Scout alfalfa fields for shy insects, says MU Extension entomologist

COLUMBIA, Mo. – Two insects are creating problems on alfalfa fields ready to cut for hay. Stealth scouting may be needed to see them, says a University of Missouri Extension entomologist

“These insects can be shy,” says Wayne Bailey. “The main message is go out and scout your fields.”

The pests are not new, but can arrive in overwhelming numbers.

Potato leafhoppers cause leaves to yellow and fall off, reducing hay yields. The other, the striped blister beetle, can kill horses if the insects are crushed and baled during haymaking.

With both, a noisy approach makes the insects drop to the ground and hide.

The first telltale sign of potato leafhopper may be V-shaped yellowing at the tip of each leaf. Hoppers suck juice from the leaves. The yellowing precedes leaves dying and falling off.

First-year plantings of alfalfa are most at risk, Bailey says. Seedlings in new stands are most vulnerable. A stand that survives the second season can withstand more damage.

The leafhoppers are small, green triangular insects. They do hop.

Economic threshold for applying insecticide is only one hopper per sweep of a 15-inch insect net. Some alfalfa varieties have hairy leaves that repel the hoppers. Threshold on those varieties is three insects per sweep.

A lot of fields are above threshold, Bailey says. The hoppers migrate into Missouri from the south, mostly coming up the western side of the state, then moving north and east.

“Already we have heavy infestations in central Missouri,” he adds.

The other beetles cause blisters on human skin if they are squashed. The body fluids contain alkaloids that are part of their self-defense.

The beetles are elusive, which helps them stay out of the way of grazing livestock. Infested alfalfa fields can have hundreds of thousands of the beetles. Most of the 20 species are elongated, with striped wings, most often orange in color. Some are gray and darker gray.

When disturbed, the beetles drop to the ground and seek cover.

“They are very alert,” Bailey says. He recalls the first time he saw blister beetles en masse was at alfalfa plots at the MU Thompson Farm near Trenton, Mo.

“There must have been a hundred thousand of them,” Bailey recalls. “They all turned and looked at me at once. Then they dropped out of sight. They were gone. I couldn’t see a one.”

Horses and pasture-raised chickens are most at risk. Cattle, goats and sheep don’t seem to be bothered.

The danger comes from crushing and baling the beetles with hay. “When we used to cut hay with a sickle-bar mower, the blister beetles could get out of the way,” Bailey says. “With haybines, they are crushed when the beetles go through the rollers.”

Horses should be fed only from first-cutting alfalfa. Mostly, blister beetles don’t arrive until the second or third cutting of hay. Alkaloids in the blistering agent can cause the lining of the horse’s digestive tract to slough off. That is fatal to horses.

University of Missouri Extension regional agronomists have recommendations on scouting and controlling insect pests.