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Blister beetles are an occasional problem in later cuttings of alfalfa and in other forages such as sainfoin. Numbers may be higher this year due to the high numbers of grasshoppers, because grasshopper egg pods are the larval food source for most species of blister beetles. The most numerous species seen in alfalfa is the black blister beetle, Epicauta pennsylvanica, although the spotted blister beetle E. maculata can also be common. Blister beetles are soft-bodied cylinder-shaped beetles with narrow "necks", generally 1/2-inch long or less. In alfalfa, direct damage to the plant is generally not an issue because blister beetles focus their feeding on pollen and nectar. To some extent they also feed on the flowers themselves, but feeding on foliage is more limited and doesn’t cause significant damage.

However, blister beetles produce cantharidin, a bitter-tasting blistering agent that can harm or kill horses that eat contaminated hay. Sickened horses may suffer from cramping, fever, sweating, diarrhea, and other unpleasant symptoms. Other livestock such as cattle, sheep, and goats are also vulnerable. Blister beetles are “reflex bleeders”, with cantharidin-laced blood that seeps from their leg joints when they are disturbed.

It takes a lot of black blister beetles to actually kill a horse, but it’s difficult to estimate a reliable number. Research done in the 1980s suggested between 550 and 1700 black blister beetles would be required to kill a horse, depending on the size of the horse. Some newer sources suggest that those estimates may be too lenient. The minimum lethal oral dose of cantharidin for horses is thought to be <1 mg/kg body weight. However, cantharidin levels in blister beetles vary by sex (adult males produce the cantharidin and transfer some to females during mating) as well as species, and individual horses vary in sensitivity. These variables, plus certain aspects of blister beetle behavior, make it difficult to establish thresholds for grazing or hay. Other considerations may further affect management decisions. For example, even moderate numbers of blister beetles may make grazing difficult or unpalatable. Some hay buyers have zero tolerance for blister beetles, making even lightly-infested bales unmarketable in some sectors.

First cuttings of hay are considered safe for horses and other livestock because they occur before adult blister beetle populations have built up. Subsequent cuttings can be made before flowering, which reduces the attractiveness of the field. Growers will have to consider their target market and treatment costs to make management decisions. Insecticides such as Sevin 4F or Warrior can be used, but beetles killed by pesticides can still end up in bales, and there are also pre-harvest and grazing intervals to consider. For these reasons, insecticides are not always recommended.

Instead of insecticides, blister beetles can be excluded from baled hay by using haying equipment without crimpers/conditioners. This prevents crushed blister beetles and cantharidin residue from contaminating the hay, and as the windrows dry the surviving beetles will leave.

Fortunately for Montana, we do not yet seem to have a significant number of aggregating species, including striped blister beetle, which can be common in neighboring states. Aggregating species represent the biggest risk because during baling they can end up concentrated heavily in some bales.

Further information about blister beetles in alfalfa:

Black blister beetle. Image: Whitney Cranshaw