Blister beetles in alfalfa hay: What to look for in the hay field and the hay rack |

Blister beetles in alfalfa hay: What to look for in the hay field and the hay rack

Ruth Wiechmann
for Tri-State Livestock News

With recent reports that hay contaminated with blister beetles may be the cause of the death of several horses in Wisconsin, it behooves hay buyers and producers alike to familiarize themselves with the wee beasties and know what to watch for in the hayfield and the hay rack.

“Blister beetles have always been around,” said Patrick Wagner, SDSU Extension entomologist in Rapid City, South Dakota. “They are present pretty much nationwide.”

There are many different kinds of blister beetles, but those most likely to be found in the northern Great Plains area are the ash gray blister beetle (Epicauta fabricii), the black blister beetle (Epicauta pennsylvanica), and the striped blister beetle (Epicauta lemniscata). Blister beetles contain a chemical called cantharidin that they secrete as a means of defense. Cantharidin is toxic and causes blistering and rashes to the skin of people who happen to come in contact with it and can cause internal injury to animals that ingest it.

“If one gets down your shirt it’s pretty painful,” Wagner said. “Horses are the most sensitive to cantharidin but other animals might possibly get sick from it.”

The inch-long winged beetles travel and mate in swarms, seeking pollen to feed on; blooming alfalfa is a favorite target.

“If they’re in the field you will see them,” Wagner said. “The grey beetles are the most common in South Dakota, followed by the black blister beetles. We have a few of the striped ones but they are more common farther south. There is a different amount of toxicity in each kind, with the grey having the least amount of cantharidin. The black ones have a little more than the grey and the striped beetles are the most toxic.”

Blister beetle larvae are predatory and feed on grasshopper eggs, so the blister beetle population depends largely on the grasshopper population. Blister beetles are present in South Dakota but are not a common problem in the hayfield. Wagner said they appear erratically, depending on the year, the field, and the conditions.

“This year I will definitely be telling producers to watch for blister beetles,” Wagner said. “Our grasshoppers got a late start last summer so we can expect egg pods to be accessible to the blister beetles.”

Blister beetles are beneficial because they eat grasshopper eggs and Wagner does not recommend spraying specifically for them, because the dead blister beetles that remain still contain the toxic cantharitin. To spray or not to spray is also a difficult call to make because blister beetles’ presence is very spotty and sporadic in fields.

“Blister beetles travel in swarms,” said Scott Schell, Extension entomologist with the University of Wyoming. “They are usually very spotty in fields. They may not be there one day but show up the next. Blister beetles congregate to feed and mate where there are flowers and where the grasshoppers are most likely to lay their eggs so that they can lay their eggs nearby. You will often see them at the edges of the fields, because grasshoppers frequently emerge from the ditches and fence rows, especially in flood irrigated fields.”

Both Wagner and Schell suggest that the best way a hay producer can prevent blister beetles from ending up in his bales is to cut the alfalfa before it starts blooming.

“The adults are attracted to blossoms,” Wagner said. “The best way to avoid blister beetle contamination in alfalfa hay is to cut it in the bud stage.”

“Unless you’re going for maximum tonnage it is advisable to cut alfalfa prior to bloom,” Schell said. “If the alfalfa is not in bloom the blister beetles won’t congregate there because they are looking for flowers.”

Schell said that studies done by the University of Kansas, where blister beetles are more common, suggest that a producer can spray for them, but using older haying equipment such as a sickle mower may also be a good option if blister beetles are present in a field.

“That way you’re not running them through a conditioner and squishing them,” he said.

“If you cut the hay and wait, blister beetles will leave,” Wagner said. “They will go looking elsewhere for flowers.”

Alfalfa/grass mixed hay is not immune to blister beetle infestations.

“If there’s any alfalfa at all it draws them,” Wagner said. “It may even be worse than straight alfalfa because the alfalfa is usually in bloom when those fields are cut. Anyone who has had a lot of grasshoppers the year before also has a higher risk of an influx of blister beetles the following year. They also seem to like any greener areas, such as those close to waterways like the Missouri River.”

University of Kansas publications indicate that there is no precise answer to the question of how many blister beetles it takes to make a horse sick or kill it. It depends on the kind and toxicity level of blister beetle present and the overall health of the individual horse that consumes them. One study found that five grams (approximately thirty striped blister beetles) could kill some horses. Cattle are far less susceptible to cantharidin poisoning but a lab study indicated that cantharidin can reduce the digestibility of some forages. The same study indicated that anywhere from twenty-five to three hundred blister beetles might kill some horses.

“It’s more a question of presence versus absence,” Wagner said. “We don’t have any special hot spots in the state where they are a big problem.”

For anyone buying hay, doing due diligence and asking questions beforehand is critical.

Rusty Lytle, a hay producer from Wall, South Dakota, said that he has not seen a blister beetle in his fields in decades, but that hay producers also need to take responsibility for communicating with buyers. Lytle sprays diligently for alfalfa weevils prior to his first cutting and says that he thinks that has helped to keep blister beetles out of his fields. “Supplementing high quality alfalfa with something else, such as quality grass hay, can also help reduce the possibility of illness from eating blister beetles.”

“As a producer, a guy should probably have a questionnaire for anyone buying hay for horses,” he said. “Anyone buying hay needs to voice their concerns before the purchase.”

He also suggests hay buyers research the person they are dealing with prior to purchasing hay. With hay values increasing in recent years, many people are trying to capitalize on their hay crop, but not everyone puts the same effort into keeping the quality high.

“Find a reputable source,” he said. “Find someone coming from a professional standpoint in raising hay, whose concept is 100 percent based on hay quality, and then don’t quibble over the price.”