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Updated Alfalfa Weevil Management Recommendations and Challenges for 2023
Minnesota Ag Connection - 05/29/2023

Mid-May to June is typically the time to begin scouting for insect problems in alfalfa. After our cold spring of 2023, alfalfa weevil adults and larvae are beginning to be found in the southern half of the state. Even if larvae are present, insecticide applications may not be needed if populations are not yield limiting or when alfalfa fields can be harvested soon. Cutting exposes the larvae to weather and can greatly reduce numbers. Remove windrows as quickly as possible. The larvae are concentrated there, and heavy feeding can delay regrowth. (Fig 1). Scouting through June will determine if control surviving larvae need to be controlled. Also remember that insecticide options are more limited compared to previous years with chlorpyrifos (e.g., Lorsban) applications no longer being allowed.

Farmers have been seeing a two-pronged challenge to managing alfalfa weevil in recent years: 1. An extended larval feeding season. 2. Reports of pyrethroids failures for alfalfa weevil. Each of these problems on their own have some solutions, but the compounding effect of both of these is leaving farmers with limited options. This article will dive into general alfalfa weevil management and what adaptations are needed as the dynamics of this pest change.

Alfalfa weevil identification and development

Alfalfa weevils overwinter as adults in nearby grassy areas and go through one generation per year. Eggs are laid in the stem of alfalfa plants in spring typically around mid-May. The larvae, rather than adults, are the stage that can cause economic injury to alfalfa.

The feeding of alfalfa weevil larvae causes round holes in the upper foliage that can skeletonize leaves so only leaf veins remain. From a distance, the defoliation causes a gray or frosted appearance of heavily infested fields. This damage window typically coincides with the end of first cutting and the start of regrowth for the second cutting. 1st instar larvae are yellow to olive with 2nd through 4th instar larvae becoming green in color with a white stripe along their back. All stages have distinctive black heads. Often, growers become aware of the larvae when they find them on their mower during the first cutting of the year (Fig. 2). If populations were high, larvae that survive mowing can end up concentrated in the windrows and feed on regrowth if hay isn’t quickly removed.

Economic crop injury ends when the larvae pupae in loose, round cocoons on the ground. While they may briefly feed, the emerging adults enter a summer dormancy (Fig. 3). Typically, these adults will be present in fields and nearby grassy areas, but will not lay eggs until the following year.

Problem 1: Extended weevil feeding season

Similar to crop development, alfalfa weevil development is affected by temperature (Fig 4). Larvae that would be found so far in May would have either just hatched or else very small early-stage larvae. Especially during 2021 and 2022, there were reports of economic damage in west-central fields for second cutting (even into early July) in what appeared to be delayed development of larvae that would normally have developed into adults. This could have resulted in more reported cases of heavy feeding under windrows


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