Spraying For Weevils May Hurt Wasp Beneficials

Increased spraying for alfalfa weevils in Michigan may be dampening the impact of wasps that provide biocontrol, says Christina DiFonzo, Michigan State University entomologist, in the university’s Integrated Pest Management Field Crop Advisory Team newsletter. Cutting is preferred to spraying for weevil control, she adds.

“We don’t know this for sure, but extension educators have indicated that producers who have sprayed weevil over the last few years seem to have increasing weevil populations,” she states. If spraying is necessary, DiFonzo urges growers to note preharvest intervals, which range from zero to 28 days depending on product, rate and crop use (hay vs. forage). There are many products to choose from, plus spraying new growth achieves the best coverage and avoids treating flowers, which could potentially kill bees.

If an infested field is cut, scout new growth carefully; the threshold is just six to eight weevil larvae per square foot after cutting.

Weevil larvae continue to be found in many southwestern Michigan alfalfa fields and will cause lower-than-normal first-cutting yields, reports Dan Rajzer, Cass County agriculture and natural resources educator. Numerous fields have been sprayed for weevil control there.

Pink Pea Aphids Found In Nebraska Alfalfa

A number of unusual aphids were recently found in alfalfa fields near David City, NE, according to the University of Nebraska-Lincoln Crop Watch newsletter. These aphids weren’t green, but more of a beige/pink color, and are thought to be the pink form not previously not found in the state. Some biological differences exist between pea aphids forms, Nebraska entomologists point out.

The vast majority (90-95%) of the pea aphids collected from a Butler County field were typical green pea aphids. The pink insect is harder to see in a sweep net and could be overlooked at first. Although the pink pea aphid has existed in Europe for some time, it wasn’t noted in the U.S. until 1979, when it was found in New York. Since then it has been identified in Ohio, Michigan, Utah, California and Missouri, and probably is present in many other states, say the entomologists.

Researchers found that parasitic wasps prefer to attack green rather than pink pea aphids. In research on the European red/pink pea aphid biotype, it easily overcame pea aphid resistance in a number of U.S. alfalfa varieties. The entomologists say these observations indicate continued need for Nebraska alfalfa crops to be monitored.

View a pink pea aphid in the most recent Crop Watch issue at cropwatch.unl.edu/.