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NDSU Morton County: Scout for blister beetles

NDSU Morton County: Scout for blister beetles

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This year’s dry weather is increasing the blister beetle threat to cattle and other livestock.

“Blister beetles tend to be more numerous when grasshopper populations are high because the immature stage of blister beetles feeds on grasshopper eggs, and we have higher numbers of grasshoppers in drought years like 2021,” NDSU Extension entomologist Jan Knodel says.

Blister beetles are a half-inch to 1½ inches long with soft bodies, short wing covers, long legs and a neck-like appearance between head and prothorax. They can be ash-gray, black, brown or striped. Most blister beetle species produce one generation per year. They lay their eggs in soil from late summer into early fall and overwinter as larvae. They typically emerge in early to midsummer. Blister beetles are very mobile and tend to congregate in large numbers in small areas of the field. Adult blister beetles are attracted to blooming alfalfa fields and weeds. Adults are active June through September and will feed on nectar and pollen, and devour leaves, stems and flowers.

“Blister beetles produce a poison called cantharidin, which is toxic to people and livestock, especially horses, but cattle and sheep also can be poisoned,” Knodel says. “Cantharidin oil is released when beetles are crushed, and even dead beetles have high levels of the toxin.” Symptoms of toxicity in horses include sores on the tongue and in the mouth, depression, colic, sweating, diarrhea, blood in the feces and frequent urination. It is estimated that about 30 to 50 blister beetles could be potentially lethal to horses. Cattle also may exhibit sores in their mouth. There is no cure for cantharidin poisoning except for supportive care which includes mineral oil, IV fluid therapy, activated charcoal and anesthetics.

Hay infested with blister beetles is a big concern for hay producers and livestock owners. Hay fields adjacent to rangeland pastures are at higher risk for blister beetle infestations due to typically higher grasshopper populations in rangeland pastures. Blister beetles have been found in Morton County.

Some ways to reduce the threat of blister beetles include:

  • Scout closely for blister beetles, swarms or defoliation immediately prior to harvest. If large populations of blister beetles are observed, producers should not harvest until beetles have moved out of field or an insecticide may be applied. Fields should not be treated at peak bloom to avoid bee kill.
  • Control blooming weed hosts near or in alfalfa field.
  • Cut alfalfa early at 10% bloom. Blooming alfalfa attracts blister beetles into the field.
  • Recheck fields 24 hours after cutting to ensure that new swarms of blister beetles have not re-infested the fields.
  • Use equipment without hay conditioners or crimpers. Cut hay with a sickle bar or rotary mower that allows blister beetles to move out of hay after cutting. If large numbers of blister beetles are observed in spots during harvesting, stop tractor and allow blister beetles to move out of the way or go around them.
  • Raking may dislodge dead beetles from hay.
  • Scout harvested hay and underneath windrows closely for blister beetles and allow blister beetles to move out of drying hay before baling. Turning the windrow may be helpful to get blister beetles to move out.
  • If blister beetles are suspected in harvested hay, either don’t feed the hay or provide alternative feeds/hay for livestock. Don’t force livestock to eat hay contaminated with blister beetles.
  • Grinding hay only dilutes the cantharidin toxin when the hay is mixed into a final ration.

Upcoming NDSU Extension events

Call 701-667-3342 for help gaining access to virtual events or visit for more information.

• Tuesday - Positive Discipline at First Lutheran, 6:30-8 p.m. Tuesdays for six weeks.

• Wednesday - Circle of Security via Zoom, 10:30 a.m.-12 p.m. Wednesdays for six weeks.

• July 28 – Morton County Plot Tour. Contact Morton County Extension for more details. 

• July 29 - Navigating Drought on Your Ranch webinar, 1 p.m.

Renae Gress is an agriculture and natural resources extension agent with NDSU Extension/Morton County. 


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