Could we see alfalfa weevil insecticide resistance in South Dakota?

BROOKINGS, S.D. — Alfalfa weevils are an annual pest to alfalfa fields, causing damage typically during the first cutting of alfalfa.

However, in recent years, weevil pressure in South Dakota and surrounding states has been continuing into the second and even sometimes the third alfalfa cutting, which can be problematic when producers are only expecting it at the beginning of the season.

“Seeing this pest continuing to be present can cause some other issues,” said Adam Varenhorst, associate professor and field crop entomology specialist at SDSU Extension. “Alfalfa weevils feed on alfalfa both as adults and the larvae. But, the larvae are the much more damaging stage because they feed on the growing point as well as the leaves. When any insect feeds on the growing point of a plant, we a lot of times will see stunting and reduced growth.”

This is especially damaging to plants like alfalfa, where they need to grow for multiple cuttings per season. These weevils impact the yield.

“So, if we have stunting, a lot of times we’ll notice that continues into the next cuttings,” Varenhorst said.

Alfalfa weevil pressure varies across South Dakota each year.

“I’ve been at SDSU now for just a little over 10 years and it seems like every year, different areas of the state will have varying levels of pressure,” Varenhorst said. “On the southeast side of the state, for years, we’ve had pretty high pressure just going along the I-29 corridor.”

This may have to do with the weather conditions.

“A lot of that has to do with we’ve had a lot of what I would consider pretty nice winters for this insect and one of the things that can reduce its populations, as we saw in 2025, is having overall just cold temperatures with not a lot of snow cover, because these will either be overwintering down in leaf litters as adults or eggs in the stems of the alfalfa,” Varenhorst said. “So, you can imagine snow cover can be really important then because that is an insulating layer from the cold temperatures.”

The more alfalfa is planted in an area, the more alfalfa weevil will be present.

“The longer those fields are in rotation as alfalfa, we will also notice that those tend to become more of a pest just because the food source is handy and they don’t have to travel to find more,” Varenhorst said.

In states like Montana, insecticide resistance with alfalfa weevils has been documented.

SDSU Extension and some of their research farms in eastern South Dakota have been working on studying alfalfa weevils resistance to insecticides.

“We’ve been doing a spray trial of various chemicals, looking to see if they’re still providing control. As far as we found, they provide control. There’s no evidence of resistance with them,” said Philip Rozeboom, SDSU Extension IPM coordinator.

In partnership with NDSU, they also conducted research where they put alfalfa weevils into glass vials and tested their resistance to substances. Through this, they did find some evidence of possible resistance in Sturgis, South Dakota.

“It is not published yet, but we found evidence of resistance. It’s not confirmed. It was a very preliminary study, only one year, 2025. But, we had reduced control. 25% of the population survived that bioassay,” Rozeboom said. “That’s, again, not proof of resistance, but a good indication that it’s coming. Since it’s already been proven in Montana, you know, that’s real close to Montana, obviously, we’re just keeping an eye out for it.”

If alfalfa weevils in South Dakota do become resistant, there will be a lot of chemical control failures, Rozeboom said. He also says that the “pesticide treadmill,” where new products are created to replace ones that aren’t as effective anymore, has slowed down.

“We aren’t getting as many new products as we used to and basically we are running out of chemical management options,” Rozeboom said.

There are other management tactics that can help combat alfalfa weevil. While crop rotation is the typical recommendation for situations like this, Rozeboom says it probably won’t be a solution in this case.

“We are very good about that in South Dakota, rotating your crops, so I don’t know if that will really work. These insects have adapted to that type of management,” he said.

Light tillage in some cases could be a solution, Rozeboom said. Trap cropping is also another possible help.

Planting dates can also make an impact.

“If you plant earlier, we found you could avoid certain insects because your crop is past the certain growth stage that is susceptible to pests,” Rozeboom said.

Scouting your fields is an important step. If you find enough alfalfa weevils in your field to justify the input costs of treatment, then you can apply chemicals to control the pests.

“If you’re not at that threshold, do not apply any treatment. There’s no need to it, you’ll be wasting money by using a chemical control when you don’t need to,” Rozeboom said.

“A lot of our insects we just have a threshold and we say if you reach X number of insects out in the field, it’s time to spray. But alfalfa is based off of the height of the plant as well as the value of the hay during that season,” Varenhorst said.

If you have any issues with alfalfa weevils, there is a survey on the SDSU extension website that asks producers to report issues.