Synergistic Effect of Imidacloprid and Two Entomopathogenic Fungi on the Behavior and Survival of Larvae of Diaprepes abbreviatus (Coleoptera: Curculionidae) in Soil

The impact of sublethal concentrations of imidacloprid and the entomopathogenic fungi Metarhizium anisopliae (Metschnikoff) Sorokin and Beauveria bassialla (Balsamo) Vuillemin alone or in combination on mobility, mortality, and mycosis of 1st instars of the citrus root weevil, Diaprepes abbreviatus (L.), was evaluated in a series of laboratory bioassays. Initial studies were conducted to determine the effect of soil moisture and depth on the rate of larval movement. Movement of 2-d-old larvae in sandy soil was not affected by soil moisture ranging from 2 to 12%, but they did not penetrate soil with <1% moisture. Movement of 2-d-old larvae was retarded as moisture and soil depth increased. Newly hatched larvae exhibited poor vertical movement, suggesting that larval age is an important factor to consider in experimentation. In all bioassays, soil application of either M. anisopliae or B. bassiana at 5 ×103, 5 ×104, and 5 × 105 conidia per gram of soil as an oil or wettable powder formulation had no effect on the movement oflarvae of D. abbreviatus in soil at different moistures. The addition of imidacloprid to the soil at doses of 5 µg/g soil or greater significantly impaired larval movement. When the chemical was applied with a fungus, however, larval movement was more severely impaired. Soil treatment with unformulated conidia of M. anisopliae and B. bassianaas wettable powder at 5× 103, 5 ×104, and 5 ×105 caused minimal larval mortality of D. abbreviatus. In fact, larval mycosis (10.2%) was found only for the B. bassianaoil formulation at 5 × 105 conidia per gram soil. When sublethal doses of imidacloprid were combined with conidia of either fungus and applied to the soil, both larval mortality and mycosis increased synergistically. When M. anisopliae at 5 × 105 conidia per gram soil were applied alone or in combination with imidacloprid at 25 µg/ g of soil, larval mortality, and mycosis decreased as soil moisture increased from <1 to 14%.