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. Webworms generally feed in the upper canopy of the plant by encasing the top leaves in a webbing and consuming the leaves within the webbing. As they grow larger, they may feed outside the webbing as well.
Telltale signs of an alfalfa webworm infestation are defoliation of the upper leaves and webbing in the upper canopy filled with black specks of fecal matter. 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. The University of Missouri does not have specific threshold data, but 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. Otherwise, an early harvest will remove the webworms habitat and control the infestation.
Several insecticides are labeled to control alfalfa webworms, but harvest restrictions can vary from 0 to 15 days after an insecticide application. Be sure to read and understand the label before applying an insecticide.
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. The fall armyworm will chew on the tender leaves of plants, causing “window-pane” like damage in corn that can progress to large holes in the upper leaves.
On forages, the damage appears as large brown spots in the field that appear to have died due to drought. This is actually the result of the plants dying due to a lack of water movement, although it is rare for armyworms to kill healthy, established plants.
Scout for fall armyworm early in the morning or late at night, as they are inactive during the day. For economic thresholds on fall armyworm damage, contact your local University of Missouri Extension office.