Examine Hay for Toxic Striped Blister Beetles

Examine Hay for Toxic Striped Blister Beetles

Striped blister beetles range in length from 1/2 inch to 1 inch, and are easily recognized by their characteristic stripes and shape and prominent “neck” area.

Photo: University of Missouri Extension

Striped blister beetles, which can be toxic to horses, are being seen in high numbers in alfalfa in some areas of the state, said University of Missouri (MU) Extension entomologist Wayne Bailey, MS, PhD.

The beetles produce a compound called cantharidin that remains toxic in alfalfa hay for at least four to five years after harvesting. Adult beetles generally do not appear in the first cutting of alfalfa. Risk exists mostly in second and third cuttings.

Beetles typically appear in alfalfa, soybean, and weed patches in July and August after emerging from the soil. They range in length from 1/2 to 1 inch. They are easily recognized by their characteristic stripes and shape and prominent “neck” area.

Bailey said striped blister beetle—which earn their name because they can cause blisters on the skin of humans and in the mouths of animals—problems often appear following years with large numbers of grasshoppers. This happens because at an immature stage, striped blister beetles feed on grasshopper egg pods in the soil.

“There will be high numbers of striped blister beetles after a year of high grasshopper numbers,” Bailey said.

They move quickly in packs to protect themselves and to mate. “They drop to the ground as a protective behavior,” Bailey said, and they scurry when they perceive a threat.

Blister beetles produce the oily, caustic cantharidin, which can cause animals to become sick or die. Bailey said studies from university researchers indicate that it takes between 25 to 225 striped blister beetles consumed in a 24-hour period to be lethal, depending on the size of the horse.

Signs of poisoning vary greatly, said Tim Evans, DVM, MS, PhD, Dipl. ACT, ABVT, veterinary toxicologist with the MU College of Veterinary Medicine. Signs include sores of the mouth and tongue, and abnormal breathing with jerking of the diaphragm. Horses might try to relieve the burning sensation by putting their muzzle and lips in water and blowing bubbles in the water, and they also might paw and stretch often to reduce abdominal discomfort.

Affected animals also might urinate more often than normal, and urine could be blood-tinged. Diarrhea might contain blood, mucus, or sloughed intestinal lining.

If you see signs of poisoning, consult a veterinarian and stop feeding the hay immediately, Evans said. A veterinarian can correct electrolyte abnormalities, provide supportive care, and help reduce the animal's pain.

Beetle populations can be controlled with foliar application of insecticides, Bailey said. Application guidelines are available online. http://ipm.missouri.edu.

MU Extension forage specialists Rob Kallenbach, MS, PhD, and Craig Roberts, MS, PhD, recommend the following management practices to lessen horses' risk of blister beetle toxicity:

  • Feed horses with first-cutting alfalfa, which is usually free of striped blister bugs, Kallenbach said. Pure alfalfa stands that are flowering attract beetles most.
  • Cut late-season alfalfa when 10% or less of the legume is in bloom, and keep alfalfa free of weeds.
  • Avoid use of crimpers and conditioners, which crush hay and promote drying, Roberts said, and avoid running tires on windrows.
  • Scout frequently. Not all fields, even on the same farm, will have beetles, Kallenbach said. Quietly walk through the field the day before harvesting and apply insecticide if needed, Bailey said. Noise causes the beetles to drop to the ground to hide.
  • Horse owners should inspect hay before buying and check alfalfa for the presence of blister beetle prior to feeding. Evans cautioned that crushed beetles are generally unevenly distributed through contaminated hay, so it's important to inspect each serving of hay before feeding it to your horse.
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