE FEMALE RED SPIDER. 
 
 Egg-Lmjing. — The mite prefers a somewhat protected location for 
 egg-laying. The place most commonly selected is on the leaf surface 
 immediately adjacent to the midrib, especially on the underside. Very 
 commonly also, eggs will be seen just beneath the edge of the leaf 
 where the surface curls slightly downward. Another favored place 
 for oviposition is the hollow on the under side where the membrane 
 cups upward between two veins. Occasionally an egg will be laid in 
 
10 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 an exposed situation; eggs are found to some extent upon the fruit, 
 being situated almost always in the hollow. 
 
 The number of eggs produced by this insect depends to a large 
 extent upon the length of life. We have repeatedly obtained as high 
 as eighteen eggs from a single female in our breeding experiments, and 
 eggs are produced as rapidly as three per day for considerable periods. 
 It is impossible to say whether the mite is under the best conditions in 
 our breeding cages, and whether more might not have been produced 
 under perfectly normal conditions. 
 
 The egg is laid while the insect is actually feeding, and the process 
 has several times been observed under the microscope. As soon as 
 the egg is deposited upon the leaf, the insect, still keeping its mouth 
 parts attached to the leaf, brings the hind legs partly under the body, 
 lifting the latter high into the air. At the same time that the body is 
 lifted the stem of the egg is emitted, and the mite rests for a moment 
 with the body held in this elevated position. Then the mite ceases to 
 feed, works the abdomen about a little at the top of the egg shell, 
 apparently bringing the feet against the stem of the eggs and moves 
 the body back and forth to and from the surface of the leaf. In this 
 way the silken guys already referred to are spun and attached. The 
 number of guys placed around the egg is considerably larger than the 
 number of trips made by the abdomen from the top of the egg stem to 
 the surface of the leaf, so that it is probable that the legs aid in the 
 placing of these latter upon the leaf surface. We were not able to 
 satisfy ourselves as to the exact position of the spinerets from which 
 the silk is produced, but have very little doubt, from the process of 
 egg laying observed, that they are situated near the hind end of the 
 body. These silken strands are emitted by the insect at other times, 
 as well as at egg laying, though not as abundantly as is the case with 
 other species of red spiders. They often form quite a complete tent 
 over a colony of young mites. The individual strands are so delicate 
 that it is almost impossible to observe them. The mite is, in all prob- 
 ability, continually emitting silk as do true spiders, because when 
 suddenly jarred from the leaf it will in most cases hang suspended 
 in the air by a silken thread, which it uses to regain the leaf, in the 
 same way that the true spiders climb the strands of their web. 
 
 Natural Enemies. 
 
 The experience of former years in reference to this mite is that it 
 suddenly increases in the fall and decreases again in the spring, and 
 may, at times, almost completely die out in an orchard and not appear 
 again for several years. This suggested at once that the natural 
 
RED SPIDER — NATURAL ENEMIES. 11 
 
 enemies of these mites might be the most important factor in the 
 problem. At the beginning of the investigation, therefore, a great 
 deal of attention was given to the subject of the natural enemies of 
 the red spider, and a large number of insects and mites were found to 
 be engaged in feeding upon them. The most important of these 
 we will discuss below. 
 
 Ladybirds. — The most conspicuous enemies of the red spider were 
 the larvae of the ladybirds. The largest of these ladybirds was the 
 common Hippodamia convergens, which was observed in all stages in 
 the orchards about Azusa and indeed in every part of southern Califor- 
 nia visited. This insect lays its eggs in small masses of about a dozen 
 eggs each almost anywhere, commonly on the orange leaves or even on 
 the fence. The eggs are elongate, lying side by side attached by their 
 ends. After hatching, the young larvae remain huddled over the egg 
 shells for about a day, before they scatter and begin to feed. After 
 scattering, each larva wanders about by itself, crawling over the leaves 
 in search of food. It does not readily find a red spider, but apparently 
 in an entirely accidental manner bumps against one while wandering 
 about. After finding a red spider, however, it will, if hungry, at 
 once pounce upon it and devour the whole mite in short order. If the 
 larva is small or if it is not so hungry, it may merely eat out the 
 larger part of the contents and leave the rest of the carcass upon the 
 leaf. The young insects experience considerable difficulty in getting 
 to the body of the mite between the hairs, but the older individuals 
 seem strong enough to force the hairs out of the way. These lady- 
 birds appear to prefer the orange plant louse which is also very 
 abundant upon the orange trees during the winter season. After the 
 ladybird larva is fully fed it proceeds to pupate upon the leaf, attach- 
 ing itself by the hind end of the body in the manner usual with the 
 ladybirds, the pupa being formed within the old larval skin, which 
 splits open as soon as the pupa is complete. Emergence of the adult 
 occurs in the same way by the long dorsal split. We have not 
 observed the adult insect feeding upon the red spider, but have noted 
 it eating the plant lice. 
 
