Grain Sorghum Response to Fall Armyworm and Corn Earworm Infestation

Abstract

In two plantings of grain sorghum, Sorghum bleolor (L.) Moench, we investigated the impact of whorl injury by natural infestations of fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), on subsequent panicle development and infestation. We also examined plant response to panicle infestation by corn earworm, Heliothis zea (Boddie) (Lepidoptera: Noctuidae). Treatments were applications of chlorpyrifos, methomyl, or Bacillus thuringiensis (Berliner) to the whorl, panicle, or both. Fall armyworm injury to the whorl caused light to moderate whorl injury in nontreated plots; injury was approximately 30% greater in the later planting, possibly reflecting differences in sorghum maturity. Fall armyworm damage delayed panicle development in the second planting by 2-4 d and may have prolonged anthesis. Fall armyworm injury did not measurably affect panicle oviposition by corn earworm. Whorl stage applications of chlorpyrifos apparently increased corn earworm survival on panicles, possibly because of toxicity to beneficials. Methomyl and chlorpyrifos were ovicidal for corn earworm, with up to 18-fold more noneclosed eggs recorded from treated than from nontreated panicles. Control of fall armyworm and corn earworm with chlorpyrifos increased yield in the second planting 106% relative to nontreated plots. Fall armyworm control with methomyl increased plant height 6.8% in the second planting relative to nontreated plots. Yield losses were significantly correlated with corn earworm density in the first planting and fall armyworm damage in the second.