Stage-Dependent Efficacy of Selected Biopesticides against Beet Armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), on Welsh Onion under Controlled Greenhouse Conditions

Abstract

Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) is a major pest of Welsh onion, and effective biological alternatives to synthetic insecticides are increasingly needed for sustainable pest management. This study evaluated the efficacy of four commercially available biopesticides—Az Tron WG (Bacillus thuringiensis), HH-SK3 (Bacillus thuringiensis + Beauveria bassiana + Metarhizium anisopliae), Sokupi 0.5SL (matrine), and Meta Green (Metarhizium anisopliae)—against first- through fourth-instar S. exigua under controlled greenhouse conditions using a leaf-dip bioassay adapted from the IRAC susceptibility-test method for leaf-eating lepidopteran larvae. Corrected control efficacy was assessed at 24, 48, 72, 120, 168, and 336 hours after treatment (HAT), and treatment effects were analyzed using generalized linear mixed models. Control efficacy increased progressively with exposure time for all products, reaching 93.3–100.0% by 336 HAT across all larval instars. Younger larvae responded more rapidly than older larvae, with first- and second-instar larvae achieving ≥90% control by 120 HAT, whereas third- and fourth-instar larvae generally required 120–168 HAT to reach comparable efficacy. Az Tron WG consistently provided the fastest early suppression, although differences among products diminished with prolonged exposure. These findings indicate that all four commercial biopesticides are effective for managing S. exigua and emphasize that early application against young larval instars can accelerate control and strengthen integrated pest management in Welsh onion production.