Different Susceptibility in the Two Sympatric Sweet Potato Weevils, <i>Cylas formicarius</i> and <i>Euscepes postfasciatus</i>, to the Entomopathogenic Fungus <i>Metarhizium anisopliae</i>

Abstract

Laboratory and field experiments were performed to evaluate the pathogenicity of an isolate of the entomopathogenic fungus Metarhizium anisopliae to the two sympatrically occurring weevil species, Cylas formicarius and Euscepes postfasciatus. In the laboratory bioassays, suspension of conidia, ≥106 CFU/mL, caused mortalities > 80% on adults of both weevils in seven days after inoculation. It took longer time 20 days for grain formulation of the isolate adhered on rice grains of ≥5 g/m2 (107 CFU/g) to attain similar mortalities of E. postfasciatus, but no evident mortality was obtained in C. formicarius. The grain formulation was thus less effective on C. formicarius than the suspension. Field trials were carried out over two years from 2013 to 2014, in which adults of E. postfasciatus were released two times during the field experiments for enhancement of damage on plants by this weevil, whereas the other weevil species was left to naturally invade the experimental plots by flying. The results of the experiments revealed in both years that two applications of the isolate in grain formulation, equivalent to 50 kg/hectare, sprayed manually over the ground surface reduced the infestation of plants and tuber damage by weevils of both species as much as the conventional chemical insecticide applications. The occurrences of weevils at harvest were not significantly different among treatments. The potential and possible uses of the fungus are discussed for the management of these two weevil species.