E-38
field crops insects
Department of Entomology
THE ALFALFA WEEVIL
C. Richard Edwards and David L. Matthew, Extension Entomologists
TIMING OF SPRAYS
Timing of sprays is the most critical factor in controlling the alfalfa weevil. It is based upon accumulated units of temperature, called heat units. Weevil development increases with higher temperatures. The occurrence of peak populations of weevils varies over the State from year to year and from south to north depending upon the season. Thus, the economic threshold will change during the season because of temperature, weevil infestation level, and alfalfa growth.
WEEVIL HABITS AND DAMAGE
Damage to alfalfa begins when growth starts in the spring and is caused primarily by the larval stage of the weevil. Young larvae are tiny yellow-green worms. Older larvae are green with a white stripe running down the back. They grow to about 1/4 inch long. All stages of the larvae have curved bodies and shiny black heads. The larvae feed first in the growing tips and developing buds, then shred the foliage, giving infested fields a gray appearance. This feeding may cause a significant loss of yield and reduce the quality of the first cutting.
After feeding for 3 or 4 weeks, each larva spins a net-like cocoon on the plant or in soil debris. From this cocoon emerges the adult alfalfa weevil. The adult weevil is about 1/4
inch long and light brown in color with a dark brown stripe down the middle of its back.
Many larvae may be present when the first cutting of alfalfa is made. These larvae, along with the newly-emerged adults, continue to feed on both the stubble and new growth, thus retarding development of the second cutting. The adults become inactive during the summer. In the fall, they become active once again and begin laying eggs.
HELPFUL MANAGEMENT PRACTICES
1.	Check with your county Extension agent for information on when fields should be evaluated. Your agent has up-to-date heat unit information .
2.	Apply management practices that will result in a dense, vigorous crop of alfalfa.
3.	Make the first cutting in the bud stage; delayed cutting allows more damage to take place. Alfalfa has a higher protein content and greater nutritive value in this stage of growth.
4.	Cut the crop clean and close
and remove it from the field as soon as possible. Lack of food and exposure to hot sunlight kill many of the remaining larvae.	j
5.	Return to the field in 4 to 5 days after cutting to determine if weevils are still present and actively feeding.
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA 47907