Alfalfa Weevil Activity

Scouts are starting to notice 1st and 2nd instar alfalfa weevil larvae in their sweep samples. That tells us that we will have the larger sized (and hungrier) 3rd and 4th instars larvae in a matter of days to weeks (weather dependant). We could get lucky with this wet weather and see a fungal pathogen start to kill these larvae but we can't count on it. Fields should be scouted twice a week for the next month to stay ahead of any injury. Larval activity tends to start first on the shallow soils or on southerly slopes of the fields. Peak activity usually coincides with the bud stage of the first crop.

To scout for larvae, collect 30 stems in a large M shaped pattern across the field. Place them inside a white pail and beat the plants against the side of the pail to knock off the larvae. Check the plants for any remaining larvae that did not drop into the pail before tossing the plants aside. Also make note of any leaf tip feeding on these plants. Leaf-tip feeding refers to the percent of plant tips showing obvious signs of damage, which is not to be confused with the percent defoliation.

Figure 1. Third and Fourth Instar Alfalfa Weevil Larvae.
(T. Baute, OMAFRA)

The first and second instar larvae that fell into the pail are not as important and should not be counted. They are the larvae that are only 3 mm or less in size and are still pale yellow to light green, with not much of a white stripe to see. Once they are in the third and forth instars, they are bigger in size (up to 8 mm) and are a pale to bright green, with a black head (Figure 1). Once they are this big you can easily see the white stripe down the back. These are the ones that you do want to count in the pale. Only count those 3rd and 4th instar larvae that appear healthy and are actively moving around. Those that are infected with fungus will be slow moving, yellow or tan and should not be counted.

If there is 40% leaftip feeding, with 2 or 3 active weevils per stem, and there is more than 7-10 days to preferred harvest date, consider applying an insecticide. Less than 1 active larva per stem does not require action, but continue to monitor the situation. Two larvae per stem requires action if the alfalfa is less than 40 cm (16 in.) high. If there are more than 3 active larvae per stem, immediate action is required.

Cutting early is the recommended management strategy. Insecticides are only recommended when cutting is impractical, i.e. when the alfalfa is in the pre-bud stage. For a list of recommended products, refer to the OMAFRA publication 812, Field Crop Protection Guide at: http://www.omafra.gov.on.ca/english/crops/pub812/4weevil.htm