David Owens, Extension Entomologist, owensd@udel.edu
Alfalfa weevil started hatching from eggs at the beginning of March. I recorded a Pest Patrol alert a couple of weeks ago regarding alfalfa weevil. If you are not a subscriber, it is free and you will get a text when a regional alert has been recorded. The last two weeks of cool, wet weather has slowed them down. If you have not scouted your alfalfa, you need to do so ASAP. Next week’s warm weather is going to allow them to grow quickly and quickly defoliate your field. To sample a field, randomly select 30 stems and bang them into a bucket hard. This will dislodge weevil larvae and let you count them. You may still want to look at the terminals, sometimes very small larvae do not come out. Measure a couple of stems to assess plant height. Thresholds vary depending on plant height, control cost, and hay value.
Now that alfalfa weevil has been counted, you need to decide whether or not to treat and with what. Our thresholds (table from Penn State found here: https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/sustainable-agriculture/pest-management/Insect_Control_in_Alfalfa_-2023.pdf) are based on PURE stands ONLY! If a stand consists of less than 50% alfalfa (or less than 50% of alfalfa in its first cut), thresholds are not applicable, and it is not worth treating. Between 100 and 50%, you’ve got to use your best judgment.
The next question is what to treat a field with. If a field is just at threshold, a pyrethroid or an organophosphate should be sufficient. In recent years, these two classes of chemistry have given us 50-70% control. I have not tested chlorpyrifos. My first years testing were in Maryland where chlorpyrifos is not available for use. Then Chlorpyrifos was banned. And now it is ‘unbanned’. EPA recently provided some additional guidance on what products are still labeled. Note that Lorsban is not on that list (See the announcement and follow the link in the below Chlorpyrifos Update 2). Delaware users can use chlorpyrifos and existing stock, and I highly recommend that labeled products be used up judiciously this season because availability next year is far from certain. If a field is well above threshold, the best product on the market right now is Steward. It can be hard to get, and it is very expensive, around 3$ per ounce (6.7 to 11.3 ounces), making this a very expensive application. However, if a field is 2+ times over threshold, Steward will do the job. Last year we applied a test in a field that was 5x over threshold. Two weeks after application, we could see exactly which plots were treated with Steward and which were not. Steward is also not going to disrupt natural enemies that feed on aphids or parasitize alfalfa weevil.