Worms in alfalfa

by Andy Luke, Field Agronomy Specialist
Many growers have noticed damage in their alfalfa fields lately, with leaves being eaten and webs in the top several inches of the plant. These signs likely mean that alfalfa webworms are present in your fields.
Alfalfa webworms are a small, green caterpillar that can infest alfalfa as well as soybeans. As they grow, alfalfa webworm larvae turn dark green and reach 1 to 1¼” long. They have stripes extending down the length of their back, with three dark spots on each side of their body on each segment. Alfalfa webworm infestations can lower the hay quality with webbing and fecal matter while severe infestations can even reduce alfalfa stands.
The easiest way to control alfalfa webworm is to harvest the alfalfa. Texas A&M University entomologists recommend treatment if 25 to 30 percent of plant terminals are infested and harvest is greater than two weeks away.
Another pest that may show up in our alfalfa fields is the fall armyworm. This insect migrates northward every year from the Gulf Coast, and its larvae will feed on alfalfa, pastures and late-planted corn. Fall armyworm are gray or brown with black bumps on their body and have a distinctive white “Y” mark on their head. On forages, the damage appears as large brown spots in the field that appear to have died due to drought. Scout for fall armyworm early in the morning or late at night, as they are inactive during the day.