Minot local Sue Christiansen reported an injury caused by a local pest, the blister beetle.
“I have no idea where it came from. I thought it was a wood tick,” said Christiansen.
Three weeks ago, Christiansen noticed, at the time, an unknown bug on her foot that left behind a painful blister. After visiting with a few doctors, it was determined the blister was caused by the toxin cantharidin, which is produced by blister beetles.
Paige Brummund, North Dakota State University Agriculture and Natural Resources Extension agent in Ward County, said that blister beetles are common pests of field crops such as alfalfa, sweet clover and canola, but she still spots the occasional urban beetle in town or in potted flowers outside of restaurants. Brummund went on to say that the risk of injury in urban areas is low as the beetles don’t swarm as they do in rural areas and they don’t seek out humans to intentionally cause harm. While generally unharmful to humans, blister beetles pose a different risk to livestock, especially horses.
According to NDSU information, the blister beetles’ attraction to alfalfa and other foraging crops makes them an ingestion risk to livestock. The blistering toxin releases when the beetles are crushed. Even dead beetles contain high levels of cantharidin, which doesn’t weaken over time.
When blister beetles are accidentally harvested with the foraging crops, it can prove harmful, or even lethal to livestock who consume them. Cantharidin oil causes blistering of internal and external tissues and attacks the gastrointestinal and urinary tracts, leading to death in horses. Sublethal symptoms include depression, sores on tongue and mouth, diarrhea and elevated temperatures.
Brummund noted that blister beetles overwinter in North Dakota by laying eggs in the soil. When there’s an increased population one year, the next there’s even more.
Brummund said blister beetles have become very common in the area.
“Especially the last few years, and we don’t know really know why. But it seems like their populations really increased during the drought years,” Brummund said. “People have also been more aware of it, so we’ve been getting more reports. But when I go out and scout fields, just about every alfalfa field that I’ve looked at in the last few months have had blister beetles in them.”
Brummund’s recommendation for dealing with the beetles is to scout fields before harvesting, particularly before putting up hay. They tend to travel in clusters, so she recommends cutting the areas of the field that aren’t affected without crimping the crops, and then waiting for the swarm to move before harvesting the rest. It isn’t recommended to spray pesticides on affected crops, as dead beetles still pose the risk of livestock ingestion.
“It just comes down to scouting that field and trying to get it cut before it’s in full bloom because that’s what’s going to attract all the beetles to it,” Brummund said. “And of course, Mother Nature doesn’t always cooperate. You might have the intention to cut it but it rains and it’s wet, and pretty soon it’s in full bloom and full of beetles.”