A few weeks ago during the latter part of our mild winter, most of us would have guessed that alfalfa weevils would have been active by now. However, the recent cold temperatures obviously have slowed down the onset of alfalfa weevil activity. Good thing, too. We will not be able to publish maps of actual and projected degree-day accumulations for two weeks while Bob Scott, Illinois State Water Survey, is out of the country. Therefore, I encourage you to keep track of temperatures locally to get a rough estimate of when to begin looking for alfalfa weevil larvae. (As I indicated in the "moth capture" article, if you want to keep apprised of weather data around the state, visit Bob's Web site, Water & Atmospheric Resources Monitoring, at http://www.sws.uiuc.edu/warm.)|
I have heard through the grapevine that some producers in southern and central Kentucky are already battling alfalfa weevils, so it's only a matter of time before activity begins in southern Illinois. John Shaw, director of the Illinois Insect Management and Insecticide Efficacy Program, found a couple of active adults in a field in Champaign County on April 2. He observed overwintering eggs (dark orange to brown) in the stems but found no spring-deposited eggs (bright yellow).
As you begin to scout alfalfa fields for alfalfa weevils, look first in areas of fields that might warm up early, such as south-facing slopes and areas of fields with lighter soils. After 200 to 250 degree-days have accumulated from January 1, you should be able to find small, first-instar weevils in the folded terminal leaves. These small, yellowish larvae with black heads feed on the leaves, resulting in observable pinholes. This injury is not economic because the larvae are too small to cause significant defoliation. However, by the time alfalfa weevils grow into third instars, they begin to cause more economic damage by skeletonizing the leaves. At this stage of development, alfalfa weevil larvae are bright green with a distinct white stripe along the center of the back.
Fully grown alfalfa weevil larva.
Skeletonization of alfalfa leaves by alfalfa weevil larvae.
If you are scouting for alfalfa weevils and symptoms of the injury they cause, let us know when you encounter them. It's possible that we could experience some heavy infestations this year, and we want to keep everyone alert.--Kevin Steffey