By Nathan Drewitz University of Minnesota Extension
FOLEY, Minn. (05/18/2022) — Shrimp boat captain in Central Minnesota was not on my 2022 bingo card, but maybe I should have written it in a square. Judging by the look of fields throughout the area we are still about a week to 10 days from planting in some places. Hopefully the lighter soils in places allow us to stay busy enough over that timeframe. Amid all this rain, we also received reports of possible alfalfa weevil larvae in southwest and central Minnesota (Stearns). While I have yet to find issues in Benton or Morrison counties, these are still one of our most important insect pests in alfalfa. Scouting for these pests is going to be important to managing them, especially since our options for insecticides is more limited this season. Let’s first discuss identification before looking at control.
Alfalfa weevils overwinter as adults and only go through one generation per year. Larvae are the most damaging stage that can skeletonize leaves where only leaf veins or holes remain. 1st stage larvae are yellow to olive with 2nd through 4th stage larvae becoming green in color with a white stripe along their back. All stages have distinctive black heads (Photo). Clover leaf weevil larvae can sometimes be confused for alfalfa weevil, but instead have a tan head capsule. Once larvae develop into pupae, they are no longer causing feeding damage, and newly developed adults enter a summer dormancy.
Scouting should occur in mid-May through June by using a sweep net to determine if larvae are present. If present and easily found in sweeps, select and cut 30 plants across the field at ground level. Record each plant’s height and shake in a 5-gallon bucket to determine the average number of larvae per stem in the field and if weevil counts exceed economic thresholds. For more information, and to view the economic thresholds table go to the Alfalfa Weevil article on the UMN Extension Crop News blog for that information.
Alfalfa weevil is typically the first pest insecticides may be used for in alfalfa during the year. Insecticides labeled for alfalfa weevil include pyrethroids and organophosphates. Parasitoid wasps and other natural enemies help suppress alfalfa pest populations, but they are also susceptible to these insecticides, so flare-ups of alfalfa weevil or other pests such as pea aphid can occur after treatment. In addition to further reducing natural enemy populations, multiple applications in a field can also increase the likelihood of alfalfa weevil becoming resistant to those insecticides, which further illustrates the need for careful use of currently available control options. Table 2 of the Alfalfa Weevil article above shows the different options that we have for control.
The above information is pulled directly from the “Preparing for alfalfa weevil in 2022” article. If you have questions on the above information, please feel free to email me at ndrewitz@umn.edu, or call at 608-515-4414. To receive future events, educational programming, and agronomic updates by email, signup at z.umn.edu/tricountysignup.