 Another ladybird, a Rhizobiid, was more or less abundant in 
 places, and the larva was repeatedly observed feeding upon the red 
 spider. It appears, indeed, that the red spider constituted the bulk of 
 the food of this creature, both in the larva and adult condition. These 
 lady-birds were much less numerous than the former species and the 
 egg stage was not observed. 
 
12 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 Lace-winged Flies. — The larvae of the common lace- winged fly 
 (Chrysopa calif omica Coq.?) were very much more voracious than 
 the larvae of the ladybirds. Had they been as abundant as the latter 
 it is possible they would have had an appreciable effect upon the 
 numbers of the red spider, but while they were abundant enough to 
 be collected in numbers they could not be considered as having any 
 noticeable effect upon the mite. The lace- winged flies could readily be 
 observed in all stages at any time upon the orange trees. The eggs 
 are laid singly or only a few together, usually on the upper side of the 
 leaf. The adult insect first exudes a long thread attached vertically 
 to the surface of the leaf, and then deposits the egg upon the end of 
 this stalk. The young insect that hatches can be readily distinguished 
 from all other larvae found on the orange leaves, by the large size of 
 the jaws. The body is somewhat spindle-shaped, the legs are well 
 developed and the insect is very active, devouring mite after mite in 
 rapid succession. The insect does not eat up the mite it attacks, but 
 merely sucks out the contents of the body. When full grown the 
 larva makes a slight cocoon and transforms into a pupa, from which 
 very soon the adult lace- winged fly emerges. 
 
 The Coniopteryx. — Rather closely related to the lace-winged fly is 
 an insect bearing the name of Coniopteryx. The members of this 
 group are very rare insects, only a single rare species being heretofore 
 known in the United States. Our species was observed in considerable 
 numbers in the orchards about Azusa and appears to be more important 
 than any other of the natural enemies of the red spider. It works wholly 
 upon the egg of the mite. The coniopteryx is a small, white-winged 
 insect, suggesting somewhat a large "white fly." The adult insect 
 moves about in much the same way as the lace-winged fly, and while 
 slightly more abundant was by no means a conspicuous object. One 
 point in the life history has not yet been cleared up, — the egg laying 
 habits not having been observed. The young insect is a beautiful 
 mottled black and white larva with somewhat the same general shape 
 as those of the lace- winged fly, but with a head having protuding eyes 
 and terminating in a slender beak instead of large, spreading, jaws. 
 The legs are very much smaller, though the insect gets about quite 
 rapidly. It uses the posterior end of the body to a large extent to 
 supplement the legs, there being a sucker-like organ at the end of the 
 abdomen. The larva is continually at work searching for the eggs of 
 the red spider, which as soon as found it proceeds to puncture. The 
 egg shell is quite resistent, so it requires the utmost effort of the insect 
 to accomplish its purpose. As soon as the shell is broken the larva 
 
RED SPIDER — NATURAL ENEMIES. 13 
 
 begins to suck out the contents, apparently projecting into it a 
 slender tongue-like organ from the end of the proboscis, by means of 
 which it laps up the last trace of the coloring matter from the inside 
 of the shell, leaving it as white as though the mite had emerged. The 
 rate of feeding depends entirely upon the abundance of eggs. When 
 placed upon an orange well stocked with eggs, and observed under the 
 microscope, one insect was noticed to remove the contents from the red 
 spider eggs at the rate of two per minute for ten minutes at one time. 
 When the Coniopteryx larva is full grown it proceeds to spin a 
 double cocoon on the under side of the leaf, consisting of an outer flat 
 layer about 6 mm. in diameter, under the center of which it makes a 
 nearly spherical inner covering. 
 
