LAUSR

BACKGROUND In soybean fields containing insect and herbicide resistant genetically engineered varieties, some weed species have increasingly become difficult to be managed and may favor the population growth of secondary pests like Spodoptera cosmioides (Walker) (Lepidoptera: Noctuidae). To test this hypothesis, we measured life-history traits, population growth parameters, and adult nutrient content of S. cosmioides reared on the foliage of four Amaranthus species, of Cry1Ac Bt and non-Bt soybean varieties, and on meridic artificial diet. RESULTS Larvae reared on A. palmeri and A. spinosus had a shorter development time (5-7 days) than on the soybean varieties and A. hybridus. The armyworm survival probability was zero on A. viridis and highest (80 and 71%) on soybeans and A. palmeri. This latter and the artificial diet produced the heaviest larvae and pupae, contrariwise the non-Bt soybean variety. Body nutrient content diverged mostly for the artificial diet reared adults in relation to those on the soybean varieties. The intrinsic rate of population increase (i.e., overall fitness) was 27.88% higher for the armyworms on A. palmeri, Cry1Ac Bt soybean, and artificial diet than on non-Bt soybean, A. spinosus, and A. hybridus. CONCLUSIONS Cry1Ac soybean fields infested by some Amaranthus weeds, especially A. palmeri, are conducive for the population growth of S. cosmioides. Integrated Pest Management programs may be needed to properly manage S. cosmioides in soybean fields, with surveillance for population peaks and judicious control measures when needed. This article is protected by copyright. All rights reserved.