By Jeff Whitworth
The first agricultural insect pest we typically confront each year in Kansas is the alfalfa weevil. This severe pest will begin feeding on alfalfa when eggs start hatching, anytime from late February through April. Alfalfa weevils overwinter either as eggs inserted in alfalfa stems (Figure 1), or adults (Figure 2).
Alfalfa weevils are cool-weather insects. Adults have been inhabiting alfalfa fields since last October where they have been doing a little feeding, but mostly laying eggs. Most of these eggs survive the winter. The adults will be active all winter, any time temperatures get over about 48 degrees F. The eggs deposited into the stems will also begin their development and continue developing anytime temperatures are greater than 48 degrees F. That is partly the reason the larvae can start hatching in late February and others can continue to hatch through April, depending upon when the eggs were deposited and how much development they have completed.
Figure 1. Alfalfa weevil eggs in the stems.
Figure 2. Alfalfa weevil larva (bottom) and adult (upper).
Since these development rates are controlled by temperatures, a thermal unit accumulation system, or growing degree day system, has been developed to help predict alfalfa weevil development. This is a neat and convenient way of monitoring weevil development without going to the field. You can calculate how many growing degree days have accumulated for the alfalfa weevil in your area, since January 1, 2018, using the Kansas Mesonet: http://mesonet.k-state.edu/agriculture/degreedays/