Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. rs i Ds. * | 4 . + - ft i t "7 “ > é 1 ™ BULLETIN No. 1088 Washington,D.C. © | PROFESSIONAL PAPER July, 1922 ZYGOBOTHRIA NIDICOLA, AN IMPORTANT PARA- SITE OF THE BROWN-TAIL MOTH. By C. F. W. MvurEseseck,* Scientific Assistant, Gipsy Moth and Brown-tail Moth Investigations, Bureau of Entomology. -_ CONTENTS. ; Page. Page. SALET OOM CtOM = su oe neat te Sr 1 Life history and biology of the para- Distribution in the United States___ 2. PS ep ee SS es 8 BS a Reta Life cycle of the brown-tail moth___ 2 | Economic importance of the parasite_- Ss: INTRODUCTION. _ One of the most effective factors in the control of the brown-tail “moth (Huproctis chrysorrhoca L.) in the United States is -the introduced tachinid fly Zygobothria nidicola Townsend. Strangely enough this European parasite had not been described at the time it was first obtained in this country. The first adults were reared at the Gipsy Moth Laboratory, then located at North Saugus, Mass., in the summer of 1906, from brown-tail moth caterpillars that had _ been received from Europe during the preceding winter. The speci- - mens were referred to Mr. C. H. T. Townsend, of the Bureau of Entomology, United States Department of Agriculture, for identi- fication. After some correspondence with European authorities, to whom also specimens were submitted for examination, Mr. Town- send concluded that the species was new, and subsequently described it under the name Zygobothria nedicola. | _ Much difficulty was experienced in rearing adults of the parasite for colonization from imported brown-tail moth larve. This was due to the great mortality among the caterpillars, particularly from 1 Special acknowledgments are due Mr. A. F. Burgess, in charge of Gipsy Moth and Brown-tail Moth Investigations, for helpful criticism of this bulletin, and Messrs. Sb tS Crossman and R. T. Webber, of the Gipsy Moth Laboratory, Melrose Highlands, Mass., _ for many suggestions during the prosecution of the work. _8 TOWNSEND, CHARLES H. T. THE TAXONOMY OF THD MUSCOIDEAN FLIES, INCLUDING “DESCRIPTIONS OF NEW GENERA AND SPECIES. In Smithsonian Miscellaneous Collections, vy. 51, p. 99-101. 1908. 104616—22 1 2 BULLETIN 1088, U. S. DEPARTMENT OF AGRICULTURE. disease, before the parasites completed their development. Despite the discouraging results of the breeding work, however, Z. nidicola became established, and by 1910 had definitely taken its place in ~ our fauna. The trying experiences of the laboratory force, in their i attempts to establish this parasite, are interestingly recounted by — Howard and Fiske.* DISTRIBUTION IN THE UNITED STATES. Because it evidently has no hosts other than the brown-tail moth the parasite must necessarily remain within the area over which this insect occurs; but within these limits it appears to be widely dis- tributed, although it is relatively less abundant in the sections where very low temperatures are reached during the winter. It has been recovered from Rhode Island to northeastern Maine—very abundantly in the former region, sparingly in the latter. This wide dissemination, within some seven or eight years, is very largely the result of natural spread, since there has been little artificial coloniza- tion of this species. LIFE CYCLE OF THE BROWN-TAIL MOTH. Before taking up in detail the biology of the parasite it will be well to review briefly the life cycle of its host. During July the female brown-tail moth deposits her eggs on the underside of a leaf of one of the favored food plants—apple, pear, oak, or wild cherry. Usually the terminal leaves of the uppermost shoots of the tree are selected for oviposition. The eggs hatch in about three weeks and the small caterpillars feed on the epidermis of the leaves, preferring the terminal ones, which they gradually tie together with a large amount of silk. This process is slow, but ultimately a firm, tough web, about 3 or 4 inches long, is formed. By this time the majority of the slowly growing larve are in the third stage and are ready for hibernation. In the spring feeding begins as soon as the buds open, and continues until the middle of June, when cocoons are formed and pupation occurs. Moths issue during the first half of July, and, after a few days, lay their eggs. There is only one generation annually. LIFE HISTORY AND BIOLOGY OF THE PARASITE. EMERGENCE AND LONGEVITY OF THE ADULTS. Adults of Z. nidicola appear during the latter half of July. They are very sturdy flies (Fig. 1) and. endure unfavorable conditions well, normally living for a period of at least several weeks. Some 8 HOWARD, L. O., and FISKE, W. F. THE IMPORTATION INTO THE UNITED STATES OF THE PARASITES OF THE GIPSY MOTH AND THE BROWN-TAIL MOTH. U. S. Dept. Agr., Bur. Ent. Bul. 91, p. 289-295. 