Manage alfalfa weevils
By David G. Hallauer
Meadowlark Extension District Agent
At any one time during the growing season, alfalfa can have from two to five insect pests causing damage. Some pests never reach problem levels and are more nuisance than anything. Others can be serious--the most common being the alfalfa weevil.
Alfalfa weevil adults are active in both fall and spring with temperature dictating activity level. As long as temperatures are above 40 degrees F, females can lay eggs in litter, stubble, or on fresh stems, doing so from October through April. A single female lays up to 1,500 eggs (800 on average) with most of the damage occurring on the first cutting.
The weevil also has the ability to adapt to changing temperatures. For example, if early spring temperatures are warm, we get early development and a continued hatch throughout the spring. If temperatures fluctuate, we could get two hatch periods--and problems over a longer period--even in to the second cutting.
Most of the insecticides labeled for alfalfa weevil do an excellent job of control according to K-State trials, making timing the critical factor. You can check out any of the projects and critical insect levels in KSU's Alfalfa Insect Management guide
If alfalfa weevils tend to be your challenge, a publication called "Alfalfa Weevils" might be of interest. It outlines the application of growing degree days to the management of alfalfa weevil. For example, weevil hatch in stems any time we have from 0 to 300 thermal units. So, at 150 to 180 growing degree days after Jan. 1, you should start scouting. It continues with the progression of weevils as thermal units accumulate. Useful information in the atypical spring weather we seem to experience any more.
If you are an alfalfa producer interested in these two publications, contact your district Extension office. To monitor growing degree days, visit the KSU Weather Data Library at http://www.ksre.ksu.edu/wdl. Click on the Weekly Station Summaries tab on the left side.