Look out for Alfalfa
Weevil Activity

It is a bit repetitive to say this but with the mild winter and summer-like temperatures this spring, insects are very active. This includes alfalfa weevil which could be starting up in alfalfa fields shortly, if they haven't already.

Alfalfa weevils overwinter as adults at the soil surface around the crown of the alfalfa plant. As spring temperatures warm up, these adults begin creating cavities in the stem of the alfalfa plant to lay approx 40 eggs per cavity. Though the adults can do some feeding it is the larvae that come from these eggs that do the majority of the damage. Unfortunately after a mild winter, larval feeding can happen even sooner than normal. Some adults will lay eggs in the fall but after a "normal" winter, the eggs usually do not survive. A mild winter increases the chance that the eggs do survive which would start alfalfa weevil off in an even more advanced stage of development. Survival of overwintering eggs could mean that larvae begin actively feeding much earlier than expected. And the warmer the days get, the quicker their development is further sped up.

It is therefore important to get out and start scouting to see what is going on in your fields. Fields most at risk of alfalfa weevil activity include those seeded last year, pure alfalfa stands, and those with south facing slopes, though all fields should be scouted to verify weevil activity. Peak larval activity usually coincides with the bud stage of the first crop so if there are fields that are nearing that stage already, scout those first.

Start scouting twice a week for at least the next month to 6 weeks. To count larvae, collect 30 stems in an M shaped pattern of the field. Place them inside a white pail and beat them against the side to knock off the larvae. Larvae (Fig. 1) are pale to bright green, with a black head and white stripe down the centre of the back. First and second instars are smaller (3 mm or less), pale yellow to light green, with the white stripe not yet distinguishable but do not include these smaller younger larvae in the count. Only count 3rd and 4th instar larvae. Measure the crop at 5 areas at least to get an average height.

Thresholds are as follows:
" Less than 1 active larva per stem does not require action, but continue to monitor the situation.
" If the alfalfa is less than 30 cm (12 in.) high with more than one larva per stem, action is required.
" Two larvae per stem require action if the alfalfa is less than 40 cm (16 in.) high.
" If there are more than 3 active larvae immediate action is required.

Insecticides are only recommended when cutting is impractical, such as when the alfalfa is in the pre-bud stage or when weather conditions do not favour cutting. Cutting can remove large numbers of these larvae from the field. Remaining larvae usually dry out, starve and are exposed to natural enemies. Unfortunately insecticide sprays will kill these beneficial insects so it is critical that spraying take place only once thresholds have been reached.

Occasionally, if weevil populations are high during the 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. With a severe infestation, be sure to monitor stubble re-growth.

For information on insecticide options, refer to the OMAFRA publication 812, Field Crop Protection Guide now available online at: http://www.omafra.gov.on.ca/english/crops/pub812/p812toc.html

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