Factors Associated with Resistance to Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) in Corn at Different Vegetative Stages

Corn, Zea mays L., germplasm with resistance to leaf feeding by southwestern corn borer, Diatraea grandiosella Dyar, and fall armyworm, Spodoptera frugiperda (J. E. Smith), has been released. Resistant genotypes sustain less leaf-feeding damage than susceptible genotypes. Larvae that feed on plants of resistant genotypes or on tissue collected from these plants grow more slowly than those that feed on susceptible genotypes. Both resistant and susceptible genotypes sustain less damage as the plants mature. This investigation was undertaken to determine the associations between various chemical and physical characteristics of resistant and susceptible corn hybrids and leaf-feeding damage and larval growth on hybrids infested at different vegetative stages of growth. Leaf feeding was heavier on plants infested 28-35 d after planting than older plants. Larvae from the field weighed more when plants were infested between 28 and 35 d. Larvae fed on lyophilized leaf tissue in laboratory bioassays were heaviest when whorl tissue was collected from plants 28-35 d after planting. Larvae fed on tissue of resistant hybrids weighed less than those fed on tissue of susceptible hybrids. Resistant hybrids completed the transition from juvenile to adult earlier than susceptible hybrids. The 7th leaf was the 1st adult leaf of the resistant hybrids, but the 10th or 11th was the 1st adult leaf of susceptible hybrids. Leaves of susceptible hybrids were less tough than leaves of resistant hybrids, and toughness increased with plant age. Relative amounts of hemicellulose were higher in resistant genotypes and in older plants of all genotypes. Hemicellulose and protein levels appear to be associated with fall armyworm resistance, but not southwestern corn borer resistance. Relative amounts of cellulose were negatively correlated with leaf feeding by both insects. Relative amounts of ash, fat, and lignin did not appear to be associated with resistance. Further investigations of resistant and susceptible genotypes at different vegetative stages of growth should be useful in identifying factors associated with resistance to fall armyworm and southwestern corn borer.