Blister Beetles in the Pacific Northwest

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Horse owners in many parts of the United States already deal with concerns of blister beetles in alfalfa hay as part of their everyday regimen. In the Pacific Northwest we have been relatively free of that worry–that is until now. Earlier this month the Pacific Northwest & Treasure Valley Pest Alert, a newsletter distributed by the University of Idaho Extension and Oregon State University Extension, identified an outbreak of these poisonous insects in the foothills of Boise, Idaho. Todd Murray an entomologist with Washington State University in Skamania County, Wash., verified this and noted that the pests have also been found in Eastern Washington as well.

Specimens identified by Idaho State Department of Agriculture in the Boise Foothills are the native punctate blister beetle (Epicauta puncticollis.) Blister beetles are associated with grasshoppers and weedy conditions. According to the alert, the blister beetle larvae serve as predators of grasshopper eggs, but adult blister beetles feed on vegetation, occasionally moving to flowering field crops such as alfalfa. That is where the problem can begin for horse owners.

Bodies of adult beetles contain the chemical cantharidin, a toxic chemical that protects them from predation by causing a painful blister when crushed. Cantharidin can be toxic, even lethal, to horses and livestock if ingested. Horses seem to be particularly sensitive to the toxins, especially when crushed beetle body parts are consumed in dried hay. According to Internet sources, Cantharidin affects the gastrointestinal and urinary tracts of horses. Typical symptoms of cantharidin poisoning in horses include abnormal breathing patterns, submerging their mouth in water for long periods of time and restless behavior. Other symptoms are blistering of the mouth, colic, pawing, frequent attempts to urinate, jerking contractions of the diaphragm, diarrhea, discarded intestinal tract lining in the stool, and reduced levels of calcium and magnesium in the blood.

blister beetle

Pacific Northwest native punctate blister beetle. Photo courtesy of the Pacific Northwest Pest Alert, July 17, 2012

Depending on the species of beetle, it is reported that ingesting someplace between several and 100 beetles will kill a horse. The toxin does not break down quickly and hay can remain infested for many years. Ruminants, however, seem to be more tolerant of the toxins.

Blister beetles have long (3/4 to 1 1/4 inch) narrow bodies, broad heads and antennae that are about 1/3 the length of their entire bodies. The front wings are soft and flexible in contrast to the hard front wings of most beetles. The punctate blister beetle occurring in the Pacific Northwest is black, however other blister beetle species range in colors from tan to gray and even stripped.

As mentioned earlier, blister beetles are associated with grasshoppers and weedy conditions. The larva stage of the blister beetle feeds on grasshopper eggs. Perhaps one way to avoid an infestation of blister beetles is to not have grasshoppers or at least not have them in excessive amounts.

Grasshopper control methods might be an option to consider, but unfortunately studies show grasshopper numbers have been on the rise in recent years. Sage-grouse eat grasshoppers, as do many other birds, reptiles and mammals. A biological control protozoan, Nosema locustae, has been primarily used by homeowners for small grasshopper infestations.

There are chemical controls for grasshoppers, such as carbaryl and malathion. These products may also work on adult blister beetles but a disadvantage of using these pesticides is that they also destroy beneficial insects. If you choose this control method contact your local Extension Office or another knowledgeable resource to discuss pesticide handling and use.

Here are blister beetle management precautions (compiled from Extension and Internet sources) which you can implement:

¥ Know your alfalfa supplier. Talk with them to be sure they are aware of the issues and are keeping tabs on the situation.

¥ Ask alfalfa producers what precautions they are taking to avoid the presÂence of blister beetles in forage.

¥ Control weeds and harvest alfalfa before flowering. Blister beetles are attracted to flowering weeds or blooming alfalfa. Weed-free alfalfa, if harvested before bloom, will be less likely to attract beetles into the hay field.

¥ Purchase hay harvested before May or after September. This will not guarantee a lack of problems with blister beetles but will greatly reduce the risk as blister beetles emerge during mid-summer months.

¥ Avoid use of hay conditioners or crimpers if blister beetles are present at harvest time. Blister beetles often congregate in large groups. Because of this behavior, conditioners or crimpers often kill large numbers of beetles in a small area and dead bodies are held in the windrow formed by the conditioners or crimpers. Using a self-propelled harvester with wide set wheels and no conditioner or crimper will prevent most beetles from being killed at the time of cutting and living beetles will move out of the alfalfa as it dries. A sickle bar mower will do the same if the operator avoids driving on freshly cut hay.

¥ Thoroughly scout fields 7 or 8 days prior to harvest. If blister beetles are present, use Sevin (carbaryl) or other recommended pesticides. Check the label, your dealer, or local Extension Office for the suggested application rates. Carbaryl must be applied 7 days prior to grazing or cutting of alfalfa. Other pesticides may have longer or shorter preharvest intervals. The application should kill most blister beetles and allow them to drop to the ground and away from harvest equipment. Care should be taken not to pick up the dead beetles with equipment during the harvesting process. Read and follow all pesticide label precautions and restrictions.

¥ If blister beetles are suspected carefully inspect your hay before feeding, pulling apart and examining each flake for dead blister beetles. If present, do not feed infested hay to horses.

¥ If you live in a heavily infested area consider switching from feeding alfalfa to some type of grass hayÑblister beetles do not feed on grass hay.

If you think your horse may have ingested a blister beetle contact your veterinarian immediately. If you discover blister beetles in your alfalfa, or have a question on identifying blister beetles, contact your nearest Extension Office. For southwestern Idaho, if the beetle is found in significant numbers contact University of Idaho entomologist Jim Barber at the Parma research station at 208/722-6701.

View the full alert and download attached files at Read an interview in the July 19, 2012 issue of the Capitol Press with Jim Barber, entomologist at the University of Idaho’s Parma research station at


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Written by:

Alayne Blickle, a lifelong equestrian and ranch riding competitor, is the creator/director of Horses for Clean Water, an award-winning, internationally acclaimed environmental education program for horse owners. Well-known for her enthusiastic, down-to-earth approach, Blickle is an educator and photojournalist who has worked with horse and livestock owners since 1990 teaching manure composting, pasture management, mud and dust control, water conservation, chemical use reduction, firewise, and wildlife enhancement. She teaches and travels North America and writes for horse publications. Blickle and her husband raise and train their mustangs and quarter horses at their eco-sensitive guest ranch, Sweet Pepper Ranch, in sunny Nampa, Idaho.

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