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Blister beetle population up due to grasshoppers

Rebecca Colnar
for Tri-State Livestock News

It’s been said that following a grasshopper infestation come the blister beetles. That’s accurate, according to University of Wyoming’s Extension Entomologist Specialist Scott Schell.

Female blister beetles lay clusters of eggs in the soil in late summer, and those hatch in about 12 days and go on a search to invade the grasshopper egg pods, eat the grasshopper larva and winter in those pods. The following summer they enter the larval pupal stage, emerging in late summer as active blister beetles.

“The more grasshopper egg pods there are, obviously the more blister beetles you will have,” Schell explained. “In many areas, this is their second bad grasshopper year so the threat was there this year for haying and will also be there next year.”

Blister beetles are of the family Meloidae, so called for their defensive secretion of a blistering agent, cantharidin. There are about 7,500 types of blister beetles worldwide. Blister beetles can be both direct and indirect pests. They are leaf feeding pests in potato, sugar beets, soybean and alfalfa, with their greatest detriment being their contamination of alfalfa hay with toxins.

Schell points out that not all blister beetles are equal. “In Wyoming, Dakotas and Montana, we don’t have the most toxic striped beetles. The ones we have are black or gray.”

Although blister beetles can be harmful to cows and sheep, the insects are especially toxic to horses. When blister beetles are ingested by livestock, blistering of the stomach and esophagus can occur, kidney and heart function can be impaired, and death may result. Dave Boxler, Nebraska Extension Educator, cites that the poison can kill horses within 72 hours so it’s essential to call your veterinarian if poisoning is suspected.

“Toxicity to horses has not been definitely determined but estimated minimum lethal dose is 1 milligram of cantharidin per 2.2 pounds of body weight,” says Boxler. “The number of beetles necessary to provide a lethal dose depends on the species of beetle and how much cantharidin it retains.”

One reason horses may have a worse reaction to the toxin is due to the way they are fed—often they are given a couple of hay flakes in their stalls where they eat alone, whereas cows will generally be fed hay out in the open with the ability to jostle each other around and go to different hay piles thus having less concentration. Dried beetles fall out of the hay and are often consumed by the horses who are still nosing around the stall and picking up more hay.

Adult beetles like to feed on flowering plants so it stands to reason that if alfalfa goes to bloom, the blister beetles will swarm it to eat and mate. When that plant is cut, the toxin doesn’t deactivate; when the grass runs through the crimper on the swather, the beetles are crushed, releasing the toxins.

Schell noted producers can minimize blister beetle numbers by not crimping the hay, and harvesting the alfalfa before it’s in full bloom. “I know a lot of people prefer to feed their horses second cutting, but the first cutting, although not ideal nutritionally, is safer,” Schell cautions.

His advice is cut pre-bud/pre-bloom to minimize the risk. “Like grasshoppers, blister beetles are mobile which is what makes control so difficult. You could spray your field and kill them, but there is no guarantee that field wouldn’t get re-infested. It used to be you might be able to keep your eye out for beetles while you were swathing, but swathers are so fast now, it’s really hard to spot insects. If you do spray, plan to kill the weeds and other vegetation outside of your hay field, as well.”

Grass hay is relatively safe because it doesn’t have blooming flowers like legumes, although weeds that contain flowering tops along the edges can pose a problem due to their proximity to the grasses. Generally, if a horse is grazing an area with blister beetles, it won’t be a problem because the insect will fly away when the horse starts eating the plant.

“The beetles being in the hay is the main concern. Unfortunately, you can never guarantee there are no beetles in a bale of hay,” shared Schell. “As long as hay is cut when it’s not in bloom, the risk is low. The later the hay is baled, the more risk there is. However, there really are no zero-risk options.”

Whether you bale the hay yourself or work with a hay farmer, planning to bale alfalfa pre-bloom is the best bet. Even if you are baling grass hay, consider spraying any weeds that flower that are alongside the field. Another option during a grasshopper/blister beetle year is to avoid alfalfa and opt for grass hay supplemented by alfalfa pellets.

“This year many areas in Montana, Wyoming and the Dakotas had severe grasshopper infestations which means next year will be a good one for blister beetles,” Schell said.


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