Scout Alfalfa for Alfalfa Weevil and Pea Aphids

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Be sure to sample alfalfa fields for small alfalfa weevil larvae feeding in the tips of plants. Early damage will appear as a round, pinhole type of feeding. Once you detect tip feeding, a full field sample should be taken. You will want to avoid treating fields too early since it may result in multiple applications. Also, be sure that you do not confuse clover leaf and alfalfa weevil larvae. Cloverleaf weevils are generally larger at this time of year and have a distinct white stripe lined with red down the middle of their backs. Although cloverleaf weevils can cause damage during cool, dry springs, controls are generally not needed for cloverleaf weevils. For pictures of cloverleaf weevil and alfalfa weevil, please refer to the following links:

http://www.ipm.iastate.edu/ipm/icm/1997/4-21-1997/icloverweevil.html
http://www.ent.iastate.edu/imagegal/coleoptera/curculionidae/0212.47alfalfalarva6in.html

You will also want to sample fields for pea aphids. Heavily infested plants may turn yellow and wilt. Pea aphids prefer cool, dry conditions and can be a problem in both the first cutting and during spring seedling establishment. This species tends to congregate on the tips of alfalfa plants where they feed on young, succulent developing shoots. To sample for aphids, clip alfalfa stems at the base of the plant and record the number present per plant. You may want to examine plants over a white bucket to collect any aphids that are dislodged from the plants. In seedling stage alfalfa, a treatment should be considered if you find 5 aphids per stem. As a general guideline, you should consider a treatment in alfalfa less than 10 inches tall if you find 40-50 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75-100 per stem.

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