Microsporidioses of coleopterous pests of soybeans

Interest in insects affecting soybeans has risen sharply in recent years with the increased economic importance of the crop. Among the coleopterous pests, the Mexican bean beetle, Epilachna varivestis, and the bean leaf beetle, Cerotoma trifurcata, are two of the most important defoliators of soybeans (L. L. Dietz, J. W. Van Duyn, J. R. Bradley, R. L. Rabb, W. M. Brooks, and R. E. Stinner, N.C. Agr. Exp. Sta. Tech. Bull. 238, l-264, 1976). Along with several species of lepidopterous defoliators, these pests have received much attention in studies on various strategies and control tactics of soybean insect pests. While much work has been accomplished on pathogens of lepidopterous pests, relatively little is known of the pathogens of the coleopterous pests of soybeans (Dietz et al., lot. cit.). Evidence of a microsporidiosis of E. varivestis was discovered in laboratory colonies being maintained for research purposes at Clemson University in South Carolina and subsequently at North Carolina State University in North Carolina. The microsporidium was a serious limiting factor in maintaining not only the E. varivestis colony but also in producing the larval parasite Pediobius foveolatus for inundative release studies. Incidence of infection in the colonies approached lOO%, and the colonies were gradually decimated by the microsporidian infection. A microsporidium-free culture of E. varivestis was finally obtained by the use of the Pasteur technique involving the selection of egg masses from field-collected, diseasefree female adults. In obtaining data on the prevalence of infection in laboratory and natural populations of E. varivestis and other coleopterous pests, we soon discovered that a second species was also present in E. varivestis and that a third species was present in natural populations of C. trifurcata. Because of their frequent occurrence in laboratory colonies and the economic importance of their hosts as pests of soybeans, brief descriptions of each of these species and their host-pathogen relationships as presently known are presented to assist other workers who might encounter these pathogens in their studies with E. varivestis or C. trifurcata. Since only limited systematic studies have been undertaken so far, these microsporidia will be referred to as MBB (mexican bean beetle) sp. No. 1, MBB sp. No. 2, & BLB (bean leaf beetle) sp. No. 1. We are proceeding on the basis that these are likely new and undescribed species of microsporidia since there are relatively few described species from the Coleoptera in general and no previous reports of microsporidia specifically from E. varivestis or C. trifurcata (V. Sprague, In “Comparative Pathobiology” L. A. Bulla and T. C. Cheng, eds., Vol. 2, pp. l-510, Plenum, New York, 1977). MBB sp. No. 1. Fresh spores (Fig. IA) are straight to slightly curved and ovocylindrical in shape. They measure about 5.3 t 0.13 x 2.1 2 0.03 pm (n = 25) with a mean polar filament length of 84 pm (n = 10). Infection in E. varivestis is generally systemic in nature with the fat body, muscles, and Malpighian tubules being the more heavily invaded tissues. Larvae and adults are easily infected per OS and the microsporidium is also transmitted transovarially at a high incidence. The species is also moderately virulent for larvae of the corn earworm, Heliothis zea, where infection is generally accompanied by an inflammatory response including extensive cellular infiltration by hemocytes and melanization.