A “fairly large” blister beetle outbreak in West Texas’ Reeves County has a Texas AgriLife Extension Service entomologist warning alfalfa growers and buyers to be on the lookout for the toxic insects.

“The outbreak could lead to statewide implications, since alfalfa from West Texas is shipped all over,” says Mark Muegge.

The beetles contain cantharidin, a blistering agent that causes severe health issues and risk of death in livestock, especially horses. Dead blister beetles are just as toxic as live ones and remain so for months, ruining any hay they are in.

“Research has shown a lethal dose in horses to be 0.5-1 mg of cantharidin per pound of body weight,” he says. “Because the toxin varies considerably among the blister beetle species, it’s hard to figure how many beetles must be eaten to kill a horse. Cantharidin concentrations average from about 0.4 to 5.2 mg per beetle in some of the blister beetles commonly found in West Texas alfalfa. So a healthy 1,200-lb horse would have to eat about 115 of the most toxic beetles for death to occur.”

Outbreaks usually are worse from early June through August, says Muegge.

“Since dead beetles are still lethal, killing them isn’t the answer.” The immature stage of the beetle, he adds, eats grasshopper eggs – a good thing in years like this when most of the state is overrun with hoppers.

“The trick here is to keep the beetles happy, healthy and alive, so they’ll move on out of the alfalfa and take their toxin with them.” Since blister beetles are a summer problem, growers producing horse hay should cut early before the pests arrive or late when they’re gone, the entomologist says.

“It’s also a good idea to keep alfalfa fields and areas around the fields weed-free. Cut and bale alfalfa when 5% or less of the plants are blooming, because the flowers are what attract the beetles. And consider using a self-propelled mower-windrower without crimper or conditioning rollers that crush the hay. Studies have shown these kill far fewer beetles, thus keeping them out of the hay.”

Spraying pesticides around field margins can sometimes be effective. But Muegge cautions against spraying the crop itself – again because the dead beetles could be baled into the hay.

For more information, read our stories, "Keep Blister Beetles Out Of Hay," and "Blister Beetle Guide Online,"