^^. >?U^±.^o<^^ /^ 'TATE PLANT BOARJ; Circular No. 57. United States Department of Agriculture, BUREAU OF ENTOMOLOGY. L. O. HOWARD, Entomologist. THE OREENHOUSE WHITE FLY. • {Aleyrodes vaporariorum Westw.) By A. W. Morrill, Special Field Agent. The damage by the greenhouse white fly to tomato, cucumber, and man}^ other plants growing under glass easily places it in the front rank of greenhouse pests. In many cases it would be impossible to grow certain crops in forcing houses without the aid of remedial measures. A specific instance is on record where, in a western Massa- chusetts town, the attacks of this insect resulted in the total loss of a greenhouse crop of tomatoes and cucumbers valued at |4,000. SB Fig. 1. — Aleyrodes vaporariorum: a, egg; b, young larva; c, pupa, top view; d, pupa, side view; e, adult— c, d, e, about 25 times natural size; a, b, still, more enlarged {a-d, after Morrill, Tech. Bui. Mass. Exp. Sta.; e, original). 20127-05 During the past few 3^ears many appeals for remedies have been made to the editors of our horticultural journals, to man}^ State exper- iment stations, and to the United States Department of Agriculture; yet, although it is not, in reality, a difficult insect to control, the remedial methods which should be followed are far from being gener- ally known among the many greenhouse men who suffer from its depredations. HISTORY, ORIGIN, AND DISTRIBUTION. The records of the greenhouse white fly date back to 1856, ^ when Prof. W. O. Westwood, of England, recognized it as a previously undescribed species. The first published record, so far as is known to the writer, of the existence of the insect in this country was in the year 1870, when Dr. A. S. Packard, at that time Massachusetts State entomologist, reported it as occurring in abundance on tomato plants at Salem, Mass. ^ Two places have been suggested as the original home of this species, viz, Mexico and Brazil, but while presumably the origin is tropical American, there is no definite information on this point. Of more importance from a practical standpoint is its present distribu- tion. Besides its occurrence in Europe, Canada, and Mexico, it is known to be widely distributed in greenhouses throughout the eastern United States, and without doubt it occurs more generally than the published records show. We have specific reports of its occurrence in greenhouses in the States of Maine, New Hampshire, Massachusetts, Connecticut, New York, New Jersey, Pennsylvania, Illinois, Indiana, and Michigan, also in the District of Columbia. FOOD PLANTS. The insect under consideration is notable for its very general feed- ing habits, having alreadj^ been recorded as breeding on over 60 dif- ferent kinds of plants. Of these the following are of the most eco- nomic importance: Aster, chrysanth'emum, salvia, lantana, fuchsia, coleus, ageratum, primula, geranium, heliotrope, rose, eggplant, bean, melon, lettuce, cucumber, and tomato. The two last named sufl'er the most serious injury from this insect, perhaps more than the other greenhouse plants together, although not infrequently there are reported serious losses in greenhouses devoted to one or more of the other plants mentioned. DESCRIPTION AND LIFE HISTORY. The mature white flies of both sexes are four-winged insects scarcely more tiian 1^ mm. or three-fiftieths of an inch in length. The adult a Gardeners' Chronicle, p. 852. & Agriculture of Massachusetts for year 1870. 8 white HioH, as well us the scalo-liko hirva', ai"C provided with suckiiij^ mouth parts. In a short time after th(^ emergence of (li(» a(hiit i'lom the pupa case, the body, legs, and winj^s become covered wilh a wliitc^, waxy substance whicli gives this, as w^ell jis othei' species of tli(^ genus, a characteristic Houry appearance. The adults fecul nearly contin- uously during their existence. If deprived of food, they will rarely live for a longer period than thiee days under ordinary temi)erature conditions. The longest recorded length of life of one of these insects in the adult condition is thirty-six days, but it seems probable that the average ength of adult life is much greater than this instance would indicate. The largest nunjl^er of eggs which an adult white fly is positively known to have deposited is 129, but this number is probably below^ the average. Indeed, the specimen which produced this imm- ber of eggs with little doubt deposited over 50 others which w^ere not recorded. The number of eggs deposited per day by an adult female white fly in a laboratory has been found to average very nearly four. Probably in the warmer temperature of a greenhouse this number is greater by one or two eggs per day. These observations, even though falling short of showing the normal increase in numbers of this