Most people are already finished with their first cutting of alfalfa throughout Iowa, but each of our 10 field agronomists reports that alfalfa weevil activity continues and has been much worse than they’ve seen in over a decade. Lance Scott in southwest Iowa took this short video to show significant injury. Alfalfa weevil only has one generation per year. The larvae are the most damaging stage, defoliating leaves and sometimes only leaving the veins behind. Fields may appear frosted, or have a silvery appearance, with heavy alfalfa weevil feeding as leaflets turn brown and die (Photo 1). Based on accumulated degree days, it is likely that larvae are done feeding and pupating throughout much of the southern half of the state. Any adults at this time are going to enter their summer dormancy period (non-feeding) soon.
Larvae may still be active in some areas and would likely be 3rd or 4th instars that do much of the feeding. Although cutting alfalfa is typically an effective management strategy for alfalfa weevil, larvae can seek shelter under windrows and feed on the crown of regrowth. Now would be an excellent time to scout alfalfa stubble for alfalfa weevil larvae, especially in northern Iowa where larvae may still be feeding. Treatment of regrowth is recommended if eight or more larvae are found per square foot or if regrowth is delayed due to feeding. Regrowth may already be significantly challenged by drought conditions across the state.
Even though alfalfa weevil activity may be wrapping up around the state, this would also be a good time to think about other alfalfa pests. Aphids have been present in alfalfa fields for some time already, but scouting and proper identification can determine if significant injury is occurring and treatment is needed. Typically, very few aphids survive harvesting, so it is possible aphid numbers will be lower in fields that have been mowed. Continue to scout for aphids and their look-alikes, the potato leafhopper. The potato leafhopper is likely beginning to migrate from southern states and is persistent in alfalfa from the second cutting through August.