Alfalfa hay is an excellent, high quality forage for horses, but it has the potential to attract blister beetles–toxic insects that can kill horses if ingested.

Blister beetles cause inflammation and blistering of the skin within hours of contact. If ingested, cantharidin, a toxic substance in the beetle, is absorbed and rapidly excreted in the animal’s urine, causing inflammation of the digestive and urinary tracts. Horses can suffer severe poisoning from even a few beetles, alive or dead (hay processing can kill the beetles and release the toxin).

If cantharidin poisoning has been diagnosed in one horse, all horses in the barn that are eating alfalfa should be examined by a veterinarian. An individual horse’s sensitivity to the toxin and the amount of toxin consumed can cause variations in the time of onset and severity of the signs of poisoning.

Horse owners might hear that it is safe to purchase alfalfa hay produced in certain areas of the country because these areas do not have blister beetles. However, there are 355 species of blister beetles found throughout the United States, so all alfalfa hay is suspect for blister beetle infestation. The biggest concentration of blister beetles does tend to be in the southwestern states, so you should be especially diligent in being an informed consumer if your hay is coming from this area.

Many owners might think that close inspection of alfalfa hay for the presence of blister beetles before feeding will protect their horses. While inspection of any species of hay before feeding is a good idea, chances of being able to identify beetle pieces and crushed beetles in baled alfalfa are extremely slim.

So, how can the horse owner protect his animals from blister beetle poisoning? The obvious answer is to avoid feeding alfalfa. However, this might not be practical, and it restricts horses from a high-quality forage. Being an informed consumer is a better option. Here are some steps you can take to minimize the risk:

1) Find out when the alfalfa was harvested. Blister beetles mature in July and August, so the first cutting is the safest in terms of reduced numbers of beetles. Later cuttings should be baled before the plant blooms to reduce beetle numbers.

2) Find out how the hay was harvested. Blister beetles often swarm alfalfa fields or sections of fields and move on in a few days. Conscientious hay producers investigate alfalfa fields for the presence of blister beetles before harvest and delay harvest if beetles are present. Beetles tend to concentrate around the margins of the fields, and hay in these areas is driven over repeatedly crushing insects in the hay. Knowledgeable hay growers segregate alfalfa bales along the margins of the field and do not market them to high-value animals such as horses. Also try to avoid harvesting equipment that kills or crushes the beetles, such as mower-conditioners and sickle bar mowers. Insecticides which kill the beetle are not recommended because the dead beetles are still baled in the hay.

Information courtesy of the Alabama Cooperative Extension System, www.aces.edu