Issue No. 2, Article 5/April 7, 2006
Time for Alfalfa Weevils Is Drawing Near
Temperatures are beginning to rise and degree-days to accumulate. As we move fully into spring, the watch for alfalfa weevils begins. Alfalfa fields in southern Illinois will soon be experiencing larval hatch; larval hatch from eggs is expected to occur after the accumulation of 300 degree-days from January 1 (base temperature 48°F) (Table 1). First instars can be found in the folded terminal leaves. Initial injury caused by the larvae appears as pinholes in the leaf terminals. As larvae continue to develop and increase in size, damage also increases. The alfalfa weevil larva has a green body with a prominent white stripe down the center of its back. When the larva reaches about the third instar, after the accumulation of approximately 325 degree-days, it begins skeletonizing the leaves. If temperatures continue as they are, alfalfa weevil feeding will be well under way in the southern third of the state by mid-April. To get current degree-day accumulations for alfalfa weevils and other insect pests, visit our Degree-Day Calculator.
Alfalfa weevil larva.
Some tips for scouting for alfalfa weevil larvae: Be sure to look for them and the symptoms of their feeding injury throughout the field, not just along the edges. Look at areas of the field that may warm up early, such as south-facing slopes and areas of lighter soil. The best way to count the larvae is to snap a stem off at ground level and place it top down into a white bucket. We recommend collecting stems at random locations by walking in a U-shaped pattern through the field. After collecting 30 stems, you can beat the stems, a few at a time, against the sides of the bucket to dislodge the larvae. Sample plant heights throughout the field, or randomly select a sample of 10 of the stems to measure the height.
One word of caution: Clover leaf weevils can be present in alfalfa fields just before alfalfa weevils are noticed. Be sure to identify the weevil larvae that are present in the field before making any treatment decisions. Growers across the state should brush up on scouting for alfalfa weevil larvae. More information on identification, life cycle, scouting, and management can be found on fact sheets for these insects--alfalfa weevil (Adobe PDF) and clover leaf weevil (Adobe PDF). --Kelly Cook
Clover leaf weevil larvae (photo courtesy of Matt Montgomery, Crop Systems educator, Sangamon-Menard counties).