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Scout for striped blister beetles

Toxins are deadly to horses.


Linda Geist
University of Missouri Extension
Phone: 573-882-9185

Photo available for this release:

Striped blister beetle.

Credit: Clemson University - USDA Cooperative Extension Slide Series,

Published: Wednesday, July 23, 2014

Story sources:

Wayne C. Bailey, 573-864-9905Craig A. Roberts, 573-882-0481Robert L. Kallenbach, 573-882-6385Tim J. Evans, 573-884-9270

COLUMBIA, Mo. – 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 Extension entomologist Wayne Bailey.

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 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 problems 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.

Striped blister beetles move quickly between fields of soybean and alfalfa, so scout alfalfa frequently to determine pest numbers, Bailey said. Damage to alfalfa is minimal, but 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, 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 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 quit feeding the hay immediately, Evans said. A veterinarian can correct electrolyte abnormalities, provide supportive care and help reduce pain of the animal.

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.

Control beetle populations with foliar application of insecticides, Bailey said. Application guidelines are available at

MU 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, Kallenbach said. 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, Roberts said. 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 purchase only first-cutting hay. Inspect hay before buying and check alfalfa for the presence of blister beetle at time of feeding.