Combined action of the entomopathogenic fungus Metarhizium robertsii and avermectins on the larvae of the colorado potato beetle Leptinotarsa decemlineata (Say) (Coleoptera, Chrysomelidae)

Synergy between the entomopathogenic fungus Metarhizium robertsii J.F. Bisch., S.A. Rehner et Humber and a complex of natural avermectins was observed after combined treatment of the Leptinotarsa decemlineata (Say) larvae. This effect was also recorded at the larval stage most resistant to fungi (5 days after molting to the IV instar). Intoxication with avermectins led to a 2.7-fold decrease in food consumption and arrest of larval development. Synergy between the fungus and avermectins was revealed in field experiments conducted in the steppe zone of Western Siberia under strong fluctuations of temperature and humidity. In particular, the median lethal time (LT50) decreased 2.8-fold after combined treatment as compared to treatment with M. robertsii alone. The mechanisms of synergy and the prospects of using the combined treatment for Colorado potato beetle control are discussed.