Evaluating methoxyfenozide efficacy against regional soybean looper populations

The soybean looper, Chrysodeixis includens (Walker), is an important migratory insect pest of soybean in the Mid-South and Texas. Soybean looper infestations in the U.S. originate from reservoir populations in southern Texas, Florida and areas south of the continental U.S. These insects do not enter diapause in sub-tropical locations and are exposed to numerous applications of insecticides. Soybean producers in the U.S. presently rely on a limited number of insecticides for effective chemical control strategies. In 2008, Louisiana producers reported unsatisfactory control of this species in soybean with methoxyfenozide. In 2009 and 2010, field collections from Louisiana, Missouri, Mississippi and Texas were exposed to discriminating concentrations (0.020 to 5.000 ai µg/ml) of methoxyfenozide in diet incorporation bioassays. All field colonies were compared to a reference strain LC₅₀ of 0.007 µg/ml for 2009 and 0.008 µg/ml for 2010. Louisiana populations exhibited LC_50¢s of 0.079 µg/ml and 0.146 µg/ml for 2009 and 2010, respectively, which were the highest values among all field collections. Insects collected from Missouri demonstrated the lowest LC_50¢s with 0.025 µg/ml in 2009 and 0.011 µg/ml in 2010. In general, results of these discriminating concentration tests indicated that all field collections showed elevated LC_50¢s compared to the susceptible laboratory colony. However considerable variability in responses to methoxyfenozide was observed, which is presumably the result of differential selection with this insecticide among cropping systems as soybean looper populations migrate north.