By Tracey Baute, OMAFRA Field Crops Entomologist
Some field scouts are finding alfalfa weevil above threshold in Niagara and Haldimand counties, though there are likely more fields in other counties at levels of concern. Alfalfa weevil activity usually coincides with first cut but in some years when the crop is slower to develop, the weevil damage gets ahead, causing significant damage before first cut can happen.
Alfalfa weevil larvae are pale to bright green, with a black head and white stripe down the centre of the back. They initially feed within the leaf buds and then move to the tips of the plant. Pinholes are the first signs of damage but can quickly progress to skeletonized leaves. Heavily damaged fields will look greyish white from a distance. Heavy feeding not only impacts yield but also reduces feed quality.
Fields should be scouted twice a week. Fields at greater risk and therefore should be scouted first include those seeded last year, pure alfalfa stands, and areas of the field with south facing slopes.
While walking an M shaped pattern across the field, collect 30 stems by cutting or breaking them off at ground level. Measure the height of the plants. Then using a white pail, shake each plant against the side to knock off the larvae. Count only the 3rd and 4th instar larvae. Smaller 1st and 2nd instar larvae are pale yellow and lack the white stripe along their bodies. Don’t count larvae that are slow moving and are tan or yellow in colour, they are infected by a fungus or parasitoids and will die shortly.
No action required:
< 1 active larvae per stem (but continue to monitor)
> 1 active larva per stem AND alfalfa < 30 cm (12 in) tall
2 or more active larvae per stem AND alfalfa < 40 cm (16 in) tall
> 3 active larvae at any height.
Cultural Control: If the timing is right and the crop is ready, cutting the crop is the preferred and effective method of control, avoiding the costs of spraying and risk to pollinators and natural enemies. Cutting removes large numbers of larvae while larvae left in the field usually dry out and starve. That said, in heavy infestations, larvae can stick around and cause damage to the early growth of the second crop so scouting is recommended again after first cut.
Cutting before the bud stage, however, can result in extensive damage to second cut regrowth and lower yield. If the crop is not ready to cut, insecticides are the recommended control option.
Insecticides: Registered products are listed here. Spray in the evenings to avoid harming foraging bees and contact local beekeepers prior to spraying so that they can move or protect the hives during the spray. Pay close attention to the days to harvest intervals when selecting and using these products.