Blister beetles are one-half to one inch. long and have comparatively
soft bodies. The head is broad and vertical. The section of
the body between the head and the wings (prothorax) is distinctly
narrower than the wings, and usually is slightly narrower than
the head. Thus it appears that the insect has a neck. The wing
covers are soft and flexible, and the legs are comparatively
long. Striped blister beetles are about 5/8 inch long and one-fourth
as wide. They are gray to brown with yellow stripes running
lengthwise of the wing covers. The ash-gray blister beetle is
about 1/2 inch long and is completely gray. The black blister
is about 1/2 inch long and is solid black. The margined blister
varies from 5/8 to 1/2 inch long and is black with a gray to
cream band around the edge of each wing cover.
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Figure 1. Margined Blister Beetle Adult
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2. Black Blister Beetle Adult
Blister beetles lay their eggs in masses in soil where grasshoppers
normally deposit eggs. The newly hatched larva searches for
a grasshopper egg pod. Upon finding one, it chews its way into
the rod and begins to feed. With each molt, its legs become
less distinguishable. At the last molt, the larva again becomes
active, crawls out of the grasshopper egg pod, and pupates in
the soil. In 10 to 20 days it emerges as an adult. These insects
winter primarily as almost mature larvae. There is probable
only one generation a year.
The larvae of the common blister beetles are beneficial, since they
destroy grasshopper egg pods. Heavy infestations often occur during
or just following a grasshopper outbreak. The black blister beetle
adults are largely pollen feeders, but they also feed on alfalfa
blossoms. The margined and ash-gray blister beetle adults feed primarily
on the flowers and blossoms of plants, but they may also feed on
the leaves. Striped blister beetles often appear in great swarms
and seem to concntrate on particular rows. In solid plantings, they
strip plants over a circular area. This species usually damages
vegetables more than forage crops. Heavy populations of blister
beetles in green-chopped forages have rendered the feed unplantable
to cattle. Blister beetles have an oily substance, cantharidin,
in their body fluide that causes large blisters t oform when a beetle
is crushed on a person's skin. Unless properly cared for, these
blisters can become infected.
Blister beetles rarely cause economic damage to alfalfa, but
they can cause problems as a contaminant in baled hay. Blister
beetles contain an oily, caustic substance in their body fluids
called cantharidin that helps protect them from natural enemies.
Cantharidin is toxic and can severely injure livestock, particularly
horses, when beetles are ingested with the hay. In fact, the
beetle itself does not have to be ingested; hay contaminated
with the body fluid of crushed beetles can be equally dangerous.
The chemical irritates the stomach lining, small intestine,
bladder, and urinary tract and reduces the calcium level in
the blood. Horses that have ingested cantharidin may exhibit
signs of colic, including excessive salivation, sweating, cramps,
and urinary straining; a fatal dose will include fever, depression,
shock, and death.
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3. Striped Blister Beetle Adult
Cantharidin concentration in beetles varies by species. Some species
may have 50 times more cantharidin than others. In addition, horses
differ in their sensitivity to cantharidin. These variables, plus
certain aspects of blister beetle behavior, make it difficult to
establish strict guidelines for determining thresholds in hay. Some
research suggests that as few as 5 to 10 beetles, when ingested,
could cause severe injury and death in horses, but other research
indicates that 30 or more beetles would be required.
Some blister beetle species tend to aggregate in clusters in a
field, while others do not. Aggregating beetles may cause more problems
because a few bales could contain many beetles, making contaminated
bales much more toxic. Because of these variables, many horse owners
require that the hay they purchase be blister beetle free. Cases
of cantharidin poisoning in horses are rare in Illinois, but that
is no solace to a horse owner who suffers a loss. The following
steps will help to avoid a poisoning.
- Use first-cutting hay to feed horses. Nearly all blister beetle
species will still be immature during the first harvest of hay.
Most adult beetles will die by late September, so the last cutting
also should contain fewer beetles.
- Harvest later cuttings of hay while the alfalfa is still in
the vegetative stage. Research conducted in Kansas indicated that
significantly higher blister beetle densities were found in bud-
or bloom-stage alfalfa.
- Scout alfalfa for blister beetle infestations before taking
the second, third, and fourth cuttings. Sweep several sites (10
to 20 or more), especially in alfalfa near field borders, ditches,
and weed spots. If blister beetles are present:
- Cut hay without crimping or conditioning so that blister beetles
are not killed, and leave wind-rowed hay as it is drying. This may
not be very practical for most operators, but it has been shown
to reduce the presence of dead blister beetles in hay.
- Do not feed this hay to horses.
- Consider the application of an insecticide. Carefully read the
label for preharvest restriction guidelines and other instructions.
- It is sometimes suggested that the purchaser inspect hay before
feeding. This may be unrealistic for most horse owners because it
requires large amounts of time to thoroughly inspect the hay for
- Horse owners who buy alfalfa hay should purchase only first-cutting
hay. If later cuttings must be purchased, request that the hay suppliers
follow the steps outlined previously.
If you suspect that your horse has been poisoned by blister beetles,
contact your veterinarian immediately. Treatment will often include
administration of activated charcoal and a saline cathartic, fluids,
and mineral oil. A veterinarian also will monitor the horse's heart
rate and control diaphragmatic flutter that is often associated
with low blood levels of calcium.--Mike Gray
Susan T. Ratcliffe (firstname.lastname@example.org)
Michael E. Gray (email@example.com)
Kevin L. Steffey (firstname.lastname@example.org)