MANHATTAN, Kan. – It’s not only cyclists and joggers taking advantage of recent unusually warm Kansas weather — alfalfa weevils are rapidly developing throughout the state, according to K-State Research and Extension entomologist Jeff Whitworth.
Whitworth said samples he’s taken and reports from others showed the first larvae were detected the week of March 5, and with the significant warm spell Kansas has experienced since then, egg and larval development has been readily apparent.
“All fields we sampled this week in central Kansas had 10-30 percent of the stems infested with 1st instar larvae,” Whitworth said. “Most infested stems had multiple larvae simply because that is where the female weevil deposited a clutch of eggs last fall and now they are all hatching together.”
Treatment thresholds vary in alfalfa, depending upon the end use, but usually delaying application until there is one larva for every two stems (50 percent infestation) is the most effective management tactic, he said.
“This warm weather may condense weevil development by increasing it so quickly, so early that the infestation may not drag out over a 4- to 6-week period as it has in the past,” Whitworth said.
“We’ve been getting questions about the use of chlorpyrifos, or products containing chlorpyrifos, relative to the legality of making a second application, if needed, per cutting. This question has arisen because of the early egg hatch and the potential length of time between this and swathing, which usually doesn’t occur until at least late April.
“If you do decide a second application is necessary prior to that first cutting, make sure you consult the label of the product you intend to use to ensure it is allowed for that product. Always read the label prior to and have the label available during application.”
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