Key Points:

• Alfalfa weevil is an important early season defoliation pest in alfalfa.
• Scouting should begin when pest degree days accumulate to 250 in the spring.
• Degree day accumulation in 2017 is significantly ahead of long-term norms.
• Consider insecticide when 25-50% of leaves show feeding injury and with 3 larvae per stem.

Identification and Life Cycle. The alfalfa weevil (Hypera postica) is the most important early season defoliation pest of alfalfa in the Midwest. Adult weevils are small brown snout beetles with a dark narrow stripe extending down the back (see Image 1). Adults become active in late March or early April and lay small oval yellow eggs inside the alfalfa stems. Eggs are laid in both the fall and the spring in warmer southern areas, but only in the spring in northern areas. Larvae are very small (3/8” when mature), legless, and yellow-green in color with a white stripe along the middle back and a very distinctive black head capsule (see Image 2). Larvae feed for 3-4 weeks, then pupate for 1-2 weeks, and emerge as adults that feed for about one week before moving to sheltered areas for aestivation (summer dormancy). During the summer season, all life stages of the alfalfa weevil may be found. Adults lay more eggs in the fall and overwinter in the crown or areas of protective vegetation, such as woods, fence rows, and ditch banks.

Injury. Both adults and larvae feed on alfalfa foliage, but the larvae are responsible for most of the damage. Early instar damage appears as pinholes in terminal leaves (see Image 3). As the larvae grow, they skeletonize the leaf and gradually move to lower leaves to continue feeding. After a cutting, larvae and adults may also feed in the crown and prevent timely regrowth. Larvae in southern areas hatch from fall eggs in late March or early April after approximately 300 pest degree days (base 48°F) have accumulated and when alfalfa is about 8” in height. Larvae can cause significant damage to first cutting alfalfa in these warmer climates. Areas farther north (north half of Illinois and most of Iowa & Wisconsin) rarely have to deal wit h alfalfa weevil damage to first cutting alfalfa because most of the weevil eggs are laid in the spring. Feeding by weevil larvae normally doesn’t normally peak in these areas until about 575 degree days (base 48°F) are accumulated. First crop harvest in northern areas often takes place in time to reduce or eliminate the weevil threat.

Scouting and Thresholds. Begin scouting when 250 degree days (base 48°F) have accumulated. Continue monitoring stands for damage through the end of June. Larvae can be counted by collecting stems and beating them against the inside of a bucket. As a static threshold, insecticide treatment may be advised when 25-50% of the stems show obvious feeding or there is an average of 3 or more larvae per stem, and when at least 7-10 days remain before the crop can be harvested.

Natural pathogens and parasitic wasps can keep populations in check, so be sure to also scout for and consider beneficial populations when making treatment decisions. Do not apply insecticide to alfalfa when in bloom to avoid damage to pollinator populations. Control may be advised after a cutting if larvae and adults are feeding in 50% of crowns and regrowth is prevented for a period of 3-6 days. Other sources, like the Penn State reference below, advise the use of dynamic action threshold tables that adjust the static threshold in consideration of the market value of the crop and cost of an insecticide application. Refer to the FS Crop Protection Handbook or web application for specific product recommendations for alfalfa weevil control. Always read and follow insecticide label directions.

References:

• University of Illinois
• Purdue
• Penn State