- Alfalfa weevil can significantly damage the foliage of alfalfa
- Fields in warmer areas should be scouted for Alfalfa weevils as soon as possible
- Use scouting information to determine if an insecticide or early harvest is needed.
Alfalfa weevils are an important pest affecting alfalfa production in many parts of the Midwest. Occasionally severe infestations cause severe economic damage to the crop. Feeding has started in parts of Southern Illinois and Iowa, which may put some people into a situation where feeding injury could pose a problem. 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. 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 larvae of different developmental stages. Note the black heads and bright yellow/white stripe. Image credit: R. Villuneva UKY.
Both the larvae and adults feed on alfalfa foliage, with larvae favoring feeding on the terminal and upper leaves, eventually skeletonizing leaves, and adults often feeding at leaf margins. If eggs were laid at significant numbers in the fall, the first cutting of alfalfa in warmer areas and in delayed harvested acres in cooler climates may be affected.
Scouting and treatment thresholds
Alfalfa fields in southern parts of Illinois and Iowa and Kentucky should be monitored closely as recent models indicate feeding is likely to begin soon or may have already started in some areas. To determine where weevil activity and development are in your part of 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. As you can see, areas in the southern part of Illinois are likely approaching 3rd instar. Below is an example from Rend Lake, Illinois:
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. 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.