Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
If you haven’t been scouting your alfalfa for Alfalfa Weevil, now is the time!
To sample alfalfa for alfalfa weevils, examine 10 random stems per field on a weekly basis until first cutting, noting the presence of alfalfa weevil larvae and feeding injury. If damage or larvae are found, a full stem sampling should be conducted by randomly collecting 30 stems throughout the field. Once the stems have been collected, separate them into 3-4 bundles and beat them against the inside of a bucket to dislodge larvae from the stems. Count and record the total number of larvae found from the 30 stems. Measure the length of the stems to determine the average stem height and note the percentage of plant in the bud or flower stage.
Here is a short Youtube video demonstrating how to do a “full stem” sampling:
https://drive.google.com/file/d/0B76Sjy23j-ZTVHBiMzRzbWNOeEU/view?usp=sharing
For more information refer to our Extension Fact Sheet for thresholds and control decisions:
http://extension.udel.edu/factsheets/alfalfa-weevil-control-in-alfalfa-2/
If alfalfa is in the full-bud stage and economic levels are present, early harvest is an option if harvest is possible within 3 days and populations are increasing. If cutting early versus spraying, be sure to check fields within one week for damage to re-growth. Re-growth can be significantly damaged from alfalfa weevils and in some cases; a stubble treatment may be needed if you find 2 or more weevils per stem. If cutting early is not an option, please refer to our Insect Control in Alfalfa Recommendations for chemical control options found at:
https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063238/Insect-Control-in-Alfalfa-final-for-2017.pdf