As spring temperatures rise and degree-days start to accumulate, the watch for alfalfa weevils begins. The accumulated degree-days (above a base temperature of 48°F) from January 1 through March 30, 2003, are shown in Figure 3. Alfalfa fields in southern Illinois will soon be experiencing larval hatch; larval hatch from eggs is expected to occur after the accumulation of 200 degree-days from January 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. Alfalfa weevil larvae have a green body with a prominent white stripe down the center of the back. When the larvae reach about the third instar, after the accumulation of approximately 325 degree-days, they begin skeletonizing the leaves. Figure 4 shows the projected degree-day accumulations through April 13, 2003. If temperatures continue as they are, alfalfa weevil feeding will be well under way in the southern third of the state by mid-April.
Some tips for scouting for alfalfa weevil larvae: Be sure to look for alfalfa weevil larvae 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 or 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 walking in a U-shaped pattern through the field, collecting stems at random locations. 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.
Image 1 - Alfalfa weevil larva in terminal leaf (Photo by Phil Sloderbeck, Kansas State University - http://www.oznet.ksu.edu/entomology/extension/visual.htm).
Image 2 - Full-grown alfalfa weevil larva and feeding injury.
As you initiate your scouting efforts and begin to find alfalfa weevils and signs of their feeding activity, let us know when and where you gathered the information. We'll keep our eyes on reports from Kentucky to give growers in southern Illinois a heads-up.--Kelly Cook