Early Insect Activity In Alfalfa And Wheat

By Ada Szczepaniec

Spring has begun on the warmer side and it sparked a flurry of insect activity, particularly in alfalfa and wheat. After a cooler week, the weather is forecast to warm up again, so here are a few notes and scouting recommendations that will be useful next week.

Alfalfa Weevils

First and foremost, according to the growing degrees accumulated throughout the state, alfalfa weevil activity should be picking up in South Dakota. For tips on using the growing degree day accumulation in making decisions about management, see our Spring is Here: Watch out for alfalfa weevils article from last year. Rather than taking a cue from weevil larvae accumulating on harvesting equipment or in sweeping nets, we recommend using the stem counting method. It is much more precise in estimating weevil populations.

Army Cutworms

Figure 1. Cutworm larva in a wheat field. | A. Chirumamilla

Army cutworms have also had an active early spring (Figure 1). They are usually a problem in wheat, but this spring we got word of them damaging alfalfa fields. This is probably related to the fact that alfalfa stands are faring better than wheat this spring, given the dry fall and winter. As far as managing army cutworm larvae in alfalfa, the recommendations are the same as in wheat, which you can find in the SDSU Extension Arthropod Pests of Wheat: Cutworms publication. Thresholds for these pests are defined as more than four larvae per square foot or healthy alfalfa and more than two per square foot of thin and stressed alfalfa. It’s important to note the size of the larvae as you are scouting: if they are close to 1 ¼ to 1 ½ inch in length it means they are done feeding and are getting ready to pupate. Applying insecticides when larvae are fully grown is not recommended since they have ceased feeding and will no longer damage the crop.