Blister Beetles Could Pose Risk

Blister beetles could be considered a beneficial insect as they feed on grasshopper eggs, however, they pose a significant risk. Blister beetles are named for a toxic in their body called cantharidin that causes blisters when it comes in to contact with the skin. This cantharidin is released when beetles are crushed, making it a concern for not only humans, but also for livestock producers, as hay containing blister beetles can be toxic to livestock.

Blister beetles feed on crops including sugar beets, canola, potatoes, and alfalfa. Defoliation in these crops caused by blister beetles is rarely economically damaging. The main concern is when alfalfa hay containing blister beetles is fed to livestock, specifically horses. There are many species of blister beetles that occur in Montana with the most common being the Black, Spotted, and Ash Gray. While these species are toxic, the most toxic species, the Striped blister beetle, has not yet been reported in Montana. It has been documented in surrounding states, however, so it is only a matter of time before it makes its way here.

When hay containing blister beetles is consumed, sors and blisters can develop in the mouth, on the tongue, esophagus, and stomach. Kidney and heart function may also be impaired, and in severe cases death may occur. Horses are the most sensitive to blister beetles. Research has indicated that it could take as many as 200 blister beetles to kill an adult horse, or as few as 30 to 50 blister beetles in the case of the Striped blister beetles. Although they are less susceptible than horses, sheep and cattle can also be affected by blister beetles. However, little information exists on toxic numbers for these species.

It is important to scout for blister beetles in your alfalfa prior to cutting. Because the toxin is still released in dead beetles, insecticides may not be the best option as the dead beetles can get hung up in the hay and still cause a threat. Blister beetles are attracted to the alfalfa blooms, making early harvest prior to the bloom stage an option to reduce the potential of beetle infestations.

If you have any questions or if you see any Striped blister beetles, please contact the MSU Richland County Extension Office at 406-433-1206 or [email protected].