In the world of pesticide-resistance management, resistance to herbicides tops the charts.

Farmers can easily get focused on the rhythm of managing weeds in the fields. Just as important is following proper insect management practices to keep insect resistance from becoming the issue herbicide resistance is today.

One MOA vs. Multiple

Herbicide-resistance management stresses the importance of rotating sites of action between each application and using multiple sites of action within one herbicide treatment. Insecticide application differs in that it uses just one mode of action.

Because there are fewer insecticide modes of action compared to herbicide sites of action, the Insecticide Resistance Action Committee recommends using only one mode of action per treatment of insecticide. This precaution preserves other modes of action for subsequent applications, if needed. 

According to Bob Koch, extension entomologist at the University of Minnesota, focusing on one mode of action is often the best approach for insect-resistance management and leaves other modes of action available as viable forms of control. 

“Using individual modes of action in each application is generally a better choice for insect-resistance management,” Koch says. “If there happens to be any survivors after you apply one MOA on the first application, you can switch to another MOA if a second treatment is needed.”

However, farmers must pay close attention. Product names can be misleading and provide little insight into the mode of action a product contains. Erin Hodgson, extension entomologist at Iowa State University, says it’s important to understand the group number of the insecticide being applied. 

“Farmers could think, ‚ÄòI’m changing the name, so I’m changing up my mode of action.’ But that’s often not the case. For example, it’s just swapping a pyrethroid with another pyrethroid,” Hodgson says. “A different product trade name does not equal a different mode of action.”

Farmers can make sure they know what mode of action is in a product with tools such as the Take Action Insecticide Classification Chart. Farmers can match the trade name of their insecticides with the mode of action group number. They can confirm their insecticide products utilize different modes of action from their previous application. 

Some Insects Left Behind

Herbicide applications focus on eliminating the entire weed population present in the field to reduce competition with the crop and eradicate weed seed production. Because insects have multiple generations in one season and travel from field to field, insecticide treatments have different recommendations. Insecticide use is only warranted when the yield loss caused by insect damage is high enough that it outweighs the cost of treatment. This is known as the economic threshold.

“With insects, there’s been a lot of research done on the relationship between the number of insects and the amount of yield loss,” Koch says. “Plants can tolerate a certain number of insects.” 

Complete elimination of insects in the field is not the goal. “If we were trying to eradicate insect pests, we would be spraying constantly,” he says. “It would have a lot of additional costs for the farmers and not be economically efficient.”

Hodgson says it’s important to apply only after economic thresholds have been met. That’s where the best return on investment from the insecticide application comes. 

“When market values are low, break-even points on insecticide costs are harder to obtain,” she says. “In these situations, I’m in favor of scouting and using thresholds so that farmers can make a profitable choice. Deciding when to apply insecticides is a tough choice for farmers to make. Thresholds offer farmers their best bet of making a profitable decision.”

It’s All in the Details

Another difference between herbicide and insecticide application is the mobility of the pest. Herbicides are applied to a weed that is static, and insects are on the move. Farmers should ensure insecticide applications make contact with the insect.

To help achieve this contact, Hodgson stresses the importance of making sure treatment configurations are right for the insect being controlled. While these details are also important for weeds, they are even more important for insect control.

“Something that I always try to stress to farmers or applicators is that pyrethroids or organophosphates are contact-based,” she says. “Things like nozzle choice, volume and pressure help make sure the product is coming in contact with the insect. If those elements are right, you have your best odds of getting a higher knockdown. Then, you might not have to spray again.”

It’s important for farmers to note the differences in the number of modes/sites of action, eradication strategy and application configurations between herbicide-resistance management and insect-resistance management. Because of the nature of the pests being controlled, taking the proper steps is crucial to mitigating resistance.

For more information on insecticide modes of action, download the Take Action app or check out the Take Action Insecticide Classification Chart