Changes in insecticide efficacy against Helicoverpa zea (Boddie) on soybean due to larval size and plant phenology

Abstract

The within-plant distribution of corn earworm, Helicoverpa zea (Boddie), on soybean, Glycine max (L.) Merrill, changes with larval size and plant phenological stage. Small (instars 1 and 2) larvae on H. zea are often found within rolled leaves and flowers when each plant part is available (vegetative and flowering stages). Medium (instars 3 and 4) and large (instars 5 and 6) larvae at all plant reproductive growth stages, and small larvae when rolled leaves and flowers are not available, are found primarily on open leaves. Because both rolled leaves and flowers may protect small larvae from exposure, insecticide efficacy may be reduced in soybean fields during the vegetative or flowering stages. To ensure that any differences observed in field experiments were not due to differences in susceptibility, the relative mortality of small, medium, and large H. zea larvae fed on soybean foliage treated with insecticides was assessed in the laboratory. No significant difference between small, and medium plus large larvae (combined) was detected in one test, but mortality of small larvae was significantly higher in another test. In the field, the number of H. zea larvae on insecticide-treated and untreated soybeans was compared 1 and 7 days after application, with separate experimental designs. The relative number of small, medium, and large larvae in each treatment varied with soybean growth stage. Differences between insecticide treatments were greater for large larvae than for small larvae when soybeans were flowering. When soybeans were in the pod-forming stages, few larvae of any size were found in the insecticide-treated plots one day after application. One week after application, greater differences in larval numbers were observed for small and medium larvae on soybeans in early reproductive growth stages than for large larvae in early stages or all sizes in later stages. It was concluded that insecticide efficacy for small H. zea larvae is low on soybeans in vegetative or early flowering growth stages. Because mortality of small larvae was higher at later plant growth stages, and as mortality of small larvae fed on insecticide-treated foliage was as high as, or higher than, mortality of larger larvae, the low efficacy may be attributed to lower exposure to insecticides.