Alfalfa Weevils in Eastern PA May Be Resistant to Pyrethroid Insecticides

In the last week, we have heard of numerous reports of populations of alfalfa weevil larvae that were treated with pyrethroids, yet the weevil larvae did not die and kept on feeding. Most of the fields were in Berks and Lehigh Counties, but nearby areas could also be affected. A researcher would have to study these situations closely to determine with certainty whether the populations have evolved resistance to pyrethroid insecticides, but such widespread reports are certainly suggestive. Regardless of the technical details, there are two challenges:

1. solving the immediate challenge of alfalfa weevils that continue to feed; and
2. dealing with alfalfa weevil populations next season.

For the first issue, alfalfa weevil has one generation per year and is typically only a problem in the first cutting, so if alfalfa can be harvested soon, a good solution would be to harvest the alfalfa and avoid another spray. If this is the best option, folks should closely watch the regrowth of the second cutting to see if remaining weevil larvae are limiting the regrowth. If fields still need to grow a bit before being harvested, then spraying with another non-pyrethroid insecticide will be necessary, but be sure to keep the pre-harvest interval in mind. I encourage folks to consult the Penn State Agronomy Guide to see the options; less expensive options include organophosphate (e.g., active ingredients malathion, dimethoate) or carbamate (carbaryl) insecticides, but these options are more broadly toxic than pyrethroids and have harsher environmental profiles. A less toxic option that would give good control would be indoxacarb (trade name Steward), but the cost will be higher.

The second, less pressing issue is finding a better way to control alfalfa weevil in the spring of 2026. Many farmers routinely treat for alfalfa weevil with inexpensive pyrethroids when they probably do not need to, often as insurance-based treatments. Such annual exposures promote the evolution of resistance. A better approach to dealing with alfalfa weevil each spring would be to use Integrated Pest Management, which involves scouting your fields for weevil populations and only managing populations only if weevil populations exceed the economic threshold, which can be found on our alfalfa weevil fact sheet. If populations are above the economic threshold, one would need to decide if the field is mature enough to consider harvesting it or if an insecticide is necessary. If you need an insecticide, you will have to consider changing the active ingredient that you typically use to help avoid any challenge posed by pyrethroid-resistant alfalfa weevils. If you would like to discuss alfalfa weevil or the challenge posed by populations that you suspect to be resistant to pyrethroid, please contact your local Penn State Extension educator or reach out to me.