Does Florivory by Helicoverpa zea (Lepidoptera: Noctuidae) Cause Yield Loss in Soybeans?

Helicoverpa zea (Boddie), corn earworm, is a damaging insect pest of many crops, including soybeans. An economic threshold for soybeans during the pod-filling stages exists to prevent economic damage to seeds. However, the impact of florivory (flower feeding) by H. zea larvae on seed yield is poorly understood and there is no economic threshold for flowering-stage soybeans. Four small plot experiments were conducted in North Carolina during 2011 and 2012 to assess the impact of H. zea feeding during the flowering stages of determinate soybeans on various yield components. Helicoverpa zea densities were manipulated with insecticides and various planting dates of soybeans and monitored weekly. Helicoverpa zea naturally infested the plots after flowering began and were allowed to feed until R3; they were eliminated from all plots from R3 to maturity. In some sites, H. zea densities exceeded the podding economic threshold during the flowering stages, but yield did not differ among treatments. During 2012, florivory from H. zea was measured directly by counting injured flowers. There was a negative yield relationship between both injured flower number and cumulative flower number. Moreover, H. zea densities were related to both a decrease in cumulative flowers and an increase in injured flowers, even though a direct linkage between H. zea density and yield loss was not observed. Without knowing the preferred tissue types and performance of early-instar larvae on soybeans, it is possible that H. zea density may not be the best measurement for developing an economic threshold in flowering soybeans.