Be Careful Of Rootworm Insecticides Interacting With Herbicides

Farmers going back to soil-applied corn rootworm insecticides, causing problems when organophosphate insecticides interact with certain herbicides.

Published on: May 2, 2013

The use of soil insecticides applied at planting time for corn will increase this year. There have been more problems the last few years with failures of certain Bt corn traits to protect against corn rootworm. The situation has prompted more farmers to go back to using soil-applied insecticides even on corn hybrids that have the Bt trait.

Farmers have many insecticide choices available, but several of these could restrict which corn herbicide should be used. "We discussed this at winter meetings, but now is a good time to remind corn growers about the potentially yield damaging interactions that can occur if the wrong insecticide/herbicide combination is used," says Clarke McGrath, an Iowa State University Extension field agronomist. McGrath writes the "Corn and Soybean Insight" column each month in Wallaces Farmer magazine.

AVOID INJURY TO CORN: Using an organophosphate insecticide at planting or after corn emergence could restrict the use of herbicide products that are either the ALS or HPPD type of herbicides. The precautions and restrictions most often appear on the herbicide label and are due to the increased potential for corn injury following the use of organophosphate insecticides and ALS or HPPD herbicides.
AVOID INJURY TO CORN: Using an organophosphate insecticide at planting or after corn emergence could restrict the use of herbicide products that are either the ALS or HPPD type of herbicides. The precautions and restrictions most often appear on the herbicide label and are due to the increased potential for corn injury following the use of organophosphate insecticides and ALS or HPPD herbicides.

Problem is returning to radar screen as more farmers are going back to using soil-applied corn rootworm insecticides

This potential problem of certain insecticides and herbicides interacting had slipped off the radar screen for nearly a decade. That's because corn hybrids with the Bt corn rootworm traits were introduced and used on more and more acres. But recent issues with rootworms becoming resistant to certain Bt traits in corn hybrids have a number of growers going back to using soil-applied insecticides with the planter again given the large amount of corn on corn now being grown in Iowa.

"You should be very careful and strategic in making your herbicide choices if you are using soil insecticides," says McGrath. "Any insecticide product that has an organophosphate in it, such as Lorsban, Counter, Aztec, Thimet, Fortress, can have a potential interaction with many of the most popular herbicides that are being used these days. That reaction can damage young corn plants."

He says the best plan to handle this is to pick the insecticide of your choice -- and there are some options without organophosphates you can consider as well -- then talk to your ag chem dealer prior to making any herbicide applications.

To avoid this potential problem, be sure your insecticide and herbicide choice is compatible

Why do certain combinations of organophosphate insecticides and ALS or HPPD-inhibiting herbicides increase the risk for corn injury? Both insecticides and herbicides are foreign compounds to the corn plant. The corn plant tries to "defend itself" against any potential injury a foreign compound might cause by attempting to render the compound inert or nonphytotoxic.

That process is called "metabolism" or breakdown of foreign compounds. A corn plant has pathways to detoxify the foreign compounds but the organophosphate insecticides and many ALS and HPPD-inhibiting herbicides share a common pathway. If both the insecticide and herbicide are present, the metabolism pathway can't effectively metabolize both compounds. When this happens, corn injury can result.