spe- cies, emphasize the importance of a remedy 'which will, above all, destroy the adults and check at once the rapid deposition of eggs. A pecularity of the egg-laying habits of this and some other species of white fly is the tendenc}^ to deposit the eggs in a circle while feeding, using the beak as a pivot. These circles, when completed, are about li mm. in diameter and usually contain from 10 to 20 eggs each. On the more hair}^ leaves groups of eggs of this kind are less frequently met with than on those which are more nearly smooth. The majority of the adults are found upon the upper and newer leaves of the food plant. They are almost invariably found upon the underside of the leaves, and it is here that nearly all the eggs are deposited, although many are found upon the tender stems and leaf petioles and a very few scattering ones on the upper surfaces of the leaves. The eggs are distinguishable with difficulty by the naked e3^e, being but one-fifth of a millimeter, or one one hundred and twenty-fifth of an inch, in length. They are more or less ovoid in form and suspended from the leaf by a short, slender stalk. With ordinary greenhouse temperatures the eggs hatch in from ten to twelve days. The newly hatched insect is flat, oval in outline, and provided with active legs and antennae. It rarely crawls farther than one-half inch from the empty eggshell before settling down and inserting into the tissue of the leaf its thread-like beak. After feeding for five or six da3^s, the insect is ready to molt its skin. The second and third stages are much alike, except in size, and difi'er principally from the first stage in that the legs and antennae are vestigial and apparently f unctionless. These two stages occupy from four to six days each. The so-called pupal stage, up to the time when growth ceases, is in reality the fourth larval stage, the fourth larval skin enveloping the true pupa. The pup^e and empty pupa skins are quite conspicuous when the insects are abundant. Their outline is similar to that of the larvae, but the}^ are thicker and box -like, about three-fourths of a mil- limeter, or three-hundredths of an inch, in length and provided with long, slender wax rods or secretions which are useful in distinguishing this from nearly allied species of the white fly. The entire stage from the insect's third molt to the emergence of the adult form lasts from twelve to sixteen days in the laboratory and greenhouse. The adult emerges from a T-like opening, leaving the glistening white pupa case attached to the leaf. At first the wings of the adult are crumpled close to the body, giving them a peculiar appearance. In the course of a few hours the wings unfold and the insect has then completed its development, which has extended over nearly five weeks, if under the ordinary temperature conditions of a greenhouse. CLOSELY RELATED FORMS. In addition to the one here discussed, there are but two other species of white flies which are likely to be met with in the greenhouse. A white fly found infesting citrus plants would be likely to be the orange white fly {Aleyrodes citrl Rilej^ and Howard), while one infest- ing strawberry plants, either in the greenhouse or in the field, would probabl}^ prove to be the strawberry white fl}^ {Aleyrodes jpackardi Mori.). The latter species resembles the one commonly found in green- houses, but fortunately its list of food plants is much more restricted, it being apparently unable to subsist on the tomato. APPEARANCE OF INFESTED PLANTS. As already stated, the upper leaves of a plant are preferred b}^ the adult females for the deposition of their eggs. Thus there is a slow but continuous migration of adults upward to keep pace with the unfolding of the leaf buds. On thoroughly infested plants we find on the upper- most leaves only adults and freshly laid eggs; a little lower on the plants we find eggs in the process of hatching; and, finally, on the lowermost parts of the plants we find discolored, shriveled leaves with manj^ pupje and emerging adults and few, if an}^ unhatched eggs or young larvae. The larvae and pupfe secrete little globules of honey- dew, so named after the material of a like nature secreted by plant lice. These globules usually either drop or are forcibl}" ejected,' and, falling on the upper surface of leaves directly below, give them a glazed appearance. This is frequently followed by the growth of a sooty fungus which hastens the complete destruction of the leaf. When overcrowding of the young occurs, this fungus growth finds favorable conditions for its development on the under surface of the leaf, resultiiio- in the dostruclioii of many of (lie iiiiniaturc insocts. Owiii*^- to the intei'fereiu'C with llic n's])iratory processes of the leaf, })otli l)y the