A Fungal Insecticide Engineered for Fast Per Os Killing of Caterpillars Has High Field Efficacy and Safety in Full-Season Control of Cabbage Insect Pests

Abstract

Fungal insecticides developed from filamentous pathogens of insects are notorious for their slow killing action through cuticle penetration, depressing commercial interest and practical application. Genetic engineering may accelerate their killing action but cause ecological risk. Here we show that a Beauveria bassiana formulation, HV8 (BbHV8), engineered for fast per os killing of caterpillars by an insect midgut-acting toxin (Vip3Aa1) overexpressed in conidia has both high field efficacy and safety in full-season protection of cabbage from the damage of an insect pest complex dominated by Pieris rapae larvae, followed by Plutella xylostella larvae and aphids. In two fields repeatedly sprayed during summer, BbHV8 resulted in overall mean efficacies of killing of 71% and 75%, which were similar or close to the 70% and 83% efficacies achieved by commercially recommended emamectin benzoate but much higher than the 31% and 48% efficacies achieved by the same formulation of the parental wild-type strain (WT). Both BbHV8 and WT sprays exerted no adverse effect on a nontarget spider community during the trials, and the sprays did not influence saprophytic fungi in soil samples taken from the field plots during 4 months after the last spray. Strikingly, BbHV8 and the WT showed low fitness when they were released into the environment because both were decreasingly recovered from the field lacking native B. bassiana strains (undetectable 5 months after the spray), and the recovered isolates became much less tolerant to high temperature and UV-B irradiation. Our results highlight for the first time that a rationally engineered fungal insecticide can compete with a chemical counterpart to combat insect pests at an affordable cost and with low ecological risk.