2005 Year in Review

From the Fields – Alfalfa Weevil

Alfalfa weevil is the first major insect pest that alfalfa producers face in the spring. This defoliator of alfalfa was first discovered in the US in Utah in 1904. The pest overwinters in Illinois and becomes active in northern Illinois in late April-early May. The insect’s activity is related to the amount of heat that has accumulated from a given point in time, typically January 1.

The adult (beetle) alfalfa weevil lays eggs inside the stems of alfalfa in the early spring and perhaps during relatively warm days in the winter.

Eggs hatch when 300-degree days accumulate from January 1 above a base temperature of 48 degrees F. As of May 1, degree-day accumulations for Freeport, DeKalb, and Monmouth were 263, 259, and 322, respectively. A projected degree-day total through May 8 for the same locations is 334, 336, and 403, respectively. As degree-day accumulations near 400, larvae leaf-tip feeding may become severe. Degree-days can be viewed and determined at the University of Illinois IPM Degree-Day Calculator web site, IPM.

The larva (worm) has a yellowish-green body with a white stripe along the middle of the back, and a black head. A fainter white stripe is also present on each side of the prominent central stripe. Larvae are approximately 3/8 inch in length when mature. Their presence and damage will often appear first on south facing slopes or in fields protected by timber or wooded areas.

Initially the larvae can be found within the folded terminal leaves of the alfalfa plant. Evidence of their feeding is pinholes (small round holes) in the tip leaves. This damage is not economic, but it’s an indication of their presence. As the larvae increase in size (result of increased heat units) the leaves are skeletonized. Damaged leaves dry very quickly giving the field a grayish to whitish cast, similar to frost.

An insecticide treatment may be warranted when 25 to 50 percent of tips are being skeletonized and there are three or more larvae per stem. Be aware of the harvest interval or the number of days required between insecticide application and harvest. Do not apply insecticides when alfalfa is in full bloom; instead, cut and remove the hay. A treatment threshold has also been established based on the number of larvae per stem at different plant heights and values of alfalfa hay.

Early harvesting is a very effective tool for the management of the alfalfa weevil. Weevil numbers can also be reduced by natural parasitoids and fungal diseases. Monitoring alfalfa fields is the only way to know what’s happening with this pest.

Scouting procedures and control recommendations are available at Extension offices. A University of Illinois fact sheet on alfalfa weevil can be viewed at IPM

by  Editor, theCity1.com
May 6, 2005