Alfalfa weevil larvae were just hatching on March 31 in south central Kansas. They had not even started feeding yet as no pin-prick sized holes in leaves or any nibbled terminals were noted. Fields in north central Kansas were also sampled on March 31, but no larvae were detected.
They are probably hatching by now, April 3, but as long as the average daily temperatures are only in the mid-50’s °F, the larval development and consequent feeding will be relatively slow. When temperatures warm alfalfa weevil activity will increase and defoliation will proceed very quickly depending upon infestation levels. So monitoring should start immediately.
Do not be too quick to “pull the trigger” on an insecticide application, however. Generally, waiting until about 33-50% of the stems (1 larva/3 stems or 1 larva/2 stems) have feeding larvae will increase the effectiveness of the application. Also, ensure that there will be at least 3 days of temperatures above 50°F without a moisture event immediately after the application.
For information on rates of registered insecticides, see K-State’s 2014 Alfalfa Insect Management Guide:
Figure 1. Pinprick feeding holes caused by early instar larval feeding. Photos courtesy of Holly Davis, K-State Research and Extension.
Figure 2. Small alfalfa weevil larvae.
Jeff Whitworth, Extension Entomology
Holly Davis, Research Associate, Entomology