GROWTH-INHIBITION OF CORN-EARWORM (LEPIDOPTERA, NOCTUIDAE) LARVAE REARED ON RESISTANT CORN SILK DIETS

Laboratory studies were conducted to determine whether the growth inhibition response of corn earworm, Helicoverpa zea (Boddie), larvae on corn silks is a result of a feeding deterrent or other factors. Neonates and 4th and 5th instars were fed either pinto bean diet (control), a mixture of silks of 'Stowell's Evergreen' (susceptible), or 'Zapalote Chico 2451# P(C3)' (resistant) corn. The silks of Zapalote Chico inhibited growth, extended development time, and reduced weight of pupae when neonate and 4th instars were tested. Both consumption and amount of feces were less for neonates fed resistant silk diets, but not for 4th and 5th instars. Consumption and amount of feces excreted by 4th and 5th instars on both resistant and susceptible silk diets were greater than that of the pinto bean diet. The amount of protein in the feces excreted by 4th and 5th instars reared on resistant silk diets was substantially more than the amount of protein in the feces excreted by larvae reared on control diets. The percentage of protein:feces was substantially higher for Zapalote Chico (91.5%) than for Stowell's Evergreen, the susceptible entry (75.1%). The growth inhibition factor in resistant silks is likely caused by an antinutritive factor causing the larvae to excrete large amounts of protein.