Insects Can Influence the Timing of Weed Management in Soybeans

Insects Can Influence the Timing of Weed Management in Soybeans

May 16, 2008

Insects and weeds are common pests that can cause major expenses for crop producers. To help producers make reasonable judgments concerning pest control and pesticide use, scientists developed the concept of integrated pest management.

This strategy uses a combination of cultural, mechanical, biological, genetic and chemical methods for effective and economical pest control. In most crop production fields there are many species of weeds and insects with different life cycles and survival mechanisms, and it is not likely that they can be managed by a single control measure.

In reality, weeds and insects interact and affect not only each other, and the crop, but also the efficacy of their respective management tactics. For example, insect-induced defoliation can significantly delay soybean canopy development, which in turn provides more sunlight for weeds to grow and compete with the crop, directly affecting the subsequent weed management plans. Learning how insects and weeds interact with each other and with the crop is essential in developing IPM strategies.

In 2003 and 2004 we conducted a study in eastern Nebraska to determine the critical time for weed removal as influenced by the three levels of simulated insect defoliation (0%, 30%, and 60%). Results indicated that insect damage to the soybean leaf area resulted in a need for earlier weed management. For example, 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 the 30% and 60% defoliation level, weeds should be removed by the V3 and V1 stages, 17 and 10 days, respectively.

From a practical standpoint, this indicates that soybeans with 30-60% of insect damage do actually have a shorter weed control window and potentially fewer weed control options. Soybean leaf damage, for example by bean leaf beetles, affects not only the final yield but also the timing of when weed control needs to be initiated to prevent further yield loss. This research also shows that producers may now have another tool to fight weeds in their soybean fields — a good insecticide. If there is a particularly bad infestation of bean leaf beetle, spraying an insecticide to control the bean leaf beetle may actually 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 Ag Lab, Concord

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