SOYBEAN YIELD INTERACTION BETWEEN HELICOVERPA-ZEA INFESTATION AND SIMULATED FROST INJURY

Soybean, Glycine max (L.) Merr., yields may be reduced by corn earworm , Helicoverpa zea (Boddie), feeding and by early frosts. One consequen ce of H. zea feeding can be a delay in pod development. A consequence of frost injury can be shortening of the seed rill duration. The objec tive of 12 experiments conducted at multiple locations from 1984 to 19 87 was to evaluate the potential for an interaction between H. zea fee ding damage and early frost. Insecticides were used to establish varyi ng levels of H. zea populations in soybean and paraquat (1,1'-dimethyl -4,4'-bipyridinium dichloride) was used to simulate frost injury. Sign ificant yield differences between levels of H. zea populations were de tected in eight experiments, and paraquat treatments significantly red uced yields in 11 experiments. A significant interaction between H. ze a population level and paraquat treatment time occurred in three exper iments. In these experiments, yield reduction due to paraquat was grea ter where H. zea populations were larger. Although soybean can compens ate for a delay in pod set caused by H. zea feeding in most years, whe n seed fill period is shortened, as would occur with early frost, a de lay in pod set can contribute to yield loss. Identifying one of the ma ny factors that lead to variation in yield loss from a particular leve l of H. zea infestation, is a step towards more effective management o f this pest.