Ranchers in two Wyoming counties have reported blister beetles this summer, although neither has reported any livestock poisonings, said Scott Schell, assistant extension entomologist with the University of Wyoming.
Still, the pests' presence is worrisome. Horses that eat blister beetles can become severely ill or die from gastrointestinal, heart or kidney trouble. The first signs of blister beetle sickness are acute colic, diarrhea and excessive salivation.
Blister beetle outbreaks often follow significant grasshopper infestations because the beetle larva eat grasshopper eggs, said state veterinarian Jim Logan.
Cattle and sheep can also be sickened, but not usually to the same degree as horses.
"Not every case is going to cause death in the horse," Logan said. "It depends on the species of the blister beetle, and how much is ingested by an individual horse. There are varying clinical signs that you'll see, and in some cases it does cause death."
Schell said the black blister beetle is the most common species of the beetle found in Wyoming. It has a relatively low toxicity: Studies have found it takes 1,700 black blister beetles to kill an 825-pound horse, he said.
"It probably occurs in low numbers most every year, but this year it's getting more attention," Schell said last week. "The numbers will probably pick up next year actually because its larva live on grasshopper egg pods, so there's going to be a lot of larva habitat after this year."
Slade Franklin, weed and pest coordinator for the state Department of Agriculture, said he sent pictures of blister beetles to the state's 23 weed and pest districts in response to a case in Fremont County, in central Wyoming. He said he hasn't fielded any other reports about them.
Grasshopper outbreaks tend to be cyclical and are influenced by climate and insect population dynamics, said Bruce Shambaugh, state plant health director for the Plant Protection Quarantine division of the federal Animal and Plant Health Inspection Service.
Wyoming has experienced outbreaks of pest grasshopper species this summer in counties that traditionally have grasshopper problems, he said. Those include eastern and northeastern counties, the Big Horn Basin and Fremont County.
Franklin said grasshoppers are an important food source for birds, but an overabundance of the insects can be problematic because they feed on the same vegetation as livestock and other wildlife.
"We have a lot of vegetation this year. In some places, there's been enough vegetation" to keep grasshoppers, livestock and wildlife happy, he said.
That's not been the case for rancher David Kane. He said the grasshopper infestation on his ranch in north central Wyoming's Sheridan County is the worst since 1988. He downsized his herd last week because of the grasshoppers' effect on food supply.
"They're eating all our forage, and then to top all that off we had a major hailstorm come through and pounded a bunch of our rangeland as well, so we kind of got a double-whammy," Kane said.
When it comes to blister beetles, infestations normally occur in late summer and can contaminate green, cured or baled hay.
Logan, the state veterinarian, said cutting hay before it blooms helps decrease the likelihood of contamination, because beetles infect the hay primarily when it's in bloom.
If the hay already has bloomed, Logan recommends farmers remove the conditioner from windrowers - mowers used to cut hay - so it doesn't crush the hay stems and the beetles. Then the beetles will leave once the hay is down.
The Wyoming Livestock Board recommends seeking immediate veterinary care if blister beetle poisoning is suspected.