Alfalfa Damage
- Alfalfa weevil can significantly damage the foliage of alfalfa
- Use scouting information to determine if an insecticide or early harvest is needed as temperatures increase
Alfalfa weevils are an important pest affecting alfalfa production in many areas where the crop is grown. This article discusses alfalfa weevil identification, scouting, and management.
Identification. Lifecycle, and Damage
Alfalfa weevils are small (1/4 in), brown beetles with an elongated snout and readily fall to the ground when disturbed. For those of us old enough to remember the Muppets, one may think weevils somewhat resemble Gonzo with their bent snout. A characteristic dark strip down the center of the back helps to distinguish this pest from other weevils that may be present in the field.
After eggs hatch, small, light green larvae emerge, which have pronounced black heads and white stripes down the center and sides (Figure 1). These larvae are similar to the larvae of the less problematic Clover leaf weevil. The difference between the two is that clover leaf weevil contains red smudges that boarder the central stripe. When mature and approximately 3/8 in length, larvae produce a white cocoon on foliage from which they will emerge as an adult weevil.
Figure 1. Alfalfa weevil damage can severely impact quality and yield. Image D. Owens, U. Delaware
Scouting and Treatment Thresholds
Alfalfa fields in warmer regions and regions where feeding is predicted to begin should be monitored closely after egg hatch. Egg hatch is predicted in the Southern portion of Illinois within 2 weeks (https://www.fssystem.com/Admin/Resources-Library/av/2000/id/467).
Insects are cold blooded, and their growth and development are driven by degree day accumulation, similar to corn. Consequently, we can use knowledge of the GDD required for insects to estimate the approximate stage of development. For weevils, we are interested in knowing when eggs have hatched, and larvae are active. Some states have insect degree day calculators that can be used to help with scouting efforts. Illinois is one great example. If you live in Illinois go to https://data.prairie.illinois.edu/warm_pdd/default.aspx and select the commodity crop, alfalfa weevil pest degree calendar for your location or the site closest to your area.
Scouting: Scout fields by making at least 20 stops across a field, using a “U” shaped path. At each stop, break off three stems by using the thumb and forefinger grip the stem (held horizontally) while allowing the middle finger to break the stem, then shake the stem into a bucket. Count the larvae and determine the average number of larvae per stem.
Management: The following are guideline thresholds for making an insecticide application: if stems are 3-7 in tall, an average of 2 larvae per plant; 8-14 inches tall, 4 or more larvae per stem; late season infestations-within 2 weeks of harvest early cutting is advisable. Ensure that label directions are followed and avoid application of insecticides during bloom to avoid contacting honeybees and other pollinators. Regrowth should be carefully monitored for continued weevil activity to determine if stubble sprays are required. If at 50% or more of regrowing crowns show signs of weevil damage and regrowth has been prevented for at least 3 days, an insecticide may be warranted. There are many effective insecticides labeled for this pest. Contact your local crop specialist for more information.