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DEVELOPMENT AND BIONOMICS OF CHRYSOMELOBIA LABZDOMERAE (ACARI: TARSONEMINA; PODAPOLIPIDAE), A PARASITE OF THE MILKWEED LEAF BEETLE (COLEOPTERA: CHRYSOMELIDAE) THOMASC. BAKER^ and GEORGEC. EICKWORT Entomology Department, Cornell University, Ithaca, New York Abstract Can. Ent, 107: 627-638 (197.5) Adult females of Chrysomelobia labidomerae Eickwort lay eggs on the upper surfaces of the hind wings of Labidomera clivicollis (Kirby), The eggs hatch in approximately 7 days and male and female larvae feed at the base of the wings and in the meso-metathoracic crevice and swell to about twice their original length,,For about the second half of the approximately 7-day larval stadium, the larvae are inactive (pharate adults) and are usually cemented to the undersurfaces of the elytra, Inactive female larvae are accompanied by adult males that apparently copulate with the newly emerged adult females,, The pharate adult is enclosed in a cuticular sac that may represent a calyptostatic nymphal instar, Adult females feed on the beetle's abdominal terga and sometimes also occur on its venter where they do not feed., Females disperse from host to host when the beetles copulate. The species is anhenotokous. Mites overwinter on the diapausing adult beetles and do not infest the immature stages of their host. Even at high population levels, the mites do not noticeably affect the longevity or fecundity of their hosts,, Introduction Chrysomelobia labidomerae Eickwort is a common subelytral ectoparasite of adult milkweed leaf beetles, Labidomera clivicollis (Kirby) (Fig. I), in North America. Chrysomelobia exhibits many characteristics that are primitive for the Podapolipidae (Regenfuss 1968, 1973; Eickwort 1975) and knowledge of the biology of its species should aid in understanding the evolution of this family of highly specialized insect parasites. There are no previous published observations on living Chrysomelobia and prior to Eickwort (1975) the genus was known only from one museum specimen of its type-species, C. mahunkai Regenfuss, that was found under the elytra of a European leaf beetle, Chrysomela graminis Linnaeus (Regenfuss 1968). The majority of Podapolipidae, including all the more primitive genera, have been found under the elytra of beetles, especially Carabidae. Specialized podapolipids also occur on roaches, grasshoppers, and bumblebees, including a few species endoparasitic in the body cavity (Regenfuss 1968, p. 239), genital system (Stannard and Vaisham- payan 1971), and tracheal system (Husband and Sinha 1970). For an excellent summary of host relations and an evolutionary analysis, see Regenfuss (1973). Detailed observations on living podapolipids have only been conducted on the primitive genera Eutarsopolipus and Dorsipes, subelytral ectoparasites of carabid beetles (Regenfuss 1968, 1972, 1973), and on Locustacarus buchneri (Stammer), a tracheal system parasite of bumblebees (Husband and Sinha 1970). All podapolipids apparently obtain nourishment by puncturing the cuticle of their hosts with their styliform chelicerae and sucking hemolymph (Regenfuss 1973). Like nearly all members of the Tarsonemina, Podapolipidae have at most three developmental stages - egg, larva, and adult. The nymphal instars characteristic of more generalized acariform mites are usually lacking. We describe in this paper the development of C. labidomerae, its dispersal from host to host, its seasonal cycle, and aspects of its population biology and effect upon its host as determined from field-caught beetles and from laboratory rearings. All instars of this new species are described in a companion paper (Eickwort 1975). 'Present address: Entomology Department, New York State Agricultural Experiment Station, Geneva, N Y 14456 627 628 THE CANADIAN ENTOMOLOGIST June 1975 FIGS 1--2 Labidomera clivicollis 1, dorsal view; 2, ventral view showing phoretic female Chr ysomelobia labidomerae Methods Our study of Chrysomelobia labidomerae extended from July 1969 to November 1972. Mites were reared on milkweed leaf beetles, Labidomera clivicollis, that were obtained from the laboratory population maintained by Dr. Kathleen Eickwort for her study on the beetle's ecology (Eickwort 1971). Studies were initiated at the E. N. Huyck Preserve in the summer of 1969 by G.C.E. on beetles and mites obtained from Albany, N.Y. and were continued at Cornell University by T.C.B . with additional beetles and mites from the Cayuga Lake region of New York. Approximately 100 mite-infested beetles yielded data in the laboratory. Beetles were reared following the techniques of Eickwort (1971). They were maintained in 12-cm-diameter, clear plastic, covered petri dishes. Filter paper lined the bottom of each dish. The beetles were fed milkweed (Asclepias syriaca L.) leaf halves at 2- to 3-day intervals, at which times deionized water was squirted onto the filter paper. In the winter, frozen milkweed was used. Since frozen milkweed quickly became infested with fungus, a fungicidal agent, Tegosept