Alfalfa Weevils Attacking Alfalfa Regrowth in the Panhandle - UNL CropWatch, June 21, 2013

Alfalfa Weevils Attacking Alfalfa Regrowth in the Panhandle - UNL CropWatch, June 21, 2013

alfalfa weevil damage   alfalfa weevil damage
Figure 1. Regrowth of first cutting alfalfa was severely curtailed in this field due to alfalfa weevil larvae feeding. (Photos 1-3 by John Thomas)

June 21, 2013

alfalfa weevil larvae

 Figure 2. Alfalfa weevil larvae

alfalfa weevil pupae

Figure 3. Alfalfa weevil pupae

Figure 4. Alfalfa weevil adult.

 

Be watching your first cutting alfalfa and the stubble after first cutting for delayed green-up. Serious infestations of alfalfa weevil have been found in several areas of the Nebraska Panhandle and regrowth may be at serious risk.

Damage in the early growth stage of alfalfa in April and May will appear as small holes eaten in the leaves. Often it may not be noticeable. As the larvae grow and continue feeding, seriously damaged fields take on a white or gray appearance as the heavily skeletonized leaves dry out. Generally, larvae damage the first cutting and adults damage regrowth as the next generation emerges. Often harvesting the first cutting takes care of the weevil by killing the larval stage. In fields with extremely high populations of larvae this may not be the case. Larval survival can be large enough to hinder or stop regrowth.

This month alfalfa growers have noticed damage in alfalfa at first cutting as well as an abundance of weevil larvae as they cut the alfalfa. Several growers have noted that the alfalfa was not starting regrowth after harvest (Figure 1). Upon closer inspection, it was noted that the crowns of the alfalfa stubble were loaded with larvae with counts ranging from 15 to 20 larvae per square foot (Figure 2). Additionally, weevil pupae were easily seen in the fields (Figure 3).

Although threshold calculators exist (see CW article), another threshold developed by Colorado State University indicates that alfalfa stubble should be treated if an average of eight or more larvae or young adults per square foot are present in the field (Figure 4) or more than 50% of the new growth is damaged. Pre-harvest treatment thresholds for first cutting alfalfa are if 30% to 50% of the terminals are damaged or 1.5 to 2 larvae are found per stem.

If the thresholds for treating alfalfa weevil are reached, apply a chemical treatment to protect your crop. Check with your chemical supplier to find the best product and be sure to check the pre-harvest interval for treatment. Always follow label instructions.

See more on the alfalfa weevil life cycle, treatment thresholds and management in this May 7 CW article.

John Thomas
Extension Educator, Box Butte County
Jeff Bradshaw
Extension Entomologist, Panhandle REC, Scottsbluff