The corn earworm (Helicoverpa armigera) and the native budworm (H. punctigera) are major pests of irrigated soybean in southern New South Wales and northern Victoria. Information on the within-plant distribution of eggs is required to assist in the development of appropriate sampling techniques for these pests and to determine possible effects on the control achieved by insecticides. Field-cage experiments were conducted to determine the within-plant distribution of the eggs of both species throughout the development of a soybean crop. The distribution of eggs in the field was also recorded from commercial crops of soybean. The results indicate that the majority of eggs of both species are laid on fully expanded leaves in the top 20 cm of the crop canopy at all growth stages, except pod fill. This distribution resulted from a preference for leaves and the top of the canopy, combined with the dominance of leaves as potential oviposition sites. There was a strong preference for the lower surfaces of the leaves, with the upper surface rarely preferred. During the reproductive development of the crop, H. armigera exhibited a strong preference for the developing flowers and pods, while H. punctigera showed a preference only for fully developed pods. The preference for flowers and pods, combined with the increase in the proportional area of leaves in the lower regions of the canopy, led to a decrease in the proportion of eggs laid in the top 20 cm of the canopy as the crop developed. Although the two species tended to lay eggs on different plant structures, the vertical distribution of eggs within the canopy was similar. The results are discussed in terms of the development of appropriate sampling techniques and possible effects on insecticidal control.
- Field cage
- Pest management