When Pesticides Don't Play Nice

ST. LOUIS (DTN) -- It's time to brush up on some old chemistry lessons before heading into the fields this spring. The potential for crop injury from herbicide-insecticide interactions is back on the farming curriculum, scientists told DTN.

"Everything old is new again," explained University of Tennessee weed scientist Larry Steckel. "Insecticide-herbicide interactions were a problem in the '80s and they're cropping up again now." As growers turn to a wider range of herbicides to control resistant weeds and return to soil insecticides to control Bt-resistant rootworms, this risk of crop injury is on the rise, he said.

At issue is the interaction of organophosphate (OP) insecticides like Lorsban with certain HPPD- and ALS-inhibitor ingredients found in common herbicide premixes such as Capreno and Halex GT, Steckel said.

New research from Steckel and University of Tennessee entomologist Scott Stewart confirms that severe corn injury and significant yield loss can result when these chemistries mix.

Corn plots treated with an OP insecticide (Lorsban) along with a herbicide premix containing an ALS-inhibitor ingredient (Capreno), suffered yield losses of up to 58% in the study. Plots with the OP insecticide applied with a herbicide premix containing an HPPD-inhibitor (Halex GT) also showed significant visual injury, although it did not translate to yield loss in these particular studies, Steckel noted.

In general, OP insecticides such as Lorsban or Counter combined with herbicides such as Accent, Capreno, Corvus, Steadfast, Halex GT, Lightening, Option or Resolve, are likely to cause crop injury. For a more comprehensive list of herbicide-insecticide injury interactions, see this chart created by University of Illinois weed scientist Aaron Hager in a 2013 university pest bulletin: http://bit.ly/….

BACK TO LABELS

Herbicide labels will always warn growers about these interactions, but in the past decade many farmers have fallen away from the practice of regularly checking them, Steckel said.

Now, with glyphosate-resistant weeds and Bt-resistant western corn rootworms filling the fields of many southern and Midwestern farmers, labels are back on the required reading list, he said.

Soil insecticide use to control the rootworm is on the rise in the Midwest, and Southern corn growers are fighting the sugarcane beetle, which defies both Bt traits and seed treatments, Steckel pointed out. At the same time, glyphosate and other herbicide-resistant weeds have driven many growers across the country back to herbicide pre-mixes with a dizzying range of ingredients.

"I suspect there will be more crop injury due to these OP/herbicide interactions in the future," University of Illinois entomologist Mike Gray said. "Growers will need to get up to speed on these possibilities."

"As you incorporate insecticides, whether they are banded in the soil or applied over the top, be sure you know what herbicide you're using and check the label," Steckel concluded.

PESTICIDE CLASH

The injury to corn plants from these pesticide interactions actually stems from similarities in the chemicals, Hager explained in his pest bulletin article.

When faced with herbicides or insecticides, a corn plant protects itself by rapidly metabolizing the chemicals, which breaks the compounds down and detoxifies them.

Plants generally use a number of different metabolic pathways to deal with the range of pesticides farmers throw at them. Trouble arises with OP insecticides and certain ALS- and HPPD-inhibitors because the corn plant uses the same metabolic pathway to process them.

When faced with both the insecticide and herbicide at the same time, something like a bottleneck occurs as the plant tries to process both with the same pathway. Like a clogged highway, the detoxifying process slows down and the herbicide has time and opportunity to stunt and injure the plant, often resulting in severe yield loss.

For more information on this interaction and the resulting corn injury, see Steckel and Stewart's study here: http://bit.ly/….

For a breakdown of the chemical ingredients in herbicide pre-mixes on the market, see this herbicide classification table from the Soy Checkoff's Take Action program: http://bit.ly/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com.

Follow Emily Unglesbee on Twitter @Emily_Unglesbee.

(PS/CZ)

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