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. Smith) (Lepidoptera: Noctuidae), were determined in laboratory and field tests. In laboratory tests, the LC50 for both methomyl and fenvalerate increased as larval age increased from second to fourth to sixth instar. Fenvalerate was more toxic than methomyl to second instars but less toxic than methomyl to sixth instars. In the 1986 field tests with whorl stage sweet corn, all treatments effectively reduced whorl damage caused by second, fourth and sixth instars compared with the untreated. Both rates of methomyl were significantly more effective than either rate of fenvalerate in reducing whorl damage caused by fourth or sixth instar fall armyworm. In 1987, both rates of methomyl and the 0.224 kg AI/ha rate of fenvalerate significantly reduced second and fourth instar fall armyworm whorl damage compared with the untreated, but only the 1.0 kg AI/ha rate of methomyl significantly reduced sixth instar 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 with the untreated. Greater whorl damage was caused by fourth and sixth instars compared with second instars. Results indicate that timing of application is as important as selection of pesticide for management of fall armyworm on whorl stage sweet corn.