Start Looking for Alfalfa Weevils in Southern Illinois

April 2, 1999
Although we grow less than 1 million acres of alfalfa in Illinois, alfalfa is an important crop for producers who feed it to livestock. And serious alfalfa growers who want to produce high–quality hay need to keep their eyes open for alfalfa weevils in the spring. The weather has warmed up significantly in recent weeks, so alfalfa weevil activity will be evident very soon.
The tale of this year's "crop" of alfalfa weevils began last fall. In southern Illinois, adult females deposited some of their eggs in clusters of 9 to 10 in alfalfa litter and stubble before they went into "hibernation" for the winter. However, alfalfa weevils also will lay eggs on warm days during the winter in southern counties. When the temperature warms sufficiently in the spring, adult females will resume egg laying, and fall–deposited eggs will begin hatching. Two distinct peaks of larval activity usually occur in southern Illinois, one from fall–deposited eggs and one from spring–deposited eggs. (See the forthcoming discussion about heat units.) In northern counties, alfalfa weevils usually do not begin laying eggs until the spring. As a consequence, alfalfa weevil damage in northern Illinois occurs later in the spring.

Whether alfalfa weevils will be prevalent this spring depends largely on the survival of eggs during the winter and the extent of egg laying that occurs when females become active this spring. A single female alfalfa weevil lays an average of 600 to 800 eggs during her lifetime. However, a high rate of mortality for the eggs reduces the potential for alfalfa weevils to cause economic damage. We should know soon about the survival of overwintering eggs. During the week of March 29, Mike Gray and I will be in Des Moines, Iowa, attending the annual meeting of the North Central Branch of the Entomological Society of America. We will make an effort to chat with our colleagues in Indiana, Kentucky, and Missouri to assess the current status of alfalfa weevils in surrounding states.

Accumulation of degree–days to estimate events in alfalfa weevil development is common practice in the Midwest. The rate of development of alfalfa weevils is regulated by temperature: the warmer the weather, the faster alfalfa weevils will develop. Based on a threshold temperature of development of 48 deg F, stages and peaks of alfalfa weevil development can be estimated with relative accuracy. Every day the average temperature is above 48 deg F, degree–days accumulate and some development of alfalfa weevil occurs. Hatching of overwintering eggs usually occurs when 200 degree–days accumulate, and we suggest that scouting should begin when between 250 and 300 degree–days accumulate. An early peak of third–stage larvae from overwintering eggs occurs after an accumulation of 325 degree–days; a second major peak of third–stage larvae from spring–deposited eggs occurs after an accumulation of 575 degree–days.

As you begin to scout alfalfa fields for alfalfa weevils, look first in areas of the field that might warm up early, such as south–facing slopes and areas of the field with lighter soils. After 300 degree–days have accumulated, you should be able to find small, first–instar weevils in the folded terminal leaves. As these small, yellowish larvae with black heads feed on these leaves, you will observe some 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.

If you want to search for alfalfa weevils before signs of leaf injury begin to appear, you can scratch around the crowns of the plants to look for the adults that overwintered. Alfalfa weevil adults are oval, light brown with a darker brown stripe along the middle of the back, and about 3/16 inch long. Like most other weevils, alfalfa weevil adults have an extended "snout," at the end of which are their mouthparts. You can also look for alfalfa weevil eggs in stems; however, you'll have to have the hands of a surgeon to split the small stems and excellent eyesight to see the small eggs. The eggs are only 1/32 inch long, oval, and initially light yellow. Just before hatching, the eggs darken, and you can see the developing larvae through the "shell."

As the season gets underway, we will rely greatly on the reports we receive from people who are roaming the countryside and keeping a watch on the health of our crops. Please keep us in mind if you see something that would benefit others if they knew about it early enough. We'll begin providing degree–day accumulations for alfalfa weevil development soon, but let us know if you observe any activity.--KS
Author: Kevin Steffey