Start monitoring alfalfa fields soon | Business

Spring is coming and with the arrival of warmer temperatures, the number one enemy of the alfalfa producer, the alfalfa weevil.

Alfalfa weevil is almost always a first cut pest of alfalfa in our area, but how do you determine when they appear and if they are at levels that warrant treatment or harvesting?

University of Kentucky entomologist Dr. Ric Bessin recently shared the following information on alfalfa weevil detection and control.

The UK Ag Weather Center degree-day model for alfalfa weevil indicates that many counties in Kentucky will likely exceed 190 DD (used as a starting point to begin scouting) by the third week of March. Once temperature accumulations reach 190 DD, growers are advised to survey their alfalfa fields and begin counting alfalfa weevil larvae. So far, the degree-day accumulations for this year tend towards the average of the last 10 years.

Alfalfa weevil eggs laid in the fall are the first to hatch in the spring. These eggs hatch earlier than those laid in the spring, and 190 DD approaches the time when the first leaf feeding damage becomes noticeable. Extreme temperatures during the winter contribute to limiting the survival of alfalfa weevil eggs that were laid in the stems in the fall. Damage from young larvae will first show up as tiny pinholes in the leaves.

To locate the alfalfa weevil, use the stem sampling method. While walking in a “U” or “Z” in a field, collect 30 stalks of alfalfa. Carefully cut off the top of each stem in one hand and break it off the crown with your other hand; place the bud down in a plastic bucket. Make sure your samples are at least 20 feet from the edge of a field so that they are representative of the entire interior of a field. Knock the stems in groups of four or five stems at a time against the inside of the bucket to dislodge the larvae. Count the number of larvae. Measure the length of 10 stalks of alfalfa at random. If the stem height is two inches, more than 27 weevils warrant treatment. If the height of the stems is six inches, this threshold increases to 100 weevils. If the field is close to harvest, harvesting may be an alternative to spraying, but growers should monitor regrowth damage.

If the field warrants treatment, apply a long residual insecticide that is labeled to control alfalfa weevil in alfalfa.

Also keep in mind that insecticide resistance has been a problem in some areas. The best strategy for managing resistance is to use an insecticide only when needed and to alternate modes of action each year. For many other pests, you would rotate insecticides more often, but the alfalfa weevil only has one generation per year.

To alternate modes of action, select insecticides that have a different IRAC group number on the label.

For more information on alfalfa weevil scouting and control, contact the Hardin County Extension Office at 270-765-4121.

Matt Adams is a Hardin County Agriculture and Natural Resources Extension Officer. He can be reached at 270-765-4121 or [email protected]

Matt Adams is a Hardin County Agriculture and Natural Resources Extension Officer. He can be reached at 270-765-4121 or [email protected]