1911. ZYGOBOTHRIA NIDICOLA, PARASITE OF BROWN-TAIL MOTH. 3 specific data with regard to the length of life were obtained from laboratory experiments in which various types of cages were used. These consisted of: (1) Plain glass cylinders, measuring 50 by 200 mm., closed at one end; (2) ordinary shell vials, 22 by 100 mm.; and (3) a wooden cage, which had been successfully used by Mr. J. J. Culver in his life-history studies upon Compsilura concinnata Meigen, a tachinid parasite of both the brown-tail moth and the gipsy moth. The glass cylinders were rather satisfactory for two to five flies each, when a bit of crunched crépe paper was placed inside to afford the flies a good footing, but 1t was necessary to change the cylinders Fic. 1—Adult male of Zygobothria nidicola. every few days because they quickly became dirty and sticky, and this involved a good deal of work. This objection applied to the shell vials as well, which in addition were found to be too small even for individual flies. The wooden cage was by far the most satisfac- tory. It measures about 12 inches square and 4 inches high, and is fitted with a cloth bottom to facilitate cleaning after each experiment ; the top is a piece of window glass of the proper size. One-inch holes bored in the sides of the box and covered with fine wire gauze insure good ventilation. Another opening of the same size is fitted with a cork and is used for introducing the flies. Feeding is facili- tated by the use of a larger opening, about 2 inches in diameter, which can be closed with a wooden stopper. After the flies were placed in this cage they were left entirely alone save for the feeding, which 4 BULLETIN 1088, U. S. DEPARTMENT OF AGRICULTURE. was done on alternate days. This process merely involved slipping | a narrow strip of blotting paper, which had been soaked in a mixture } of honey and water, into the cage, and removing the old strip which | had been placed there two days before. | Only a small proportion of the flies confined in the glass cylinders } or shell vials lived from 25 to 28 days, and then only upon receiving } particularly good care; those in the wooden cages which were given } comparatively little attention lived five weeks and more. Two males } and one female, which were confined in one of these wooden cages, } were apparently in as good condition at the end of a 40-day period, when the experiment was discontinued, as when first placed in the } box; several larger lots did about as well. The data obtained cer- J tainly demonstrate the ability of Z. nidicola to live a long time; and — since even at best the artificial methods of the laboratory probably can not provide the equivalent of natural conditions, it seems safe to assume that in nature the average life of Z. nidicola is at least 25 to 30 days. EMBRYONIC DEVELOPMENT. Mating takes place within a few hours after emergence, sometimes even before either fly has fed. Following impregnation the uterus of the female fly gradually becomes much elongated and coiled, ulti- mately attaining a length of 7.5 to 8 mm., which is about the length of the entire insect. This enlargement results from the stretching of the walls of the organ as the enormous numbers of fertilized eggs pass into it and arrange themselves in more or less regular spiral layers. Embryonic development requires from seven to eight days. At the end of this period the lower part of the uterus contains a con- ‘siderable number of maggots, each still enclosed within its egg- chorion. From 12 to 16 days after impregnation two-thirds of the 600 or more eggs in the uterus have fully formed first-stage maggots within them, if the fly has not been ovipositing as rapidly as they have developed. | OVIPOSITION. The female fly prefers as its victims brown-tail moth caterpillars that are from several days to two weeks old, but even those just out of the egg are often successfully