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Pest Alert: Alfalfa Weevil and Leafhopper Numbers High

Alfalfa weevil and potato leafhopper (PLH) are being reported as extremely high in many fields.  Hot dry conditions have made it more favourable for these pests than their natural enemies (especially entomopathogenic fungi). Scout fields to determine if cutting is possible or if an insecticide application is necessary if thresholds are reached. Consider fields at threshold if both pests are present but populations are slightly below threshold for either pest.

Monitoring recently cut fields is extremely important as well, as infestations can overwhelm the young regrowth. Dry bean growers should also keep watch of their newly emerging dry bean plants for PLH as seed treatments will only protect the plants for a few weeks after planting.

Alfalfa weevil larvae 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 at greater risk of alfalfa weevil include those seeded last year, pure alfalfa stands, and areas of the field with south facing slopes.

Alfalfa weevil scouting involves using a pail and tape measure. 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.  Shake each plant against the side of the pail 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.

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