Beauveria bassiana transmission in western corn rootworm (Coleoptera :Chrysomelidae)

摘要

A key stage in the development of an insect epizootic, either natural or induced, is the transmission of a pathogen from an infected to an uninfected host (Andreadis 1987). The establishment of secondary infections can augment long-term control by increasing the amount of inoculum present within the environment (Thomas et al. 1995). The ease of disease transmission from an infected to a healthy host is an important consideration in evaluating the potential of an entomopathogen as a microbial control agent. Currently, we are field-testing the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin (Moniliaceae), for the suppression of adult western corn rootworm, Diabrotica virgifera virgifera LeConte, which is a key pest of corn, Zea mays L. (Gramineae), throughout the north-central United States and eastern Canada (Mulock and Chandler 2000). During our investigations we have often observed sporulating cadavers on corn plants, particularly within the leaf collars, following fungal application. In this study, we estimated the effect of the density of infected cadavers and the length of exposure time on disease transmission in adult western corn rootworm. Field studies were conducted at the Eastern South Dakota Soil and Water Research Farm (44 deg 19 min W, 96 deg 46 min W), Brookings, South Dakota, in July 1999. The B. bassiana isolate (BbPC-1) used in this study was originally obtained from corn rootworm adults collected from corn in Illinois. Mortality due to B. bassiana was determined within cohorts of beetles released into screened, walk-in field cages (3 X 3 X 3 m, 20 mm2 mesh) containing corn plants artificially infested with five densities of sporulating cadavers. Infected cadavers were obtained by suspending for 30 s adult western corn rootworm in a wetting agent (0.04 percent polyalkyleneoxide heptamethyltrisiloxane) containing 5 X 107 conidia/mL. Beetles were maintained in cages (8 cm high X 8.5 cm diameter, 10 beetles/cage, 26 +- 2 deg C; 50-60 percent relative humidity (RH); 16L:8D) with an artificial diet and water agar for up to 16 d. Upon death, beetles were incubated in a moist chamber (80-90 percent RH; 26 +- 2 deg C) for 7 d to promote sporulation and confirm infection.