Influence of Soybean Planting Date and Leaf Age on Resistance to Corn Earworm (Lepidoptera: Noctuidae)

Abstract

Soybean, Glycine max (L.) Merr., resistance to insects has been shown to vary with planting date and leaf age. This study determined effects soybean genotype, planting date, and leaf age had on feeding behavior of corn earworm, Helicoverpa zea (Boddie). Soybean genotypes ‘Cobb’ (susceptible) and ‘GatIR81-296’ (resistant) were planted early (29 May) and late (28 June) in 1989 in the field. Trifoliolate leaf samples from the top, middle, and bottom one-third of the plant were sampled four times throughout the season. Leaves were arranged in a free-choice fashion and presented to third-instar H. zea in the laboratory. Leaf area removed was calculated with a computer-based leaf area analysis system. Larvae preferred ‘Cobb’ over ‘GatIR81-296’ in each sampling date. Leaves from the late planting had greater leaf area removed on the first two sampling dates compared with leaves from the early planting (15.8 versus 11.7 and 34.5 versus 22.9 cm2), but leaves from the early planting had more leaf area removed compared with leaves from the late planting on the last two sampling dates (30.9 versus 22.7 and 21.3 versus 16.2cm2). Thus, H. zea exhibited a bimodal feeding preference for early- and late-planted soybean. Older leaves (middle or bottom one-third) of all plants were typically preferred over younger ones (top one-third) throughout the season. Because resistance may vary during plant development, an accurate identification of soybean resistance should be done only with genotypes at a similar stage of development.