It's Time to Start Scouting Fields for Alfalfa Weevils
Published: Friday, May 1, 2026
The northern tier of counties in Indiana just accumulated 300 heat units (base 48 degrees), which means we now expect to see alfalfa weevil hatching from their eggs. Within days, damage to alfalfa will begin to show up. It is a good idea to check your alfalfa fields a couple times a week.
Alfalfa weevil larvae can cause significant defoliation, which reduces the protein levels of your hay. Up close, you will see small holes in the tips of the upper leaves. If you can see a silvery tip damage from a distance, the feeding is quite severe in the field, and you have probably lost significant yield already. Early detection is important.
Between 300 and 400 heat units, if you find more than three larvae per plant and 50% damaged tips, you should consider treating with a residual insecticide. Current projections are we will be in that range of heat units until the middle of the first week of May.
Between 400-500 heat units, treatment is recommended if 50% of the tips show feeding.
Between 500 and 600 heat units, the threshold for treatment shifts to 75% or more of the plants showing tip damage before treatments are done.
Once we exceed 600 heat units, we are nearing harvest. At that point, the decision of whether to spray depends on if you are going to cut the hay soon. if cutting will be delayed more than five days, consider treating the alfalfa. But, if you chose to cut the alfalfa, be sure to return to the field in four to five days after harvest.
Why? Alfalfa weevil can lay on the ground and eat the new sprouts as they come up from the stubble. When stubble feeding occurs, the field will appear to remain barren for days or weeks longer than it should, which affects not only your hay yield, but also the long-term survivability of the stand. You will need to get down on your knees to find them feeding in the stubble. They tend to hide down there in the crop residue, so push that residue aside to find them hiding during the day.
Several farmers found out the past two seasons the importance of examining the field after the hay is removed. The fields had no alfalfa regrowth for weeks after the first cutting was removed!
There are a number of insecticides labeled for use on alfalfa weevil, including synthetic pyrethroids, organophosphates, carbamates, and oxadiazines. For those growing organic hay, there are several OMRI approved products, including spinosad, bacillus thurengiensis, neem oil and pyrethrin. Always follow label directions, including pre-harvest intervals and application rates.
It's a good idea to mix up your use of insecticide types. Many of the common insecticides being used the past few years are synthetic pyrethroids, a resistance code of 03A. These products are cheap and effective. However, using the pyrethroid products over and over again will increase the chance of resistance to that class of insecticide, just like the resistance we saw in the herbicides. If you used a product with a 03A resistance code last year, select a different family of products this season. It's not enough to just switch brand names. You need to change the chemistry you use.
The resistance to pyrethroids has not been documented locally yet, but they are seeing it in the western United States. But, like the glyphosate herbicide resistance that spread across the nation a few years ago, it's just a matter of time.
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