bodies of the insects tlieinselves and ))y the fundus o-rowths due to them, Ijadly infested plants have a tendency to wilt wlien (»xposed to the sun's rays. In seriously infested o-reenhouses the h'aves of the plants gradually die, the lower leaves first, and if unchecked the insects greatl}' impair the value and vftality of the plants, even though they do not actually cause their total destruction. PREVENTIVES. The importance of preventive measures in combating the white fly in greenhouses is not due to the inefficiency of properly applied reme- dies, but to the fact that in man}^ cases the tiny depredator is unob- served until considerable injury has been accomplished. With little trouble and expense one may, in a large measure, preclude the possi- bility of this and other pests appearing in the winter in greenhouses which are unused during the summer months. The introduction of the insect into noninfested floral establishments may be prevented by avoiding the introduction of infested plants' unless first subjecting them to a fumigation in a tight fumigating box, based on the directions hereafter given for entire greenhouses. Vegetable houses, which are not used during the summer months, allow of a practice which not only greatly reduces the chances of the white fly appearing in the house during the growing season, but ma}^ result in the house being success- fully kept free from thrips and other insect pests during the whole or a considerable part of the season. The practice referred to consists in removing from the house all vegetation, even the smallest weeds, and fumigating the tightly closed greenhouse with hydrocyanic-acid gas at the rate of 5 or more ounces per thousand cubic feet of space for a ten hours' exposure. REMEDIES. Fumigatioyi vnth hydrocyanic-acid gas. — This has been found to be the most successful means of controlling the white fly in green- houses. Its success in this case is due to the susceptibility of the adults and larva? of these insects to a comparatively long exposure to a small amount of the gas. Man}^ experiments have been conducted with a view to determining the usefulness of this gas against the greenhouse white fly, the amount of gas to be generated, and the length of exposure necessary to produce the best results. ^' Experi- ments of this kind thus far have been with tomato and cucumber plants, but as these plants are among those most liable to injury from improper fumigation with hydrocyanic-acid gas, a wide range of use- fulness is indicated by the success thus far obtained. «Conn. Station BuL, No. 140; Xew Hampshire Station Bui. , No. 100; Mass. Station •Tech. BuL, No. 1; Maine Station BuL, No. 96; Can. Entomologist, XXXVI, p. 35; American Gardening, XIX, p. 741. ' 6 Annnint of potassJinn cyanide to use and length of exposure. — Experiments and practice have shown that the white % is destroyed in all except two stages {<^gg and late pupal) b}- an amount of potas- sium cyanide which is extremel}- small as compared with the amounts generally recommended for other insect pests. As small an amount as 0.005 gram" per cubic foot of space, or between one-fifth and one- sixth ounce per 1,000 cubic feet, for three hours' exposure, has been used with success, * while as large an amount as 1 ounce per 1,000 cubic feet for an "all-night exposure," in a house containing infested toma- toes, has been reported to have given, in one instance, a like result.^ On the other hand, Mr. E. C. Rittue, of the Bureau of Plant Industry of the Department of Agriculture, in attempting to control the white fly infesting tomatoes in a greenhouse on the grounds of the Department, found that 0.01 gram per cubic foot, or one-third ounce per 1,000 cilbic feet, slightly injured the plants when the exposure exceeded thirty minutes. This treatment for thirty minutes destro3^s onl}^ the adults. The greenhouse is a new one and, judging from the great difference in the results obtained there and in other houses whose fumigation has been recorded in various publications, it is tighter and does not allow the gas to escape as readih^ as does the average forcing house. This shows that the greatest diflSculty attending the use of h3^dro cyanic-acid gas, in greenhouses containing plants as susceptible to injury by it as are the cucumber and tomato, is the difference in the tightness of different greenhouses. It is consequently impossible to give specific directions which will be suitable under all circum- stances. A fumigation with hydrocyanic-acid gas which will kill adults only is not effective for practical use in checking the multipli- cation of the white flies. Rather than this, the method of control described under the headipg, "Treatment when the use of h^^drocyanic- acid gas is undesirable," is greath^ to be preferred. In most greenhouses, probably 0.007 gram of potassium cyanide for each cubic foot of space for an exposure not exceeding three hours represents the amount which will prove most effective for treatment of the insects without injur}^ to tomato or cucumber plants. In many cases 0.01 gram per cubic foot has been found suitable for the same exposure, but this should not be used except in loose greenhouses where, after trial, a smaller amount is found ineffective. In all green- houses when an attempt is to be made to control the white fl}^ with hydroc\'anic-acid gas, it is advisal)le to first use not more than 0.005 gram per cubic foot of space for a three hours' exposure. If this amount is sufficient for the house, none of the adults will recover after the fumigation, though in the course of two or three days many more 0 28.35 grams =1 cfiince. 6 Mass. Station Tech. Bill., No. 1, p. 46. c American Gardening, XIX, p. 741. will emerge from the pupa cases. Th(^ larvii', when destroyed, as they should be by the fumigation, change in two or three days froni their normal glistening, greenish color to a yellowish or brownish color. When this result is not obtained by the first test, one or more further tests should be made, increasing the amount of potassium cyanide 0.001 gram per cubic foot of space for each test, with three days inter- vening to note results, until an amount is reached which is sufficient to destroy the larva^, or until the tender leaves of the plant shovv^ injuries as a result of the fumigation. Tests, thus far, with other greenhouse plants likely to be attacked by the white fly, according to available records, have all been for a much shorter exposure than three hours and with a much larger amount of potassium cyanide, but it is probable that in case plants other thart the cucumber and tomato recpiire treatment for this insect, preliminary tests in a fumigating box or in a small greenhouse will show that the amount of chemicals and length of exposure recom- mended for these two can be used without the slightest danger to other plants. In most cases much larger rates of potassium cyanide per cubic foot can be used. Time to f amig ate ^ preparation of greenhouse^ and method of gener- athuj gas. — Fumigation of plants with hydrocyanic-acid gas should be at night and the foliage of the plants should be dry. The greenhouse to be treated should be made as tight as possible, all entrances but one closed and locked, and arrangements made to open a few ventilators from the outside at the expiration of the period of exposure. A house when fumigated should not be unnaturally tight as a result of I'ain or snow, otherw^ise the greater amount of gas confined in it under these conditions may injure the plants. The materials used for the generation of the gas are 98 per cent potassium C3'anide, commercial sulphuric acid, and water, the proportions generall}^ used being one- half more acid (liquid measure) than potassium cyanide, and one-half more water than acid. Having determined the cubic contents of the house and the total amount of potassium cyanide, sulphuric acid, and water to be used, these should be divided into parts representing each 25 feet of length of the greenhouse. Owing to the small amounts of the acid and water, small receptacles must be used. Six or 8 inches is a desirable height for the receptacles, while the diameter should be as small as possible to use, preferabl v^ not more than 2^ inches. They should be either of earthenware or glass. In many cases, ordinary glass tumblers will be suitable, though the diluted acid should never more than one-fourth fill the receptacle; otherwise the violent chem- ical action whCch follows the introduction of the potassium cyanide might result in the loss of considerable of the material. Each lot of the potassium cyanide should be pulverized or broken up into small pieces, w^rapped in thin paper, and laid beside one of the receptacles, these being placed at intervals of about 25 feet on the floor of the house. In each receptacle tirst pour the proper amount of water and then the acid. Beginning with the end of the house farthest from the exit, drop into each receptacle, in succession, the package of potas- sium cj^anide, proceeding as quickly as possible toward the exit. During the few seconds the operator is in the house after the genera- tion of the gas is started, the breath should be held to prevent even the least bad efl'ects. Close and lock the door of the house and, after three hours, partially ventilate it b\' opening the. ventilators, pre- vioush^ arranged for opening from the outside. One ventilator for every 25 or 30 feet, opened for ten or fifteen minutes, is sutiicient to protect the plants from possible bad efl'ects of overexposure