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The Weekly News Source for Wyoming's Ranchers, Farmers and AgriBusiness Community

Watch for Blister Beetles in Hay

by Wyoming Livestock Roundup

High populations of grasshoppers on a property or in hay fields are the best indicator to be watching for blister beetles. Hay contaminated with blister beetles or the potential of blister beetles could affect the market and/or value of hay. Here is what hay producers should know about blister beetles. 

Blister beetle is the common name given to members of a family of insects native to the United States which contain the chemical cantharidin as defense against predators. There are more than 300 species of blister beetles in the United States. Many blister beetle species can be found feeding on the leaves and flowers on alfalfa, beans, potatoes and various horticultural plants. 

However, the black, ash-gray and spotted blister beetles are the most common species found in Wyoming’s alfalfa fields. The major concern is when mating swarms of blister beetles contaminate alfalfa hay during harvest. 

Cantharidiasis or blister beetle poisoning, from eating contaminated hay is reported most often in horses. However, sheep and cattle can be affected as well. 

Adult blister beetles of the problematic species are slender, soft-bodied beetles with fairly long legs measuring approximately one- half inch in length. The head, thorax and abdomen are clearly visible. 

Adults of the pest species emerge in late spring to early summer and are most active during the summer months. Typically, first cutting hay in Wyoming escapes blister beetle infestations because their peak swarming activity occurs later in the growing sea- son. This is because the larvae of the pest species that mate and feed on blooming alfalfa must find grasshopper egg pods to develop in, and grasshopper egg pod numbers increase through the summer months. 

Blister beetle populations usually increase within areas that had high grasshopper populations the year prior because their larvae found more grasshopper eggs to survive on. High grasshopper populations in or around a producer’s alfalfa field, even on native rangelands, can indicate a hay producer should be on the lookout for blister beetles the following growing season. 

USDA grasshopper surveys can be found at plant_pest_info/grasshop- per/downloads/hazard.pdf. 

Toxicity levels of blister beetles vary among the different species. The most toxic blister beetle contaminating alfalfa is the three- striped species Epicauta occidentalis. Just 120 of this species of beetle is required to cause mortality to an 800 pound horse. 

In comparison, the black blister beetle requires an estimated 1,700 beetles to cause mortality to the same size horse. Luckily, the three-striped beetle has not yet been documented in Wyoming. The black blister beetle is the most commonly found in the state. 

The spotted blister beetle, being the most toxic of the three species common to Wyoming, requires an estimated 520 beetles to cause mortality to an 800 pound horse. 

So what can a producer do if they have blister beetles in their alfalfa field? 

The best management is to avoid trapping or crushing blister beetles during hay harvest. The risk of including blister beetles in hay can be reduced in two ways. 

The first way would be harvesting alfalfa hay prior to bloom because blister beetles swarm into fields to feed on the blossoms. The second way would be to harvest the hay with equipment that does not have a hay conditioner, such as an old sickle bar mower or dis- engaging the conditioner on a swather. 

Crushing the beetles will contaminate the hay with the bodily fluids, which contain the toxin, even if the beetle carcass does not remain in the bale. This method cuts the hay without crushing or injuring the beetles, allowing them to crawl away and leave the windrow as it dries prior to baling. 

Both methods can be combined to achieve the highest mitigation of blister beetle contamination. 

There are labeled insecticides that are effective at controlling blister beetles, however this is usually not the most effective management strategy. This is especially true with insecticides that have long pre-harvest intervals. 

This interval is the amount of time required between application of the insecticide and the harvesting of the crop and is usually measured in days. During the pre-harvest interval, more of the alfalfa will come into bloom and new populations of blister beetles can re-infest the field negating the insecticide control. 

This hay season, keep an eye out for high populations of blister beetles within alfalfa hay fields prior to and during harvest. Contact a local Weed and Pest Office or University of Wyoming Extension Office for assistance identifying blister beetles. 

Jeremiah Vardiman is an agriculture and horticulture Extension Educator at the University of Wyoming. He can be reached at

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