Conidial attachment of Metarhizium anisopliae and Beauveria bassiana to the larval cuticle of Diaprepes abbreviatus (Coleoptera: Curculionidae) treated with imidacloprid

A series of experiments was conducted to determine the effect of imidacloprid on the number of Metarhizium anisopliae and Beauveria bassiana conidia found on the cuticle of first instar Diaprepes abbreviatus following different treatments. Larvae treated with M. anisopliae conidia and imidacloprid by dipping removed significantly fewer conidia from their cuticle when in contact with soil or a food source compared with fungal-treated larvae alone. In addition, more M. anisopliae and B. bassiana conidia were found on the cuticle of larvae treated with imidacloprid while exposed to soil at 7 and 14% moisture resulting in higher larval mortality and mycosis. Conidial attachment to cuticles of untreated larvae was higher at < 1% compared with 7 and 14% soil moistures. M. anisopliae conidia were distributed uniformly over the pleural membrane of the larval cuticle of both untreated and imidacloprid-treated larvae. However, fewer conidia were attached to specific sites such as setae and setal sockets of treated larvae. At 12 h after treatment, imidacloprid-treated larvae had fewer conidia removed from exposed cuticle, setae, and spiracles than did untreated larvae. Cuticular exposure to imidacloprid at doses >0.01% (AI) affected conidial attachment of M. anisopliae negatively. Conidial number decreased sevenfold at 0.1% (AI). Comparative data on the effect of imidacloprid formulation on conidial attachment showed that components of the inert ingredient were responsible for lower conidial attachment on larval cuticle at higher insecticidal doses. (C) 1998 Academic Press.