parasitized. Most of the oviposi- tion by this species takes place during the first three weeks of Au- gust, in normal seasons. Oviposition was readily obtained in the laboratory by confining a fertilized female fly in a shell vial with a few brown-tail moth larvee that had been placed upon a small piece of cherry leaf. Hav- ing found the caterpillars, the parasite manifested much interest, passing slowly from one larva to another and inspecting each minutely. Then, with her face but a few millimeters from one of ZYGOBOTHRIA NIDICOLA, PARASITE OF BROWN-TAIL MOTH. 5 the caterpillars, she slowly and deliberately pushed her abdomen downward and forward until the ovipositor plates were even with } her face. With a quick movement the ovipositor was then pushed beneath the larva, and an egg with a first-stage maggot within it _was deposited. The egg is almost invariably placed on the venter _of the host, and usually occupies a transverse position between two | pairs of true legs, or, less frequently, between two pairs of prolegs. _ Occasionally an egg is placed on the dorsum by accident, but in such _ cases the parasitic maggot is unable to enter its host—at least this | was true of instances under observation in the laboratory. The ex- | planation probably is to be found in the thicker skin of the dorsum, _ which is not so easily pierced by the small maggot. Although only one parasite can complete its development in one _ host larva, the fly uses no discrimination when depositing her eggs; she places eggs as readily upon larve already having eggs upon _ them as upon those not yet attacked. From five to eight eggs have _ been found on one caterpillar. That this takes place under field con- _ ditions as well as in the laboratory has been disclosed by dissections; from 6 to 10 first-stage maggots of the parasite have been not un- commonly dissected from single field-collected brown-tail moth caterpillars. EGG. As deposited, the egg measures from 0.42 to 0.45 mm. in length by 0.11 to 0.12 mm. in width; the maggot within is 0.33 to 0.85 mm. long and 0.08 to 0.09 mm. broad. In form the egg is elongate-oval, somewhat narrowed at the posterior end, and concave on the lower _ side; in color it is whitish. The thin and delicate chorion is trans- parent. When viewed from above the egg appears opaque; this is due to what seems to be a special layer of protecting tissue just in- side the chorion; it is limited to the posterior three-fourths of the egg, and occurs only above the maggot. It is peculiarly reticulated, being marked off into very slender hexagons, the outermost of which are incomplete. Its position suggests its function to be that of afford- ing protection to the young maggot before the latter succeeds in bor- ing into its host. ENTRANCE OF MAGGOT INTO HOST. ‘ Having been placed upon its host the parasitic maggot begins to cut through the thin egg chorion that confines it, and as soon as this is done it bores into the caterpillar. The posterior end of the para- site remains inside the eggshell until the opening into the host has been made; when this has been accomplished it requires but a fraction of a second for the maggot to pull its whole body into the caterpillar. In one case under observation the entire process of cutting through the egg chorion and the host skin and entering the 104616—22 2 6 BULLETIN 1088, U. S. DEPARTMENT OF AGRICULTURE. brown-tail moth caterpillar was completed within 10 minutes after the egg had been deposited. From many observations it appears that normally 20 to 30 minutes elapse between oviposition and the entrance of the parasitic larva into its host. Often the caterpillar makes vigorous attempts to destroy the maggot before the latter has made its way inside, and occasionally these efforts are suc- cessful, particularly if the egg of the parasite was deposited near the posterior end of the host. In this case the brown-tail moth larva, by doubling its body, can reach the parasite and crush it with its mandibles. FIRST-STAGE MAGGOT OF THE PARASITE. The most striking thing about the parasitic mag- got at the time it enters the caterpillar is the strongly chitinized mouth hook (Fig. 2, a,b). It is simple in this stage, consisting of a single apical tocth and comparatively narrow, divided, posterior plates, the entire mouth hook being one solid structure.