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Lee Standley: Horse owners beware of blister beetles in alfalfa hay shipments

All horses require long-stemmed roughage in their daily diets.

Some horse owners opt to fill this need with alfalfa hay. Since this area is not generally known for alfalfa production, most will be trucked in from other locations.

As in past years, horse owners are forewarned to inspect alfalfa for deadly blister beetles before feeding. Also, make sure that you ask a few questions before buying, such as where was it grown and if it was cut during the early, mid- or late season.

Blister beetles sometimes enter alfalfa fields and can end up in the harvested product. These beetles contain a toxin called cantharidin. Although the beetles are dead, the cantharidin in their bodies is very stable and when consumed presents a hazard to horses. This toxin severely irritates the horse’s digestive tract and often causes colic and can be deadly within 72 hours of consumption. Although the exact number of beetles required to produce toxic reactions in horses is variable, ingestion of just 15-20 beetles can cause illness in horses.

The larvae of blister beetles feed on grasshopper eggs. Therefore, the populations of blister beetles tend to correlate with the populations of grasshoppers.

There are very few acres of alfalfa in this part of the state. If the hay was grown here, careful handling by the producer can ensure a safe supply. However, most alfalfa has been trucked in, and the consumer would be advised to ask the supplier where it was produced and if grasshoppers and/or blister beetles have been detected there.

A quick call to the county extension office in the county in which it was produced or call to the state’s department of agriculture can give you a perspective on whether there is a cause of concern. It is generally thought that the early cuttings of hay (first or second) are reasonably safe, depending on the climatic conditions of that production year.

Information from Oklahoma indicates adult blister beetles typically do not begin to emerge until after the first cutting (late April or early May). For this reason, many believe the first cutting has the lowest risk of contamination of blister beetles. Also, blister beetles become inactive in the early fall making cuttings at this time a lower risk.

Horse owners feeding alfalfa should be able to recognize this insect. The most common species in the U.S. are the striped, gray, spotted and black blister beetles.

The striped blister beetle is the biggest threat and can be easily identified by long yellow and black stripes down the length of its body.

Spraying insecticides does not help; even the dead beetles can cause injury. Many producers will cut fields without conditioning or running hay through a roller crimper during suspect periods. This allows the beetles to leave the field before the hay is baled. Alfalfa is high-quality forage and serves as a great source of roughage for horses, so don’t overlook its feeding value, just be aware of the beetle.


LEE STANDLEY is County Extension Agent — 4-H and Youth Development for the Texas AgriLife Extension-Denton County. He can be reached at 940-349-2880.