Alfalfa Weevil Warning

Alfalfa weevil numbers have been high in some fields. Scout to determine if management is necessary. Cutting is the best control method but if the crop is not ready to be cut, spray may be necessary. If the crop has already been cut, continue to scout to ensure the larvae do not take out the re-growth.

Several areas of each field should be scouted twice a week over the next month. Peak larval activity usually coincides with the bud stage of the first crop. To count larvae, collect 30 stems in an M shaped pattern. Place them inside a white pail and beat them against the side to knock off the third to fourth stage instar larvae. Larvae are pale to bright green, with a black head and white stripe down the centre of the back (Fig. 1). First and second instars are smaller (3 mm or less), pale yellow to light green, with the white stripe not yet distinguishable. They may be in the upper leaves, but do not include these younger larvae in the count.

Figure 1. Alfalfa weevil larva. (photo credit: University of Nebraska)

Threshold

If there is 40 percent leaf tip feeding, with 2 or 3 active larvae per stem, and there is more than 7 to 10 days to preferred harvest date, consider applying an insecticide. 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.

No Action is Required if there is:

• less than 1 active larva per stem - but continue to monitor the situation.

Action Must be Taken if there are:

• more than one larvae per stem AND alfalfa is less than 30 cm (12 in.) tall
• Two or more larvae per stem AND alfalfa is less than 40 cm (16 in.) tall
• More than 3 active larvae per stem at any plant height.

Management

If cutting is an option, it is preferable over insecticide application for a number of reasons, including the cost of spraying and damage to beneficial parasitic wasps and honeybees. Cutting removes large numbers of these larvae from the field. Remaining larvae usually dry out, starve and are exposed to natural enemies. Cutting before the bud stage can result in extensive damage to second cut regrowth and lower yield. Insecticides are recommended only when cutting is impractical, such as when the alfalfa is in the pre-bud stage.

Table 1. Recommended registered insecticides for alfalfa weevil control.
(Source: OMAFRA Publication 812, Field Crop Protection Guide 2007-2008)

Refer to product labels for re-entry periods and other restrictions. Local beekeepers should be contacted so they have an opportunity to move colonies out of the danger area.

Second Cut

Occasionally, if weevil populations are high on a late first cut, surviving larvae will feed on the re-growth. Such feeding can eliminate alfalfa re-growth which may lead to a loss of the stand. The presence of 2 or more active larvae per crown, or 4 to 8 larvae per square foot indicates a need to spray the stubble with insecticide.

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