The real problem with blister
beetles is their toxicity to livestock, especially
horses, when accidentally eaten in feed. The
beetles produce cantharidin, an irritant that
causes painful blistering when the insects are
handled. The cantharidin remains in the beetle's
body even after it dies.
That becomes a problem when
alfalfa is swathed, during which dead blister
beetles can be incorporated into the hay as it is
made. Thus, livestock can accidentally consume
whole beetles or their squashed remains when the
hay is fed. Blister beetles are slender insects 12
to 19 mm long. They have prominent heads and may
be black with yellow margins or black and yellow
striped.
Blister beetles are found
throughout the continental U.S. and agricultural
areas of Canada. Although fairly common in North
Carolina, they are infrequently pests of
importance. Blister beetles have a wide host
range. Important vegetable hosts include potato,
tomato, melon, eggplant, sweet potato, bean, pea,
cowpea, pumpkin, onion, spinach, beet, carrot,
pepper, radish, corn, and cabbage.
Horses are especially susceptible
to blister beetle poisoning. Consumption of 25 to
300 beetles can kill a mature horse. Cattle and
sheep are much less susceptible, but blister
beetles will reduce digestibility of hay and may
throw cattle off feed. Also, cantharidin is a
stable compound that withstands decomposition even
when it is dried or heated; the hay will keep its
toxicity in storage.
Because the toxin is absorbed and
then excreted with the urine, severe inflammation
of the urinary tract is a common sign of
poisoning. Death may occur within 24 hours, and
treatment with mineral oil by stomach tube is
recommended but not always very effective. If
blister beetle poisoning is suspected, contact a
veterinarian immediately.
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