I have been getting a few calls and have seen a good many blister beetles the last couple of weeks throughout the county.
Blister beetles are not necessarily uncommon in Texas, but we do not often see large numbers of them, just a few scattered around. But this year’s population appears to be larger than normal.
Adult blister beetles feed on many types of flowers and foliage; they really like legumes like alfalfa.
Immature stages of blister beetles feed on grasshopper eggs, which is something they have had a ready supply of these last two years, hence the larger than normal population. Because the larval stages do feed on grasshopper eggs, they are considered to be beneficial insects. However, to alfalfa producers and horse owners, they are certainly not considered beneficial.
In the adult stage, blister beetles’ bodies contain a toxin called cantharidin. This toxin can cause blisters on the skin if you manage the blister beetles.
Apparently, cantharidin was historically used as an aphrodisiac in ancient times, so there is that. I would certainly not recommend this practice as it seems near death experiences were usually the result. Horses that digest blister beetles or blister beetle parts in contaminated feed or hay will become extremely ill, and in many cases they will die. Most all cases of horses ingesting blister beetles occur in alfalfa. Since blister beetles are attracted to alfalfa and feed on the blooms, they can inadvertently be baled up in the hay intended for horse feed.
The chance of horses ingesting blister beetles in Bermuda grass hay are very small since they do not feed on the grass and have no reason to be there in the first place.
However, if the hay is from a weedy field with many flowering weed species, the risk does go up as these weeds may attract the beetle.
To manage for blister beetles, be sure to buy alfalfa that is guaranteed to be blister beetle free or just use some good weed free Bermuda grass hay from here in the county. While I can find no scientific evidence that blister beetles are attracted to light, I have found more of them around barns that have night lights on all night.
If you have a night light at your horse barn, either turn it off or be sure that feed barrels are sealed and check feed bunks before feeding.
If you have horses grazing in a weedy field, it would be a good idea to shred the weeds; this will remove the blister beetles’ potential feed source.