 Predaceous Mites. — The red spider is attacked by three or four kinds 
 of predaceous mites. One large red species was extremely active and 
 devoured the female mites very voraceously. All of the mites are 
 sucking animals, therefore the body of a red spider remains on the leaf, 
 after the attack of these predaceous species, or falls to the ground. 
 Predaceous mites were never common enough anywhere to become an 
 important factor in the life of the red spider. Certainly none of the 
 enemies of the latter, as observed in southern California, could accomp- 
 lish anything towards checking the increase of this creature. The 
 fact was notable that the orchards first to become freed from the red 
 spider last spring were not the ones in which we found the predatory 
 forms most abundant. Under the conditions existing they constituted 
 no appreciable factor in the decrease of numbers that occurred in the 
 latter part of May, and in June. 
 
 Diseases. — In the orchards where the rapid decrease of the red 
 spider was first observed, the presence of a fungus on dead mites was 
 noted and collections were made in other orchards in order to determine 
 its presence there. While the fungus was noted almost everywhere, 
 still it was by no means so prevalent where the mites had not yet begun 
 to die so rapidly. After the mites had almost disappeared from an 
 orchard the remains of the red spider could be collected quite abundantly 
 by beating the branches over a glazed paper tray, and their mummies 
 almost invariably showed the presence of the fruiting threads of the 
 fungus, extending out from the body. The full account of this fungus 
 will be reserved for the final report on the red spider. The presump- 
 tive evidence that we have here a fungous disease seemed quite strong. 
 Death was certainly not brought about by the condition of the weather 
 directly, for in two orchards close together one may be badly infested 
 and the other almost freed from the mite at the same time. We were 
 
14 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 not able to produce the disease at will, however, either in the orchard 
 or in our breeding cages; and the fungus is not one supposed to pro- 
 duce insect diseases, but belongs to a group ordinarily found on 
 decaying substances. It would seem that if the death of the red spiders 
 had been caused by any infectious disease, our experiments should 
 have given more definite results. Further experiments with this possi- 
 ble disease of the red spider were of necessity deferred until another 
 appearance of this mite gives opportunity. The definite relation of the 
 red spider to the season of the year indicates that if there is a disease 
 that is accountable for the diminution in the spring, it is one very 
 dependent on weather conditions. 
 
 Remedies for the Red Spider. 
 
 The red spider of the citrus trees is so dependent upon the proper 
 external conditions that it would seem that the most promising line of 
 investigation would be to determine what the cause of death is at the 
 time of their sudden decline in number, in the hope that one might 
 artificially bring about those conditions. Much of our work has been 
 directed toward that object, but the results have as yet been chiefly the 
 elimination of the possibility of the natural enemies being any large 
 factor in the problem. The approaching season of rapid increase may 
 throw a good deal of light on the subject. For the present at least we 
 must depend on the use of insecticides. 
 
 Fumigation. 
 
 The effectiveness of cyanide fumigation against most kinds of 
 insects might lead one to expect that it would prove useful in com- 
 bating the red spider. Hydrocyanic acid will destroy the mite if used 
 in large enough doses and for a long enough time. It appears to be 
 decidedly more resistant than the scale insects, however. Now, for the 
 destruction of the latter, the practice is to use about as much as the 
 tree will stand without serious injury to the foliage; but this strength 
 is not sufficient to do satisfactory work in the killing of the red spider. 
 It is the uniform experience of those who have observed the effects of 
 the gas when fumigating trees infested with red spiders, that the latter 
 are not appreciably diminished in numbers by the treatment. What 
 is still more curious, fumigated trees appear to be more favorable to 
 the red spider than are adjacent untreated trees, and are commonly 
 more injured by the mite during the months immediately following the 
 fumigation. The experience seems to be extensive enough to warrant 
 the statement that fumigation is of no value whatever as a means of 
 controlling the red spider. 
 
RED SPIDER — REMEDIES. 15 
 
 Sulfuring. 
 
