May 8, 2009

In most crop fields there are many species of weeds and insects with different life cycles and survival mechanisms. They interact and affect not only each other and the crop, but also the efficacy of their respective management tactics. With soybean, for example, insect defoliation can significantly delay canopy development, which in turn provides more sunlight for weeds to grow and compete with the crop.

Understanding these pest and crop interactions can help you integrate the most effective pest management strategy for your situation. While it's unlikely that a single control measure can manage all pests, an integrated pest management strategy can combine cultural, mechanical, biological, genetic, and chemical methods for effective and economical pest control.

Nebraska Research

Research was conducted in eastern Nebraska in 2003-2004 to determine the critical time for weed removal as influenced by three levels of simulated insect defoliation (0%, 30%, and 60%).With no insect damage to the soybean canopy, weeds could remain in the crop up to the V4 stage (third trifoliate) or about 20 days after crop emergence without significantly affecting yield. However, at 30% and 60% defoliation, weeds needed to be removed by the V3 (17 days) and V1 (10 days) stages, respectively.

From a practical standpoint, this indicates that soybeans with 30%-60% insect damage have a shorter weed control window and potentially fewer weed control options. Early season bean leaf beetle feeding and defoliation can reduce yield two ways: directly, through soybean plant damage, and indirectly, by moving the criticial period of weed control forward -- from 20 days after emergence to 10 days. Both sources of potential yield loss should be considered when deciding whether to treat early-season bean leaf beetle infestations.

This research also highlights an extra weed-fighting tool — a good insecticide. With a particularly bad infestation of bean leaf beetle, spraying an insecticide to control the bean leaf beetle may widen the herbicide application window and increase weed control options.

Funding for this project was provided by the Nebraska Soybean Board.

Stevan Knezevic, Extension Weeds Specialist
Haskell Agricultural Laboratory, Concord