Alfalfa weevils are out in force this spring in parts of Missouri. “Alfalfa fields face a double hit by weevils this spring. There are more adult weevils than I’ve ever seen,” says Duane Bailey, a University of Missouri entomologist with 29 years of experience in scouting the pests.
His advice to growers: Scout fields often. “Small weevils are easy to miss. A small pocket of them can expand rapidly and do lots of damage.”
This year’s cold weather, however, has lowered the effect of insecticides used to control the weevils. “With ground rigs, apply mixes with 20 gallons of water per acre. 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 actively laying eggs. He thinks it has to do with last winter’s extreme cold weather, although past perception contends 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 stays active laying eggs. But this year, the winter brood is out now, laying eggs with high survival rates. Winter-laid eggs often die before they can hatch in 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.
That means alfalfa faces a big threat; every new female lays up to 500 eggs. Each egg produces a small larva that hides in, and eats, the growing tips of emerging alfalfa.
Once the tiny larvae move out of the leaf folds, they defoliate an entire plant. Larvae have large appetites, Bailey says, and lost leaves reduce hay quantity and quality. “Weevils cause a lot of damage in a short time.”
Larvae are ripe for fungal outbreaks that can kill worms or, once the worms are infected, stop them from eating.
Although the fungus needs warmth and wet weather, only rain is in the forecast. Bailey advises growers to prepare for spraying and hope for warm weather.
Another option is to cut near-mature alfalfa for hay. The potential downside is that haying allows weevils to concentrate on regrowth from the alfalfa stubble. The worms nip new leaves as they emerge. Spraying for weevils probably will be needed after first cutting. If regrowth is not protected, the stand thins and weeds grow to shade the alfalfa. At that point, weeds crowd out alfalfa.