Laboratory evaluation of selected biopesticides against fall armyworm, Spodoptera frugiperda (JE Smith) larvae in Botswana

Abstract

The efficacy of Beauveria bassiana (Bb), Bacillus thuringiensis (Var. Kurstaki) (Btk), and Nucleopolyhedrovirus (HearNPV) on fall armyworm (FAW) on maize was studied in the laboratory at the Botswana University of Agriculture and Natural Resources, Sebele, Botswana. The biopesticides were applied at three concentrations, including the label rate, and replicated three times. Treatments were assessed at 24 h intervals until 192 h post treatment. The results revealed significant treatment effects across all biopesticide concentrations compared to the untreated control (p<0.0001). Mortality increased with both higher concentrations and longer exposure times. Initial larval mortality was observed following 96 h exposure, at higher concentrations, with Btk showing the highest early mortality (33.33% and 22.22%), followed by HearNPV (22.22%) and Bb (22.22%). By 168 h, HearNPV at highest and label concentrations together with Btk and Bb at highest concentrations achieved 100% mortality, while Btk and Bb label concentrations achieved complete mortality at 192 h. The study revealed that biopesticides like Bb, Btk and NPV do not need to cause high mortalities of insects to effect adequate protection of the crop from damage. The insect may not be dead but unable to feed, therefore preventing damage. Bb, Btk and HearNPV can be used to achieve effective control of FAW larvae provided high concentrations are used, and longer durations of exposure are permitted.