Mortality of Grasshoppers Exposed to Beauveria bassiana during Oviposition and Nymphal Emergence

Abstract

In a controlled environment, Beauveria bassiana conidia in a sand oviposition substrate caused extensive mortality in ovipositing grasshopper ( Melanoplus sanguinipes) females, associated males, and subsequently, in emergent nymphs. Population densities of viable conidia at the time of oviposition ranged from 7.2 × 10 6 to 4.2 × 10 7 colony-forming units/g dry weight of sand. Abdomens of ovipositing females were heavily infested and many of the conidia were aggregated near or in abdominal intersegmental folds. Mating activity was noticeably reduced 5 days after exposure, and by 9 days, 81 to 100% of the females were dead. Mortality of males due to B. bassiana was delayed relative to females but reached 50 to 100% after 14 days. Females exposed to sand amended with viable or nonviable conidia laid similar numbers of eggs during the first oviposition period (1 to 7 days). However, very few eggs were laid by treated females in the sand containing viable conidia during the second oviposition period (8 to 14 days) due to high mortality. Over the 14-day experimental period, the females exposed to viable conidia produced 57.0% fewer eggs. After emergence, exuvia from vermiform larvae were heavily infested with conidia. Newly emerged nymphs were also infested and the incidence of mycosis was 63.9% after 10 days, compared to 0.13% in the control treatment. Unhatched eggs and egg chorions in sand containing viable conidia were frequently colonized by B. bassiana but the hatchability of the eggs was not affected. This study shows that adult grasshoppers of both sexes and emerging nymphs, but not eggs, are susceptible to infection by B. bassiana in sand and that soil inoculation might provide an alternative method for the application of this entomopathogen for control of grasshoppers.