 The common red spider of deciduous trees has been quite satisfac- 
 torily controlled by the persistent use of sulfur. The practice is to 
 dust the trees with the dry powder, applying it at the rate of about 
 twenty-five pounds to the acre at each application. The method varies 
 from throwing the sulfur into the trees by the hand, to the use of the 
 seed sower or a rotary bellows geared to the wheel of a heavy farm 
 wagon. The application is repeated whenever there is evidence of the 
 beginning of a new attack, which may be several times in a single 
 season. The action of the chemical is supposed to be the same as its 
 action on mildew, for which it is used in the same way, — that is, the 
 vapor produced by the heat is fatal to them . The effectiveness of this 
 treatment depends, therefore, upon the occurrence of hot days. The 
 species for which it is used does its worst work in the hottest 
 part of the season. Where the summers never get hot, as at 
 Berkeley, the red spider of the deciduous trees never does any serious 
 injury though it is able to live, and appears to be always present. 
 The red spider of the citrus trees requires some degree of dryness for 
 its best development; this is usually not great near the coast, but 
 still it is not able to do much during the hotter part of the year. 
 Sulfur cannot therefore be expected to be as reliable against this mite, 
 because of this difference in season. We know of no experiments on 
 a scale large enough to fairly test the matter, but smaller experiments 
 have given entirely negative results. It is probable that a few warm 
 days in mid-winter would make sulfuring quite efficient, but as warm 
 weather cannot be depended upon, sulfuring can at best be con- 
 sidered simply as a supplementary measure, useful only under favorable 
 conditions. 
 
 Spraying. 
 
 As matters now stand the only really effective method of treating 
 the red spider is the use of an insecticide applied as a spray. There 
 are several substances that will kill it quite readily, at least in its 
 active condition, and it is possible to prevent its injury almost 
 entirely by this means. To do the best work it has been found 
 necessary to make two or three applications, and it is therefore quite 
 expensive. In the case of injury to a tree produced in the way this 
 creature works, it is very difficult to make more than a rough guess 
 as to the amount of loss occasioned, and therefore of the amount it is 
 profitable to expend for the control of the pest. It is certain that 
 where there is no scarcity of water and the orchard receives the 
 proper attention, the mite is not likely to materially decrease the 
 
16 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 growth of the tree. How completely the injury to the crop can be 
 mitigated by giving the trees some extra care remains to be deter- 
 mined. Probably under most conditions the killing of the spider 
 would result in a saving, far more than enough to pay for the 
 number of sprayings necessary. 
 
 Citrus trees are extremely difficult to spray effectively. To 
 do really effective work every leaf of the tree, as nearly as possi- 
 ble, should be thoroughly wetted on both sides. The web made by 
 these mites, and their habit of chosing protected points for their trans- 
 formation and egg-laying, requires that the spray should be applied 
 with good force. The way the leaves are arranged on an orange tree 
 makes it almost impossible to spray the tree thoroughly from the out- 
 side. The red spiders are wholly on the foliage, so that one does not 
 have the same problem as when attempting to spray for scale insects; 
 but the problem is none the less difficult. 
 
 The difference between thorough and careless sprajdng is very little 
 in time or material but amounts to a great deal in efficiency. One 
 should clearly appreciate all the difficulties and work intelligently and 
 with a well thought-out plan. 
 
 Method of Spraying. — The following plan of operation is thought 
 to provide for quite thorough work, more thorough than any of the 
 methods now in use. The essential feature is the provision for as 
 careful inside spraying work as is the practice on the outside of the 
 tree. Many at present recognize the importance of inside spraying, 
 and have adopted the practice of inserting the extension rod into the 
 tree at several points for this purpose. It is possible for this method 
 to result in a thorough treatment of the inside, but one cannot be sure. 
 It is something like spraying the tree in the dark. The practice here 
 proposed is to actually enter a tree and see what one is doing. It will 
 be most convenient to spray the interior first, and for this a hose and 
 nozzle without extension rod will be necessary. One man may confine 
 his attention to the interior, and the other to the outside. The plan 
 to be followed for inside spraying is shown in figure 4. The sprayer 
 forces his way in between the branches to the center of the tree and 
 begins to spray at, say, his left side, spraying thoroughly from the 
 ground up to a line somewhat above the level of his head. The dotted 
 line in the figure indicates approximately the course of the nozzle as he 
 sprays the far side of the tree working around among the branches. 
 The coures of the nozzle on the near side of the tree is indicated by the 
 solid lines. After the lower part of the tree is thus thoroughly sprayed 
 the upper part may be treated as indicated, the operator of the nozzle 
 
RED SPIDER — REMEDIES. 
 
 17 
 
 finally backing out while doing the portion that stands directly over 
 his head. 
 
