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Blister beetles pose risk of contaminating alfalfa hay

by Wyoming Livestock Roundup

Drought tends to force many producers to purchase hay, which can increase the risk of blister beetle contamination and livestock exposure. Blister beetles are a relatively common insect across the United States. However, there are specific types of blister beetles, toxic to livestock, which can contaminate hay. 

University of Wyoming Extension Entomology Specialist Scott Schell discusses the threat blister beetles present for alalfa hay consumers.  

Baling beetles in hay 

The blister beetles species Epicauta swarm on blooming plants to mate, Schell shares. The beetles are often found on alfalfa and other legumes.  

“The female is more interested in eating and the male is more interested in mating,” states Schell, noting this behavior is problematic because the insects are harvested with hay.   

Blister beetles contain a poisonous chemical in their body called cantharidin, Schell says, which serves as a defense mechanism when crushed. The chemical can blister skin and mucous membranes.  

“If the blister beetles are crushed into the hay, even if they fall out before it’s made, it can contaminate the hay,” Schell explains.  

The blister beetle species which cause problems in alfalfa hay utilize grasshopper eggs for food in the larva stage. 

 “In our region, the beetles have a one-year life cycle. The adult female will lay eggs, the eggs hatch into mobile larva where they are then able to search out grasshopper egg pods to feed on,” explains Schell, noting larva will lay dormant for the winter in the grasshopper egg underground and emerge the next year as an adult. 

 “Their population surges when grasshopper populations are high,” says Schell. “In years following grasshopper outbreaks, producers have more problems with blister beetles because more have successfully found egg pods, survived and beat the cycle.” 

Beetles to watch for 

There are four blister beetles in the region to be on the lookout for, Schell shares. The striped blister beetle is the most toxic, as it contains a greater amount of cantharidin in the body.  

“No specimens have ever been collected in Wyoming, but there’s no geographic barrier to keep them from entering the state from Nebraska or South Dakota,” Schell says.  

There are two gray-colored species of blister beetles which are rated with an intermediate toxicity. Schell shares, one species is spotted and the other is plain.  

“This toxicity rating means it would take more beetles of this species to kill a horse than it would for the number of striped blister beetles,” Schell says. 

The black blister beetle is rated the least toxic of all. This species is most abundant in late summer, and are often found congregating on foreign weeds such as goldenrod. 

“It is relatively common, and I have had them submitted from hay that was fed to horses in Wyoming,” Schell says, noting he was unsure of the hay’s origin.  

Toxicity to horses 

Since there are four different specimen of blister beetles which all have different levels of cantharidin, Schell shared results from a toxicity study conducted with striped blister beetles – the most toxic of the four.  

“It has been shown in some scientific studies the amount of cantharidin necessary to kill a horse is contained in 120 of the striped blister beetles,” explains Schell. “The striped are two to three times more toxic than gray beetles, and five times more toxic than black blister beetles.” 

He continued, “It can take quite a few blister beetles, but no amount of the cantharidin is safe to consume. Cantharidin is a potent blistering agent.”  

Schell recommends keeping the risk of feeding blister beetle-infested hay in perspective, as there is the risk of mold contaminated hay, botulism poisoning, poisonous weeds and the ingestion of twine or net wrap. 

“Horses are more sensitive to the toxin than other common classes of livestock, but it’s not good for any of them,” Schell shares.  

Schell explains how hay is fed to different classes of livestock can cause differences in the sensitivity to the toxin. Most horses are fed in mangers and will lick the mangers clean, he notes, whereas cattle usually get their feed unrolled to them.  

“Cattle jostle around the hay, which can make blister beetles fall out and there is less chance of one cow getting an entire lethal dose,” he continued. “Whereas, if one flake of hay was infested with a mass of blister beetles and it was thrown into a manger, the containment increases the horse’s chance of exposure.”    

Reducing contamination 

Grasshopper control is one way hay producers can fight blister beetles, Schell shares. Fewer grasshoppers mean fewer egg pods provided to blister beetles for reproduction.  

“Weed control around alfalfa fields can also help reduce the amount of blister beetles that may move into the field because they don’t have the initial flowering weeds to attract them,” explains Schell.  

“The best thing producers can do is to time their harvest when there is very little bloom in the crop, especially for horse hay. The risk is greatly reduced because there is nothing to attract the blister beetles into the field,” Schell notes.  

Schell states reducing crushing of the hay is another key practice to reduce contamination. Opening the crimpers or conditioners on swathers can help, he shares. Any way producers can avoid crushing will be most beneficial so beetles do not release the toxic chemical.  

Information in this article was sourced from a University of Wyoming Extension seminar dated Feb. 2.  

Delcy Bayles is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to 

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