Diagnostic Techniques for Detection of Permethrin-Resistant Soybean Loopers, Pseudoplusia Includens (Walker), Determination of the Impact of Insecticide Treatments on Age-Specific Mortality and Feeding Activity, and Estimation of Field Control.

Abstract

A glass vial bioassay was developed to detect permethrin resistance in soybean looper, Pseudoplusia includens (Walker), larvae. Third, fourth, fifth, and sixth instar soybean loopers from laboratory (susceptible) and field strains were exposed to several concentrations of permethrin in 20 ml glass vials. LC$\sb{50}$'s for third, fourth, and fifth instars from each field strain were not significantly different. A discriminating concentration of 0.5 $\mu$g permethrin per vial was identified for detection of permethrin resistance in soybean looper field populations. Field strains from cotton-soybean agroecosystems exhibited higher levels of resistance than strains from soybean-corn agroecosystems. The reliability of the vial bioassay was confirmed by conducting topical bioassays on populations tested in the vials. A comparison of vial mortality values at the discriminating concentration (0.5 $\mu$g) with efficacy of permethrin (0.11 kg (AI) /ha) against field populations demonstrated that this bioassay has potential to estimate field control. The glass vial monitoring technique also was used to detect permethrin resistance in soybean looper adults. Male and female moths from laboratory strains responded similarly, and a discriminating concentration of 2.5 $\mu$g permethrin per vial was identified and used to test male moths captured using wire cone pheromone traps at three locations in Louisiana. Mortality results indicated that all strains were resistant to permethrin, but levels of resistance varied within locations over time. The feeding activity and mortality of permethrin-resistant and -susceptible third and fifth instar soybean loopers on insecticide treated foliage were investigated. Mortality of resistant larvae was delayed with thiodicarb and Bacillus thuringiensis. All insecticide treatments significantly reduced leaf area consumption of both resistant and susceptible larvae, regardless of instar, when compared to feeding on untreated foliage. Field studies were conducted to determine the efficacy of several insecticides against small ($<$third), third, fourth, fifth, and sixth instar soybean loopers in field plots. All insecticide treatments significantly reduced the total number of larvae compared to the untreated plots on most evaluation dates. Control of individual instars varied among tests and insecticide treatments, but, in general, small larvae comprised the majority of the soybean looper populations remaining in the insecticide treated plots.