Scouting and Managing Alfalfa Weevil

Each spring brings with it the excitement of a new growing season. For alfalfa producers, the new growing season starts as their alfalfa fields break dormancy and begin to grow. The alfalfa weevil is an occasional, but important pest of alfalfa. It becomes active shortly after alfalfa greens-up.

The alfalfa weevil can be found throughout the United States. Adult alfalfa weevil beetles are about ¼ inch in length, light brown in color, and have a broad, dark brown stripe that extends down the center of their backs. The adults also have an elongated snout, which is a common characteristic of weevils.  Although the adults feed on alfalfa foliage, it is the larval stage that is considered the economic pest.

Adult female alfalfa weevils chew holes and insert 2 to 25 eggs inside alfalfa stems4 and lay between 800 to 4,000 eggs during their lifetime.2 Eggs can be laid in the fall, winter, or spring, depending on latitude.1

Alfalfa weevil eggs laid during the fall and winter months develop and hatch after 200 to 300 growing degree days (GDD) have accumulated since January 1st (Base 48°F).2,4  Peak hatch for alfalfa weevil eggs laid during the spring occurs after 575 GDD have accumulated.4 A full-grown alfalfa weevil larva is about 3/8 inch in length, has a black head capsule, and a yellow-green to green body with a white stripe running down its back.

Alfalfa weevil larvae are defoliators as they skeletonize alfalfa leaves by eating everything except the veins (Figure 1). High populations of alfalfa weevil larvae can cause severe damage to an alfalfa field leaving it with a silver or frosted appearance.2 This damage can reduce forage tonnage, quality, and weaken the alfalfa stand.2,3

Skeletonized alfalfa leaves from alfalfa weevil. Figure 1. Skeletonized alfalfa leaves from alfalfa weevil.

Depending on location, it is recommended to start scouting for the larvae after 200 to 250 GDD have accumulated by first confirming their presence using a sweep net.2 When the larvae are found, collect 5 alfalfa stems from each of 6 locations in the alfalfa field, vigorously shake them in a pail to dislodge the larvae, and then inspect the alfalfa stems afterwards to check for any remaining larvae.2 Average the number of larvae collected per 30 stems and use Table 1 to determine if the economic threshold has been met.

If the economic threshold is met, an insecticide application can be used to manage the alfalfa weevil infestation. However, if the economic threshold is met near the time the alfalfa needs to be harvested, rather than spraying an insecticide, cutting the alfalfa early can be an effective way to manage the alfalfa weevil larvae and help maintain forage quality. This action eliminates the need to delay harvesting the alfalfa while waiting for the pre-harvest interval for an insecticide to expire.

Alfalfa weevils are mainly a threat to the first cutting of alfalfa because depending on air temperature, it takes approximately 8 weeks from the time the larvae hatch to the time the adults emerge from pupation.2 The newly emerged adults feed on alfalfa for a short period of time, then go dormant during the warm summer months.3 As a result, there is usually only one generation per year.

If you have any questions regarding identifying or scouting for alfalfa weevil, or any other agronomic questions, please contact your Channel Seedsman.

Alfalfa weevil economic thresholds. Table 1. Alfalfa weevil economic thresholds. Treatment cost per acre includes insecticide and application costs. (Contents of table courtesy of and use of granted by Dr. John Tooker, Alfalfa Weevil, Insect Advice from Extension, The Pennsylvania State University).
References:

1Degooyer, T. A. 1993. Phenology and population dynamics of the alfalfa weevil, Hypera postica (Coleoptera: Curculionidae), in central and southern Iowa. Retrospective Theses and Dissertations. Iowa State University. https://lib.dr.iastate.edu/rtd/18121.

2Hodgson, E. 2019. Alfalfa weevils active in Northern Iowa. Integrated Crop Management. Iowa State University Extension and Outreach.  https://crops.extension.iastate.edu/.

3Tooker, J. 2013. Alfalfa weevil. Insect Advice from Extension. The Pennsylvania State University. https://ento.psu.edu/.

4Alfalfa weevil. Extension & Outreach. Department of Crop Sciences. University of Illinois. http://extension.cropsciences.illinois.edu/.

Web sites verified 3/8/2020

Paul Parcher

Agronomist

ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields. Channel® and the Arrow Design® and Seedsmanship At Work® are registered trademarks of Channel Bio, LLC. ©2020 Bayer Group. All rights reserved.

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