Farmers should watch their fields after the first cutting for delayed or lack of green up due to activity of alfalfa weevils (both larvae and adults). Reports from northeastern and northwestern Iowa indicate that this pest is causing problems this year in alfalfa stubble. Heavy populations of weevil adults and surviving populations of larvae can delay new growth by feeding on the stubble and new buds as they break. This feeding may reduce yields and forage quality in the second and possibly third cuttings.
What do alfalfa weevil larvae look like?
Alfalfa weevil larvae have a dark head that is almost black and are pale green with a white stripe down the back. The young larvae are about 1/16-inch long and may be light yellow in color. After feeding for several days, they turn green. They are 5/16-inch long when full grown.
What do alfalfa weevil adults look like?
The adult weevils are light brown with a dark brown strip down the back that tapers to a narrow point. They are 1/4-inch long and have a narrow snout.
How do I scout for larvae and adults in alfalfa stubble?
Start monitoring regrowth 4 to 5 days after the first cutting has been removed from the field. Check 20 1-square-foot areas in the field. Look for larvae and adults on the soil surface and around the alfalfa crowns.
Alfalfa weevil larva and damage to alfalfa leaf. (Marlin E. Rice)
Alfalfa weevil adult. (Marlin E. Rice)
What are the treatment thresholds?
Alfalfa weevil larvae usually cause economic damage only on the first cutting of hay. As a result, little information is available on treatment thresholds for the alfalfa stubble. Kansas and Minnesota recommend that growers consider treating fields when 4 to 8 larvae per square foot are present and regrowth is being delayed. In addition, Kansas recommends that if adult weevils have scraped the outer tissue from the stems and damage is widespread, treatment should be considered.
For more information on insecticides labeled for alfalfa weevil, refer to the April 11, 2005, ICM Newsletter article Alfalfa weevil: Scouting and economic thresholds. Please read the product label carefully because some products have different rates of application for larvae and adults.
Are there other insects that prevent regrowth?
Other alfalfa pests that may also prevent regrowth include clover leaf weevils and variegated cutworms. Clover leaf weevil larvae are much larger than alfalfa weevil larvae and have a light brown head. Often clover leaf weevil larvae have a white stripe edged with pink down the back. Variegated cutworms vary in color, ranging from tan to greenish-yellow to almost black. A row of small yellow, dagger or diamond-shaped spots occurs down the center of the back. An orange stripe appears along each side.
Clover leaf weevil larva. (Marlin E. Rice)
Variegated cutworm. (Marlin E. Rice)
This article originally appeared on pages Page 2-3 of the IC-494(14) -- June 13, 2005 issue.