Alfalfa weevils have been found in central and northern Missouri alfalfa fields according to Dr. Wayne Bailey. In fields he scouted in central Missouri the week of May 7th, he found foliar damage on 100 percent of the plants. Most larvae were in early stages of growth, meaning that damage was limited to the upper whorls. Fields had alfalfa weevil at two to three times the economic threshold. The economic threshold is one larva per stem, plus 30 percent of plants showing signs of feeding damage.

Alfalfa weevil larvae grow through four larval stages with most damage caused by the last two instars. As temperatures warm, larval growth accelerates and there is increased potential for damage. Producers should first scout their fields to determine if weevils are present and to assess the potential for damage. To scout for larvae a 3 to 5 gallon bucket and sharp knife are useful. Carefully cup the terminal of each alfalfa stem with your hand and cut the stem near the soil surface and carefully place in the bucket. Cupping the stem prevents the larvae from dropping off during the process. Tap the stem against the side of the bucket to dislodge any larvae. Ideally, sample ten stems at each of five random locations in a field. If the alfalfa weevil population has reached the economic threshold of one larva per stem plus 30 percent of the plants showing feeding damage, then treatment is justified.

There are several management options available. The most common control method is foliar insecticides. Read the insecticide label and use only those recommended for alfalfa weevil. Early harvest is another possibility if alfalfa is within seven to ten days of the normal harvest stage. Fields will still need to be scouted after harvesting to check for the return of alfalfa weevils. Other control methods include grazing and biological control. Joni Ross, Agronomy Specialist 573-378-5358