to the gas. Before inhaling air in the house, however, the ventilation should be more thorough, so that no odor of the gas, which is much like that of peach pits, can be detected. The morning after the fumigation the contents of the receptacles should be buried. Time for subsequent fumigations. — A single fumigation, according to the directions given above, will destroy practicall}^ all of the insects except the eggs and some of those in the late pupal stage. Although one such treatment might check the insects so that they would not cause noticeable damage for weeks, in many cases it would be the part of economy to give two more fumigations at times which a knowledge of the life histor3^of the white fl}' indicates would be most advantageous. Knowing that the ^gg and late pupal stage of the insect are not to any great extent aft'ected h\ the treatment recommended, while all the other stages may be destroved, and knowing the duration of each stage, we can outline a plan of treatment which will practically eradi- cate the pest in the worst-infested greenhouses. Two subsequent fumigations two and four weeks, respectiveh^, after the first will sub- ject to the gas all of the white flies in the house in stages wherein, under ordinary circumstances, they are unable to withstand its destruc- tive effects. Treatment vjhen the use of hydrocyanic-acid gas is undesirahle. — Fumigation with tobacco fumes, made b}^ burning the refuse stems and leaves, has no effect on the greenhouse white fly bej^ond tempo- raril}^ stupefying the adults. The adults may be destroyed, however, by vaporizing in the infested house certain tobacco extracts which are sold in liquid form. To accomplish this result preliminary tests should be made, first using the amount recommended in the directions accompanying the preparation. The attempt to control the green- house white fly by means of tobacco extracts alone has never, to the writer's knowledge, proven successful, while many cases of failure have been reported. The frequent fumigation necessary to control the insect when once it has become abundant would be impractical and costly. However, in connection with s3^ringing the plants with a soap solution such a treatment nmy sometimes bo of value, althou^^h only when the use of hydrocyanic-acid gas is impossihh; or for some reason undesirable. Among the sprays, the best brands of whale-oil soap, used in the proportion of 1 to 1\ ounces per gallon of water, have been found to destroy all of the Avhite Hies except the eggs, a small percentage of the nearly mature pupte, and from 25 to 50 per cent of the adults which escape the spray by flying from the plants. It is not advisable to syringe tomato plants in greenhouses at an}^ time, when avoidable, as syringing interferes with pollination and produces a damp atmosphere which promotes rot, but the injury b}' syringing may be as nothing compared with that which is caused daily by the insects. When the use of h^^drocyanic-acid gas is impractical, an all-night fumigation with a tobacco extract is recommended, followed during the next da}^ by a syringing with a solution of whale-oil soap or its equivalent. Comparative cost of the treatment. — Potassium cyanide costs from about 30 to 50 cents per pound, according to the amount purchased. Commercial sulphuric acid costs from about 2^ to 10 cents per pound. The entire cost for a single fumigation of a greenhouse containing 20,000 cubic feet is between 20 and 30 cents. The labor required is scarcely, if any, greater than for fumigation with other materials. A single fumigation with a suitable tobacco extract would cost, in a house of the same size, at least $1.50, and the cost of labor for the syringing which is recommended to follow such fumigation would prob- ably not be less than $1. The soap required would cost only a few cents, though possibW as much as the materials used in a fumigation with hj^drocyanic-acid gas. CAUTIONS. Hydrocyanic-acid gas is one of the deadliest poisons known, and should always b,e handled with the greatest care. Never hold the potassium cyanide in the bare hand when pulverizing, but wrap up the lumps in two or more thicknesses of cloth before breaking with a hammer. Dust or small pieces of potassium cj^anide should not be thrown awa}^ in exposed places, but alwa3^s buried, as should the con- tents of the receptacles of the chemicals after the fumigation. Never inhale air in a greenhouse after the generation of gas has begun, and provide against anyone entering the house before it is properly ventilated. Approved: James Wilson, Secretary of Agriculture. Washington, D. C, February 15^ 1905. Digitized by the Internet Archive in 2013 http://archive.org/details/greenhousewhitefOOunit UNIVERSITY OF FLORIDA 3 1262 09228 3737