Observations on the Ecololgy of a Polyhedrosis of the Great Basin Tent Caterpillar Malacosoma Fragilis

The termination of large-scale outbreaks of certain lepidopterous and hymenopterous insects by virus diseases is a long-recognized phenomenon (Steinhaus 1949). In the case of the tent caterpillars, Malacosoma, one of the first records of the effectiveness of such diseases appears to be that presented by Davis in 1903. Davis, concerned with populations of the eastern tent caterpillar, Malacosoma amtericanwunl (F.), believed the infectious agent of a widespread epizootic he observed to be a bacterium or a fungus. From his description of the infected insects it is probable that Davis was actually dealing with what we now know to be a polyhedrosis virus (Clark and Thompson 1954). Following the early observations by Davis, several investigators have considered the role of wilt or polyhedrosis disease in the natural control of the tent caterpillars (Howard & Fiske 1911: 98; Glaser 1927; Bergold 1951). The development of the information on the occurrence of polyhedroses in most members of the genus Malacosoma has been traced elsewhere (Clark and Thompson 1954) ; the present paper is concerned with the survival and transmission of the virus causing a polyhedrosis in the Great Basin tent caterpillar, Malacosoma fragilis (Stretch). The isolation and identification of this virus was reported by Steinhaus in 1951. The mechanisms of transmission of viruses afflicting insects are imperfectly known and understood. Bergold (1953) regards this subject as one of the most controversial in the whole field of insect viruses. Of particular interest in this study is the survival and transmission of the virus between generations of insects that are separated by periods during which no susceptible stage of the insect occurs. In the case of M. fragilis, as well as other tent caterpillars, this period is relatively prolonged. The adult moths oviposit in August and September and embryonic development is completed in September and October. The first instar larvae overwinter within the egg chorion. Hatching occurs with the flush of new growth on the host plants in May or June. Thus, there is a period of 9 to 10 months during which no stage of the insect capable of ingesting the virus is present. During an investigation of the possible use of microorganisms in the control of the Great Basin tent caterpillar, an opportunity was afforded to follow the course and effects of a virus epizootic in a population of M. fragilis in nature.