Evaluation of insecticides for the control of Helicoverpa armigera (Hubner) and Helicoverpa punctigera (Wallengren) (Lepidoptera : Noctuidae) on soybean, and the implications for field adoption

Seven insecticides were evaluated against first-, third- and fifth-instar H elicoverpa armigera (Hubner) and Helicoverpa punctigera (Wallengren) larvae using a leaf-disc bioassay of field-treated soybean. Significant differenc es were detected in age-specific mortality, time to effect (LT50) and field persistence of the insecticides tested. With the exception of trichlorfon, the mortality of first-instar larvae was similar in the different treatmen ts to the commercial standard, methomyl. The mortality of third- and fifth- instar larvae in the spinosad and chlorpyrifos treatments was also similar to that in the methomyl treatment, while the age-specific mortality of the insecticides based on Bacillus thuringiensis and nucleopolyhedrovirus decli ned more rapidly, resulting in low mortality to fifth-instar larvae. The LT 50 Of the biological-based products were significantly greater than those o f the conventional products. Only chlorpyrifos showed significant activity 1 week after treatment. The results suggest the need for a user education p rogram to highlight the likely differences in the field performance of the newer biological-based products compared with the conventional chemicals wi th which growers are familiar.