Issue No. 2, Article 4/April 1, 2004
Start Thinking About Alfalfa Weevils
It's once again that time of year--the snows have melted, temperatures are on the rise, and spring is on its way! But so are the alfalfa weevils. As
degree-days start to accumulate, the watch for alfalfa weevils begins. Larval hatch from eggs is expected to occur after the accumulation of 300 degree-days from January 1. Alfalfa fields in southern Illinois will soon be experiencing larval hatch. Accumulated degree-days are shown in Table 2. View statewide maps and the most current degree-day accumulations at locations near you with our degree-day calculator.
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, they begin skeletonizing the leaves. If temperatures continue as they are, alfalfa weevil hatch will be well under way in the southern third of the state by mid-April.
Alfalfa weevil larva in terminal leaf. (Photo by Phil Sloderbeck, Kansas State University - www.oznet.ksu.edu)
Alfalfa weevil larva
Here are 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 field areas that may warm up early, such as south-facing slopes or areas of lighter soil. The best way to count 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 them, 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.
Please keep us posted on alfalfa weevil activity in your area.--Kelly Cook