Delaying a spring alfalfa weevil insecticide application until temperatures rise may bring better results, suggests Jeff Whitworth, Kansas State University Extension entomologist.

“During cool weather, alfalfa weevil larvae will move down into any crop residue that is on the soil surface for protection from the cold,” says Whitworth. “As a result, insecticide applications made in early to mid-April, for example, may miss more alfalfa weevil larvae than an application made just a week later when temperatures are warmer.”

That was verified in a study he conducted in a third-year alfalfa field last spring. He applied one set of insecticide treatments on April 13 and another set at another location in the same field on April 18. The first treatment only reduced weevil numbers by 15-30%. On April 18, when a much higher percentage of the weevils were exposed to the insecticides, treatment reduced larvae numbers by 60-65%.

“When deciding on insecticide application timing for alfalfa weevils, the level of insect pressure also plays a role, of course,” says Whitworth. “If weevil pressure is higher than the economic threshold early in the season, it will pay to spray at that time even if the insecticides are not as effective as they would be a week or two later. But applicators should have realistic expectations about the effectiveness of the insecticide applications. If control does not seem as good as expected, the reason may be related to temperatures and the location of the larvae at the time of the application.”

He compared several insecticides and application rates and found no significant differences in this study.