THE RELATIONSHIP BETWEEN FALL ARMYWORM (LEPIDOPTERA, NOCTUIDAE) INSTAR AND SUSCEPTIBILITY TO INSECTICIDES APPLIED TO SWEET CORN

Toxicities of 2 concentrations of fenvalerate and methomyl to second, fourth and sixth instar fall armyworms, Spodoptera frugiperda (J. E. S mith) (Lepidoptera: Noctuidae), were determined in laboratory and fiel d tests. In laboratory tests, the LC50 for both methomyl and fenvalera te increased as larval age increased from second to fourth to sixth in star. Fenvalerate was more toxic than methomyl to second instars but l ess toxic than methomyl to sixth instars. In the 1986 field tests with whorl stage sweet corn, all treatments effectively reduced whorl dama ge caused by second, fourth and sixth instars compared with the untrea ted. Both rates of methomyl were significantly more effective than eit her rate of fenvalerate in reducing whorl damage caused by fourth or s ixth instar fall armyworm. In 1987, both rates of methomyl and the 0.2 24 kg AI/ha rate of fenvalerate significantly reduced second and fourt h instar fall armyworm whorl damage compared with the untreated, but o nly the 1.0 kg AI/ha rate of methomyl significantly reduced sixth inst ar whorl damage. All insecticide treatments resulted in significantly fewer second instars recovered from the whorls, but only the methomyl treatments resulted in significantly fewer fourth instars compared wit h the untreated. Greater whorl damage was caused by fourth and sixth i nstars compared with second instars. Results indicate that timing of a pplication is as important as selection of pesticide for management of fall armyworm on whorl stage sweet corn.