 
 
 Fig. 4. 
 plan for inside spraying. 
 
 The object of spraying the lower part of the tree first is that in this 
 way the sprayer will not be troubled by dripping from the leaves above 
 him and will be more likely to do careful work. 
 
 The outside procedure is represented in figure 5. The letters a, b, 
 and c indicate successive positions during the process. The extension 
 rod is furnished with a nozzle giving an oblique discharge. The first 
 attention is given to the top, which is the most difficult. The operator 
 moves about the tree swinging his rod from d to e or from f to g 
 until he is sure that everything is thoroughly wetted. He is then ready 
 to begin on the lower part of the tree. The rod is now swung from 
 the position h to i, back and forth, as he goes around the tree wetting 
 everything in his progress. 
 
 The above plan is not expected to require any more material or 
 time than spraying would ordinarily require if done without a definite 
 plan, but should certainly be much more likely to wet every leaf with 
 the spraying material. 
 
 Spraying Materials. — Only two substances have been used for 
 spraying for the red spider. One of these, distillate emulsion, has 
 apparently accomplished good results but is still in the experimental 
 stage. We have given considerable attention to the subject, but have 
 
18 
 
 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. 
 
 not yet arrived at results that will enable us to make definite recom- 
 mendations. There is no question of the killing power of the sub- 
 stance, but the effect on the tree is the matter that requires more 
 attention. The product is not uniform, but could be if we had any 
 definite idea of exactly what we want, as the refining companies are 
 anxious to produce an article that will be satisfactory. We have 
 obtained many samples of crude oils and distillates and have done 
 considerable work in testing them, and have obtained some important 
 results. These will be reserved for another bulletin. 
 
 Fig. 5. 
 plan for outside spraying. 
 
 A material that is both safe and effective is the sulfid of potash 
 spray mixture. This mixture is usually made according to the following 
 formula : 
 
 Potash 32 lbs. 
 
 Sulfur, finely ground 37 lbs. 
 
 Salt 2 lbs. 
 
 Water 50 gal. 
 
 This makes the stock-solution which is diluted with about a hun- 
 dred times as much water for spraying. The potash, sulphur, and 
 salt may be mixed together in a large metal tub with a little water, 
 
RED SPIDER — SUMMARY. 19 
 
 when chemical action will at once set in and the whole mass will dis- 
 solve and begin to boil very vigorously . After the boiling has ceased, 
 the water is added and the stock-solution is made. It is very doubtful 
 whether the salt is of any value in this mixture, but it can at least do 
 no harm as it is in such small quantity. 
 
 This sulfid of potash is of very little value, at least at this strength, 
 as an insecticide, but is effective enough against the active stages of 
 the mite. In order to kill the eggs and moulting forms it will be 
 necessary to make three applications, separated by intervals of one 
 week. This will make a very clean sweep. 
 
 Summary. 
 
 The red spider of citrus trees, Tetranychus mytilaspidis, does not 
 produce the spotting of the fruit usually credited to it, but does cause 
 dropping, and also injures the leaves. The eggs, which are very 
 peculiar, may be found abundantly on the leaves and to some extent on 
 the fruit. They hatch after a week or two, disclosing a six-legged 
 mite, which after about two days, one of which is spent in feeding, 
 changes its skin and becomes eight-legged. Two more moults occur, 
 requiring five or six days each before the adult mite appears. It feeds 
 on the leaf by making a slit, out of which it sucks the contents of the 
 cells beneath. The male is very active and is found mostly on the 
 older leaves. The female is much larger than the male, and lays a 
 great many eggs. 
 
 The natural enemies most abundant are the ladybirds, the lace- 
 wing flies, the coniopteryx and a number of species of mites. They 
 do not accomplish much toward the checking of the red spider. A 
 fungus was studied that may be the direct cause of the spring decrease 
 of the mite, but the weather is the really important item. 
 
 Until we learn more about the reasons of the natural decrease of 
 the red spider we will have to depend on the use of insecticides. 
 Fumigation is of no value for this purpose. Sulphur is not as valu- 
 able as it is for the red spicier of deciduous trees. Spraying is a 
 difficult operation on dense trees like the orange, and is done best by 
 spraying both from the inside and the outside. Distillates may prove 
 to be the best material, but until we know better how to avoid injury 
 to the tree, sulfid of potash is to be recommended.