Anthony Hanson (hans4022@umn.edu), Extension Educator - Integrated Pest Management
It's been a relatively slow alfalfa weevil season compared to previous years, though some reports of high population fields have trickled in during June. Most are mature larvae (i.e., fourth instar) that should cease feeding soon, but there are still some younger larvae present in samples (Fig. 1).
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| Figure 1. Alfalfa weevil larvae collected from Morris, MN on June 17, 2025. Older third and fourth instar larvae are bright green, while first and second instars are smaller and pale yellow. Photo: Anthony Hanson. |
While I've been finding larvae regularly in west-central MN, I haven't seen large enough populations that quite reach an economic threshold and financially justify a insecticide application. This has been the case this year across the state when I've checked in with other extension educators and crop professionals. I would not be surprised if there are scattered fields that have reached threshold, so be sure to scout individual fields. Many growers should not need to spend money on insecticide this year though.
However, if we're following the forecast models for alfalfa weevil development using degree-days, it was forecast to have completed the larval and feeding portion of the life cycle around June 16 in much of Minnesota (Fig. 2). With most of the larvae being close to developing into pupae in Fig. 1, this could be normal variation in the forecast model, but seeing less mature larvae brings in an additional concern.
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| Figure 2. Eastern strain alfalfa weevil degree-days as of June 9, 2025. Forecast maps can be viewed at https://vegedge.umn.edu/alfalfa-weevil. As of this posting, maps cannot be updated further due to the temperature data source being down. |
During warm falls and winters, it's possible to have staggered generations where some otherwise less cold tolerant larvae overwinter in addition to overwintering adults. Those overwintering larvae develop into adults and then lay eggs later than overwintering adults. That doesn't seem as likely this year due cold winter soil temperatures. However, the western strain of alfalfa weevil develops about one to two weeks later than the eastern strain. The western strain may have been moving across Minnesota from western states in recent decades, and this may explain why alfalfa weevil seasons have seemed to extend to the end of June in the past few years.
If you are finding younger pale yellow-green larvae in alfalfa fields at this time, continue scouting through the end of June. If you find primarily older larvae, you likely are nearing the end of the weevil season for your fields and alfalfa weevil is following a more typical developmental schedule.
Scouting and treatment recommendations have not changed much since last year, so for more information on scouting tips, treatment thresholds (Table 1), and insecticide options, visit last year's article “Alfalfa weevil season begins in 2024”. If you have any suspected fields where a pyrethroid was applied and did not reduce weevil numbers, please contact Anthony Hanson (hans4022@umn.edu).
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Hay value ($/ton) | |||||||
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Treatment | $50 | $75 | $100 | $125 | $150 | $175 | |
| Stem height | Management tactics | cost/acre | Average larvae per stem | ||||||
| 10-15 inches (mid-vegetative) |
Long or short PHI insecticide |
$7 $8 $9 $10 $11 $12 |
3.6 4.1 4.7 5.3 5.9 6.4 |
2.2 2.6 3.0 3.4 3.7 4.1 |
1.5 1.8 2.1 2.4 2.7 3.0 |
1.1 1.4 1.6 1.8 2.1 2.3 |
0.9 1.1 1.2 1.4 1.6 1.8 |
0.7 0.8 1.0 1.2 1.3 1.5 | |
| 16-20 inches (late vegetative) |
Short PHI insecticide or mow early |
$7 $8 $9 $10 $11 $12 |
3.8 4.4 4.9 5.5 6.1 6.7 |
2.4 2.8 3.2 3.6 4.0 4.4 |
1.8 2.1 2.4 2.6 2.9 3.2 |
1.4 1.6 1.8 2.1 2.3 2.5 |
1.1 1.3 1.5 1.7 1.9 2.1 |
0.9 1.1 1.2 1.4 1.6 1.7 |
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| >20 inches* (early bud) |
Mow early | $7 $8 $9 $10 $11 $12 |
4.0 4.6 5.2 5.8 6.3 6.9 |
2.7 3.1 3.5 3.8 4.2 4.6 |
2.0 2.3 2.6 2.9 3.2 3.5 |
1.6 1.8 2.1 2.3 2.5 2.8 |
1.3 1.5 1.7 1.9 2.1 2.3 |
1.2 1.3 1.5 1.6 1.8 2.0 |
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*If >50% of plants are at bud stage, it's more beneficial to mow.
Remember there are not very many pesticide options for use in alfalfa. If you have any existing stocks of chlorpyrifos products registered 2025, these can be used on alfalfa until June 30, 2025. Old chlorpyrifos products with expired registrations cannot be used this year. One common trade name farmers often ask about that falls in this latter category is Lorsban. That product was not registered for this year and cannot be applied to field crops this year even before June 30. Check the MN Dept. of Ag. registered product list for more information. There may be products with new labeling that can be used after this time provided they comply with updated EPA guidelines. For more details see Chlorpyrifos products and their use in Minnesota.
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