Surge in striped blister beetles a potential horse threat – specialist

The striped blister beetle. Photo: Clemson University - USDA Cooperative Extension Slide Series,
The striped blister beetle can be toxic to horses. Photo: Clemson University – USDA Cooperative Extension Slide Series,

Striped blister beetles, which can be toxic to horses, are being seen in high numbers in alfalfa crops in some areas of Missouri, an insect specialist says.

University of Missouri Extension Entomologist Wayne Bailey says 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 cuts.

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

Bailey said striped blister beetle problems appeared after 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.

Striped blister beetles move quickly between fields of soybean and alfalfa, so he urged those with alfalfa cops to scout it frequently to determine pest numbers.

Damage to alfalfa is minimal, but the risk to horses increases when large numbers of beetles are found in small areas of a field.

They earn their name because they can cause blisters on the skin of humans and in the mouths of animals.

The oily, caustic cantharidin in striped blister beetles 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.

Striped blister beetle problems are not new to Missouri. Problems mainly occur due to changes in harvesting equipment and methods.

Signs of poisoning vary greatly, said Tim Evans, a veterinary toxicologist with the university’s College of Veterinary Medicine. Signs include sores of the mouth and tongue, and abnormal breathing with jerking of the diaphragm.

Horses may try to relieve the burning sensation by putting their muzzle and lips in water, and blowing bubbles in the water. They also may paw and stretch often to reduce abdominal discomfort.

Animals may urinate more often than normal and urine may be blood-tinged. Diarrhea may 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.

Crushed beetles are unevenly distributed through contaminated hay. Two horses can eat from the same bale and one may be poisoned while the other is not, Evans said.

Bailey said beetle populations can be controlled with foliar application of insecticides.

University of Missouri Extension forage specialists Rob Kallenbach and Craig Roberts recommend the following management options:

  • Feed horses with first-cutting alfalfa, which is usually free of striped blister bugs. Pure alfalfa stands that are flowering attract beetles most.
  • Cut late-season alfalfa when 10 percent or less of alfalfa is in bloom. Keep alfalfa free of weeds.
  • Avoid use of crimpers and conditioners, which crush hay and promote drying. Avoid running tires on windrows.
  • Scout frequently. Not all fields, even on the same farm, will have beetles. Quietly walk through the field the day before harvesting and apply insecticide if needed. Noise causes the beetles to drop to the ground to hide.
  • Horse owners should purchase only first-cut hay. Inspect hay before buying and check alfalfa for the presence of blister beetle at time of feeding.

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