M, was dissolved in the water to help retard a mold growth. Eggs laid by the beetles were placed on leaves in separate petri dishes where eclosion of larvae and development to the final instar occurred. These fully developed larvae were then placed in dirt-filled dishes where they burrowed, pupated, and emerged as adults. To examine the mites, the elytra and hind wings of the host beetle were raised and impaled to a cork with number 0 insect pins to expose the abdominal terga (Fig. 3). Observations were made under a stereo microscope with a water-filter placed between the lamp and beetle to minimize desiccation. When a beetle was examined more than once, pinning holes in the elytra were reused so that injury to the beetle was minimized. Mites were transferred from one beetle to another with a damp camel's-hair brush or a curved minuten-nadel probe. On some beetles the mites formed large stock cultures of from 10 to 50 adults per beetle. Beetles bearing stock cultures or awaiting infestation were usually kept in groups of two to seven beetles per dish; those beetles used Volume 107 THE CANADIAN ENTOMOLOGIST 629 experimentally were maintained individually or in pairs. A 16:8 1ight:dark photoperiod was maintained during the winter to retard ovarian diapause in the beetles, allowing 8 reproduction throughout the year, although at a lower rate in winter. The beetles were reared at room temperature. Length of stadia and oviposition rate were determined by transferring single adult female mites onto previously uninfested beetles and recording mite eggs at 1- to 3-day intervals. These females were removed before their first eggs hatched, and the subsequent eclosion (noted by the disappearance of intact eggs) and development of larvae recorded. Observations were terminated when all mites had emerged as adults or when the beetles were damaged or killed by the observational procedure. The effects of heavy infestations upon the beetles and maximum population levels of mite colonies were investigated by infesting beetles with one or two adult female mites and foregoing observations until the colonies were at least 25 days old. To determine dispersal, an infested male or female beetle was paired with an uninfested beetle of the same or opposite sex in a petri dish, and the uninfested beetle was examined at 1- to 3-day intervals. The longevity and fecundity of a single female mite was determined by placing her alone on a beetle and removing and counting her eggs until the mite died. Parthenogenetic development was investigated by transferring young larvae to the meso-metathoracic crevices of beetles and allowing the mites to develop into adult females. The sex of the mites developing from eggs laid by these females was recorded. Pinned specimens of L. clivicollis in the Cornell University insect collection were relaxed and examined for mites to obtain distributional data and infestation rates. Mites were mounted on slides following the techniques of Eickwort (1975). Alcohol- preserved specimens and slide-mounted representatives of all instars are deposited in the Cornell University insect collection. Life Cycle of Host The following account of the life cycle of Labidomera clivicollis (Fig. 1) In central New York is summarized from Eickwort (1 97 1). Adult beetles are first encountered as early as 15 May, when the first sprouts of their food plant, milkweed (Asdepias), are appearing. Eggs are laid on the undersides of milkweed leaves. While the beetles FIGS 3-4 Chrysomelobia labidomerae on Labidomera clivicollis 3, beetle with wings raised to expose adult female mites on abdominal terga and eggs on hind wing; 4, scanning electron micragtaph of eggs on hind wing 630 THE CANADIAN ENTOMOLOGIST June 1975 develop equally well in the laboratory on the five species of milkweed tested by Eickwort (1971), they prefer to oviposit on swamp milkweed (A. incarnata L.), their most common host in nature. The eggs hatch in 5 to 9 days and the larvae commence feeding on the milkweed leaves after consuming their chorions. The fourth instar larvae leave the food plants and burrow 3 to 5 cm into the soil, where they become inactive prepupae for about a week. The prepupae then ecdyse to expose the pupae, whose stadium lasts about 2 weeks more. The emerging adults leave the soil and feed on milkweed for a week or more before ovipositing. Adults live several months and the generations overlap. Second (and possibly third) generation adults enter diapause in the soil or other protected locations in the fall; the latest collection date for active adults is 9 October. Life History of Mite The life cycle of Chrysomelobia labidomerae consists of an egg stage, active and inactive larval stages, and adult. Eggs are first laid by the adult female in a single layer on the upper surface of the hind wings of the beetle between the cubitus and second anal vein (Fig. 5). After this area is filled, eggs are also deposited between the radius and cubitus as well as between the second and third anal veins and on the underside of the wings.