Seven insecticides were evaluated against first‐, third‐ and fifth‐instar Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren) larvae using a leaf‐disc bioassay of field‐treated soybean. Significant differences 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 treatments 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 declined more rapidly, resulting in low mortality to fifth‐instar larvae. The LT50 of the biological‐based products were significantly greater than those of the conventional products. Only chlorpyrifos showed significant activity 1 week after treatment. The results suggest the need for a user education program to highlight the likely differences in the field performance of the newer biological‐based products compared with the conventional chemicals with which growers are familiar.
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Duffield et al. (2000) studied this question.
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