INFO-BUG

Crop Selected: Alfalfa
Crop Development: Second cutting [early summer]
Pest Selected: Alfalfa Weevil (AW) Hypera postica

Biology

Alfalfa weevil is an annual pest of alfalfa which appears in thespring and feeds primarily on the growth of first cutting.

In the spring, as temperatures exceed 48 deg. F., overwintering adults become active, lay eggs, and a single generation of larvae pass though four larval stages. The 3rd and 4th larval stages which exceed 1/4 inch in length may cause heavy foliar injury, which sometimes warrants rescue treatment.

A complex of beneficial parasites and predators maintain the weevil populations at sub-economic levels approximately 90% of the time eliminating the need for periodic rescue treatment.

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Alfalfa weevil damage Alfalfa weevil larva

Assessment

To assess weevil larvae abundance, collect 30 alfalfa stems and place top down in a bucket. Then shake stems vigorously into bucket and count number of larvae present in bucket. Determine the average number of weevil larvae per stem and evaluate the need for action based on the following rules.

If less than 1 larva per stem
then no action is warranted.

If more than 3 larvae per stem
then corrective action is needed.

If more than 1 but less than 3 larvae per stem
then action may be needed depending on stand
height, value of stand, or other relavent factors.

Control

When weevils are abundant and stand height exceeds 16 inches early harvest is preferred over spraying to preserve beneficial parasites, which generally maintain the weevil at low levels.

If treatment is warranted, the following are recommended:

Ambush* 2E @ 0.1 lb a.i./A
Baythroid* 2 @ 1.6 to 2.8 fl/oz/a
Furadan* 4F @ 1/2 to 1 pt/A
Guthion* Solupak 50WP @ 3/4 to 1 lb/A
Imidan* 70-W @ 1.4 lb in 10-50 gal/A
Lannate* SP @ 1 lb/a and LV @ 3 pt/A
Lorsban* 4E @ 1 to 2 pt/A
Malathion 57EC @ 1 1/2 to 2 1/4 pt/A, 5 @ 1 1/2 to 2 pt/A & ULV @ 16 oz/A
Mustang* @ 2.4 to 4.3 oz/A
Pounce* 3.2 EC @ 0.1 lb a.i./A
Sevin 80S @ 1-7/8 lbs/A
Warrior* 1CS @ 2.56 to 3.84 fl oz/A

* Use is restricted to certified applicators

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