Contact behaviour and mortality of wireworms exposed to six classes of insecticide applied to wheat seed

Insecticide-treated seed is commonly used to manage wireworms, with insecticide toxicity generally being deduced from crop stand protection rather than from directly observed wireworm responses. We observed the behaviour of larvae of two economic elaterids exposed to wheat seeds treated with 11 insecticides at various rates or combinations in a soil environment. Wireworms were exposed for 3 or 24 h, and the post-contact health and mobility of 1030 larvae that contacted seeds were assessed (bi)weekly for 12–42 weeks. Considerable repellency was observed when wireworms were exposed to bifenthrin, tefluthrin, λ-cyhalothrin, and some repellency was also observed at high rates of various other insecticides. A high proportion of wireworms were moribund after 24 h when exposed to treatments containing thiamethoxam, fipronil, or high rates of ethiprole, cyazypyr, chlorpyrifos, and spinosad, but not after exposure to bifenthrin, tefluthrin, chlorantraniliprole, spirotetramat, or low rates of ethiprole, cyazypyr, and chlorpyrifos. High mortality was observed in all treatments containing fipronil, but none after exposure to bifenthrin, tefluthrin, λ-cyhalothrin, chlorantraniliprole, spirotetramat, spinosad, or low rates of cyazypyr. Combining thiamethoxam with fipronil or a high rate of chlorpyrifos decreased the toxicity of the second compound. These findings largely explain why we observe stand protection without wireworm population reduction in efficacy studies with wheat seed treated with various pyrethroid and neonicotinoid insecticides, and suggest a similar result for other insecticides that only induce temporary morbidity. This bioassay allows for rapid screening of insecticides proposed for wireworm management before these are evaluated in labour-intensive and costly field trials.