Cumulative impact of a clover cover crop on the persistence and efficacy of Beauveria bassiana in suppressing the pecan weevil (Coleoptera: Curculionidae).

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan. Endemic levels of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin can occur in pecan orchards and contribute to natural control of C. caryae. Commercial formulations of the fungus can also be applied for suppression of C. caryae. We hypothesized that a clover cover crop enhances B. bassiana efficacy and persistence (e.g., by protecting the fungus against abiotic environmental stresses). The hypothesis was tested by conducting field trials in a pecan orchard in Byron, GA, in 2009 and 2010. The study included four treatments arranged in a factorial with two levels of fungus (endemic fungus only, and application of a commercial B. bassiana product), and two levels of clover (white clover, Trifolium repens L., and no clover). Fungal persistence was measured by determining the number of CFUs per gram of soil over time (during 42 d postapplication of B. bassiana in 2009 and 29 d in 2010). Efficacy was measured by capturing naturally emerging C. caryae and subsequently determining mortality and mycosis (over 24 d in 2009 and 17 d in 2010). In 2009, greater prevalence of B. bassiana conidia was detected in plots receiving fungal applications compared with no fungus applications, and no clear effect of clover was observed in plots receiving B. bassiana applications in either year. In 2010, B. bassiana prevalence in the endemic fungus plus clover treatment was higher than fungus without clover, and was similar to plots receiving additional B. bassiana applications. Given that we observed enhanced persistence of endemic B. bassiana in 2010 but not 2009, the impact of clover appears to be a cumulative effect. Mortality of C. caryae (averaged over the sampling periods) ranged between 68-74% in plots receiving B. bassiana applications and 51-56% in plots with endemic fungus only. C. caryae mortality and mycosis data also provided evidence that endemic B. bassiana efficacy was enhanced by clover relative to plots without clover (with no clear clover effect on plots receiving fungus applications). Thus, we conclude that natural control of C. caryae can increase when clover is grown in pecan orchards with endemic populations of B. bassiana.