Alfalfa Weevil Activity

Tracey Baute - Field Crop Entomologist/OMAFRA Ridgetown
23 May 2008

Surprisingly, despite the cold weather, reports are coming in of alfalfa weevil feeding in alfalfa fields in some regions. Alfalfa weevil damage will show up initially as pinholes, progressing to skeletonization of the leaves at the top of the plant. Fields at high risk include those seeded last year, pure alfalfa stands, and those with south facing slopes. Peak larval attack usually coincides with the bud stage of the first crop. Several areas of each field should be scouted twice a week for the next month or so. To scout for larvae, collect 30 stems in an M shaped pattern in the field. Place the plants inside a white pail and beat them against the side to knock off the larvae. First and second instar larvae are small (3 mm or less and should not be included in your count. Only count the larger 3rd and 4th instar larvae. Also measure the height of the crop. Thresholds for alfalfa weevil are when 40% of stems show damage and more than one larva per affected stem can be found when the alfalfa is 30cm (12") tall and more than 2 larvae per affected stem when alfalfa is 40cm (16") tall. Insecticide treatment is only recommended when cutting is impractical such as when the alfalfa is in the pre-bud stage. For information refer to the OMAFRA Forage Website.

Figure 1. Alfalfa Weevil Larva. Photo credit University of Nebraska