Corn farmers need to think twice when using soil insecticides in corn this spring.

Due to the commercialization of corn hybrids with resistance to certain insect pests, the use of soil insecticides has declined greatly, says Aaron Hager, University of Illinois Extension weed specialist. However, some farmers plan to use soil insecticides at planting this season to control “other” insect pests and suppress certain corn nematodes.

While many insecticide choices are available, Hager warns that several could restrict options to use certain corn herbicides so it’s important to understand herbicide-insecticide interactions.

“Using an organophosphate (OP) insecticide at planting or after corn emergence could restrict the use of herbicides which inhibit either the ALS or HPPD enzymes,” Hager says. “The precautions and restrictions which appear on the herbicide label are due to increased potential for corn injury occurrence following their use on corn previously treated with an OP insecticide.”

Most herbicides are systemic in plants, moving extensively from the site of uptake. Often, these trans-located compounds accumulate in areas of the plant undergoing active cell division. Because insecticides and herbicides are foreign compounds, the corn plant tries to “defend itself” against any potential injury by changing it to a nontoxic form.

Corn plants have several different pathways to “detoxify” foreign compounds, but OP insecticides and many ALS- and HPPD-inhibiting herbicides share a common metabolic pathway.

When an insecticide or herbicide is present within the plant, the plant typically metabolizes the compound before it causes problems. However, if both insecticide and herbicide are present, the metabolism pathway cannot effectively metabolize both compounds and corn injury can result.

To see a summary of herbicide label information with respect to the potential for corn injury caused by various OP-herbicide interactions, check out the April 8 edition of The Bulletin.