August 20, 2002
Increased Blister Beetle Numbers Could Spell Trouble for Horses
LINCOLN, Neb. — Alfalfa growers need to be on guard this summer and early fall for blister beetles, University of Nebraska specialists said.
Since blister beetle larvae feed on grasshopper eggs, this summer's grasshopper invasion may be causing blister beetle populations to reach toxic levels in alfalfa through mid-September, said John Campbell, entomologist at NU's West Central Research and Extension Center at North Platte. This is of particular concern if the hay will be fed to horses.
Although blister beetles appear in Nebraska alfalfa fields every summer, populations usually aren't high enough to cause problems, said Bruce Anderson, NU forage specialist.
"There is no sampling method that can detect toxic levels of blister beetles in cured hay," Anderson said. "The best way to deal with blister beetles is through management practices that keep fields from being attractive to beetles."
Blister beetles feed on alfalfa leaves and flowers and occasionally feed on soybeans, goldenrod and musk thistle, Campbell said. Animals are poisoned by ingesting dead beetles in cured hay.
Blister beetles produce a chemical – cantharidin – to which horses are especially susceptible. Cantharidin remains toxic in dead beetles. Even a small amount can cause discomfort and colic in horses, Campbell said. Small doses also can reduce milk production and weight gains of cattle and sheep. In people a crushed beetle causes blistering of the skin – hence its name.
In horses cantharidin can cause inflammation, frequent urination, straining, elevated temperature, depression, increased heart rate and respiration, dehydration, sweating and diarrhea.
"Since animals can die within 72 hours, it is imperative to contact a veterinarian as soon as blister beetle poisoning is suspected," Anderson said.
Cantharidin concentration varies with the beetle species and sex. The chemical is produced by males and some is passed to females during mating.
In Nebraska, the three striped, gray and black blister beetles are most common, Campbell said.
The striped blister beetle contains five times more cantharidin than the black blister beetle. It takes about 25 striped blister beetles to kill a 275-pound horse and more than 100 to kill a 1,200-pound horse. It would take 250 and 1,100, respectively, of the less toxic black blister beetles to be fatal at the same two animals weights.
Anderson said timely harvest can reduce beetle numbers.
"Since blister beetles are attracted to flowers, alfalfa needs to be cut on a schedule that keeps it and weeds from producing flowers," Anderson said. "Growers need to check fields for blister beetles before harvest."
A sweep net, which is used to sample for other insects, may not work with blister beetles because large groups of swarming beetles can concentrate in small areas of a field, Anderson said.
"The worst thing that can be done is to crimp or crush hay if beetles are present," Anderson said. "Since blister beetles tend to swarm, crushing can deposit many dead beetles into a single flake of hay."
Sickle bar mowers and some disk mowers can lay the hay down without crushing it. The beetles will then crawl out of the hay and leave as it dries.
Other management tips Anderson recommends:
– Do not drive on hay shortly after cutting because tires can crush the beetles.
– Avoid using outer edges of fields for horse hay, especially if adjacent to weedy strips.
– Use a short residue insecticide around field margins; follow directions on harvest intervals.
– Don't use a hay conditioner when harvesting blister beetle-infested alfalfa.
08/20/02-SA
Bruce Anderson - Ph.D
Agronomy
Professor
(402) 472-6237
Jack Campbell - Ph.D.
Entomology
Professor and Extension Specialist
(308) 696-6702
Sandi Alswager Karstens IANR News and Photography (402) 472-3030
Department: Agronomy & Horticulture|Entomology
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