Comparative efficacy of emulsifiable-oil, wettable-powder, and unformulated-powder preparations of Beauveria bassiana against the melon aphid Aphis gossypii

Abstract

ABSTRACTAphidicidal efficacy of two formulations of Beauveria bassiana strain GHA conidia, an emulsifiable paraffinic oil dispersion (OD) and a clay-based wettable powder (WP), were compared to unformulated conidia (technical material, TC) in laboratory bioassays with adult melon/cotton aphids. For the initial 24 h post-treatment, aphids were incubated under differing humidity conditions: high (100%) versus 'low' (75 ± 3%) relative humidity (RH), and over the subsequent 6 days, all aphids were incubated under the low RH conditions. Mortality from the OD and TC treatments did not differ significantly under any test conditions, and ANOVA revealed no synergism of B. bassiana activity by the OD carrier ingredients. In contrast, formulation as a WP had a small, but significant, negative impact on B. bassiana efficacy. Mean LC50 across formulations was nearly 6.5-fold higher (3457 vs. 539 conidia/mm2) at low vs. high RH. The OD formulation was significantly more effective than the WP, increasing mortality a maximum of 27 percentage points. There were no significant effects of formulation on probit regression slopes. The equivalent slopes and small differences in efficacy of the OD formulation vs. TC preparation support a hypothesis that oils function primarily as spray stickers and spreaders, increasing the efficiency of spray applications.