• By Adam Probst, Extension Agent

Be ready for the alfalfa weevil

Source: Dr. Lee Townsend; UK Extension Entomologist

Alfalfa weevil is the key pest of the first cutting. Populations have been above normal over much of the state during the past two years so it is important to be watchful this spring. High populations may last for two to three years before natural enemies, diseases, and climatic factors begin to take their toll.

Temperature drives insect development so they may appear early or late, depending on how the spring unfolds. Fortunately, an alfalfa weevil degree-day model can indicate when to start checking fields for tip feeding, the signature damage of this key crop pest.

The table here shows the variation in degree-day accumulation that can occur in consecutive years, along with the predicted values for 2018.

The accumulation of 190 degree-days (base 48 degrees F) signals the time when early tip damage can appear in fields. Check degree-day accumulations for your area at the U.K. Ag Weather website, http://wwwagwx.ca.uky.edu/dd.php.

The second critical time to check for damage should occur when 225 degree-days have accumulated. At this time, spring-laid eggs should have begun to hatch. Pay particular attention to fields that had significant weevil damage last spring.

Just looking for feeding holes or using a sweep net will indicate the presence of weevil larvae in fields, but stem sampling will give a more accurate assessment of the situation. Using degree-day accumulations, average alfalfa stem length, and numbers of alfalfa weevil grubs per 30 tips is a reliable way to assess the need for treatment. A weevil scouting procedure is outlined in Alfalfa Weevil Sampling Program (EntFact 127). If alfalfa weevil numbers exceed treatment guidelines early in the season, application of the high label rate is recommended to provide longer residual control. Many insecticide options exist for alfalfa weevil control; however, some products should not be applied to mixed stands of alfalfa and grass.

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