Alfalfa weevil and you

As spring begins and my family starts to ask about turning on the air conditioning, the hay fields are starting to jump out of the ground. Nothing else makes me embrace the new growing season like this. As the hay is coming up, so is our most damaging pest: alfalfa weevil. The adult weevil is very distinctive, being approximately ¼ inch long beetles with a blunt “snout,” thick golden hairs covering the body, and a distinct dark, narrow stripe down the back. 

These are the harbingers you will find in your fields earlier in the spring. The female beetles will be laying eggs in the stems of your alfalfa. These eggs will then hatch, and the more destructive part of the weevil’s life cycle will emerge.

The alfalfa weevil larvae will crawl up the stems and begin to voraciously eat the alfalfa leaves around the growing point. The larvae will grow through the spring up to around 3/8 of an inch in length. 

For identification purposes, the alfalfa weevil larvae are legless with a prominent black head. The larvae are wrinkly and yellow green in color with a white stripe along the middle of the back.

If you leave them alone, these larvae will pupate, and a new adult will emerge. This adult does not begin to repeat the cycle immediately; rather, it feeds and slows its maturations to try and survive a winter and begin the cycle anew.

A lot of study has gone into the association between alfalfa weevil and temperature. Many studies looking at this link show that you should expect the eggs to hatch around 310 growing degree days (GDD), with the first instars at 350 GDD and second instar 425 GDD. 

This is a great model, but is consistent with field conditions. Current Wyoming forecasts are based on weather stations located outside of your alfalfa field. Use this as a guide for field scouting on the ground. Results will vary.

Using the model provided by uspest.org, I ran the forecasts for Worland where I am working and Torrington. 5/23/25-Worland-Egg Hatch; 5/31/25-Worland-2nd instar; 5/17/25-Torrington-Egg Hatch; 5/26/25-Torrington-2nd instar/

As with all farming endeavors, control can be accomplished in myriad ways. Early harvest and insecticide treatments are going to be the main avenues most hay growers use. Each has its advantages — early harvest is the most natural and doesn’t introduce chemicals into the field. 

It should be noted, though, that just because you cut the hay, there will be larvae that fall to the ground and start munching on your next cutting. In contrast, should you decide to spray, you will need to factor the economic benefits and the harvest restrictions into the equation.

With an eye on the forecast and your hay field, you should be able to get ahead of your weevil before they become a problem. If you have questions, please reach out to your local extension agent and we will be happy to work with you.