By Christine O’Reilly, OMAFA Forage & Grazing Specialist and Tracey Baute, OMAFA Entomologist, Field Crops

Warm fall temperatures has many pests still active and hungry, looking for food resources before overwintering or being killed off by a hard frost. We are aware of at least one significant sighting of blister beetles clustering on the lawn of a horse farm and neighbouring grassy pastures in Kawartha Lakes. Not knowing if this is an isolated incidence, scouting before cutting or putting horse and ruminants out to pasture is advised. Blister beetles especially enjoy legume and solanaceous plants (examples: potatoes, tomatoes, and nightshades), but will feed on many others.

Though blister beetles feeding is not a major risk to the crops they feed on, they contain a burning and poisonous agent called cantharidin, which is released from their joints when they are startled, pestered or squished. Cantharidin causes blisters to the skin on contact and inside the digestive tract if eaten. Horses are particularly sensitive to cantharidin, though ruminants are also affected. Symptoms of blister beetle poisoning include blistering on the skin (especially around an animal’s lips), mouth ulcers, colic, diarrhea, bloody feces and bloody urine. If enough cantharidin is ingested, death may occur within 72 hours.

The three most common species are the black blister beetle (Epicauta pensylvanica), the ash grey blister beetle (Epicauta fabricii) and the striped blister beetle (Epicauta vittata). They range from 1 – 2.5 cm in length and are cylindrical shaped with their abdomens extended out further than their wings. They are most abundant in July and August but can be found as adults any time from May to October. They move around in swarms, aggregating, especially along field edges.

First cut hay is typically the safest for avoiding blister beetles. Given the poor weather conditions at harvest, there may be issues sources “horse quality” first cut hay this year. Some stables may be relying on a third cut this fall to meet their forage needs. Scout for blister beetles before cutting or letting horses out to pasture, focusing on the field edges and any areas with flowering weeds. If blister beetles are found, delay hay harvest until the beetles have left the field. If the field has already been cut, thoroughly inspect the cut hay and the ground for any signs of alive or dead beetles. Any dead beetle carcasses or body parts left on feed plants or in cut hay can pose a risk to livestock, particularly horses or those handling the feed.  Avoid crimping the crop, as crimpers can crush and kill blister beetles in the hay. Segregating field perimeters from the centre of the field when harvesting may reduce risk. Be aware that hay from the headlands is higher risk: equipment frequently turning around in the perimeter areas may crush or carry beetles into the cleaner centre of the field. Swathing without crimping may allow some of the blister beetles to leave the field before baling.

For more information on blister beetles, refer to the OMAFA Publication 30, Guide to Forage Production. Dr. Erin Hodgson, Extension Entomologist at Iowa State University Extension and Outreach has a very thorough video about blister beetles, impact to horses, scouting techniques and considerations when harvesting hay.