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Alfalfa Weevil Survived Winter, Larvae Threaten First-cutting Hay

April 30, 2014
Alfalfa Scouting
Scouting fields for alfalfa weevils will help producers understand what measures are needed to protect their fields.  

Profitability for alfalfa growers is at risk thanks to a wave of insects, particularly weevils.
By: Duane Dailey, University of Missouri Extension

"Alfalfa fields face a double hit by weevils this spring. There are more adult weevils than I've ever seen," says Wayne Bailey.

In 29 years of scouting alfalfa fields, the University of Missouri Extension entomologist has seen lots of weevils.

His advice: Scout fields often. "Small weevils are easy to miss," Bailey says. "A small pocket of them can expand rapidly and do lots of damage."

There is a problem with alfalfa-weevil control this year. Cold weather lowers the effect of insecticides used to control them.

"With ground rigs, apply mixes with 20 gallons of water per acre," Bailey says. "Ask aerial applicators to apply as much as they can carry."

Some pilots prefer to drop rates to a half-gallon of water per acre. "That won't give control against this population," he says.

Bailey isn't sure why so many adult alfalfa weevils are out laying eggs. He thinks it has to do with the extreme-cold weather last winter.

The perception was that below-zero weather kills overwintering adults. "The bugs didn't read the book," Bailey adds. "It seems they just dug deep in ground litter and hibernated."

In normal years, the overwintering population stay active, laying eggs. Not this winter. The winter brood is out now, laying eggs. Those fresh-laid eggs have high survival rates. Winter-laid eggs often die before they can hatch in the spring.

The new brood of weevils will be laying eggs for weevils to emerge in May and June. Good control now will reduce that second generation.

The alfalfa faces a big threat, as every new female lays up to 500 eggs. Each egg produces a small larva that hides, and eats, in the growing tips of emerging alfalfa.

Once the tiny larvae move out of the leaf folds, they defoliate an entire plant. Larvae have a big appetite, Bailey says. Lost leaves reduce hay quantity and quality.

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