Baseline susceptibility of codling moth (Cydia pomonella) to spinetoram and chlorantraniliprole

Codling moth (CM), Cydia pomonella (L), is the key pest of walnuts in California.  Control of CM has relied on repeated applications of broad-spectrum insecticides.  However, because of secondary pest outbreaks and environmental concerns, the agricultural chemical industries have recently developed a number of reduced-risk insecticides, e.g. spinetoram and chlorantraniliprole.  Spinetoram is an excellent larvicide with some adulticide activity and chlorantraniliprole is an excellent ovicide and larvicide for control of CM in walnuts.  Since resistance to these two insecticides is likely in the future, studies were undertaken to determine their baseline susceptibility of CM in walnuts before widespread use.  In untreated orchards, CM adults and larvae were collected and exposed to various concentrations of each insecticide. Dose mortality lines were developed for spinetoram (adults only) and chlorantraniliprole (mature larvae only).  The adult male LD₅₀ for spinetoram was calculated as 0.15 mg tech ai/mL.  The LD₅₀ for spinetoram is about the same as the typical field rate.  The mature field collected larval LC₅₀ for chlorantraniliprole was calculated as 0.023 mg tech ai/mL.  The use of mature larvae as the target stage was necessary since chlorantraniliprole was not effective against adults and it is not possible to collect eggs or neonate larvae in the field.  The LC₅₀ for chlorantraniliprole is about 4 times lower than the typical field rate.  The ratio of LC₅₀ to typical field rate is far less than desirable and indicated that field resistance of CM to chlorantraniliprole may occur in a short period of time.  However, the use of mature larvae may underestimate the toxicity of chlorantraniliprole.