Black Blister Beetle

Black Blister Beetle Blister beetles get their common name from the irritating reaction of their body fluids with animal skin or delicate membranes. There are more than 300 species of blister beetles in the United States. Their shape, size and coloration vary.

Black 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.

Mouthparts are for chewing. Blister beetle species feed on flowers and foliage of a wide variety of crops including alfalfa, ornamental plants, potatoes, soybeans, garden vegetables and other plants. Immature stages feed on grasshopper eggs, live in solitary bee hives or are predaceous, depending on species. Adults can be found on flowers or infested crops. Care should be taken to not handle them. Never handle blister beetles preserved in alcohol because the cantharadin dissolves in alcohol and will cause blisters on the skin.

Life Cycle : The eggs of striped blister beetle are whitish in color and average about 1.8 mm in length and 0.7 mm in width, though the length ranges from 1.4-2.1 mm. They are elongate oval, with rounded ends. The eggs are deposited in the soil within a tubular chamber at a depth of 3 to 4 cm, normally in clusters of 100 to 200 eggs. The female covers the eggs after oviposition, and hatching occurs in 10 to 16 days. Eggs can be found from June through September in Arkansas. Females are reported to produce several hundred eggs in captivity, though this estimate may be low because many closely related species produce 2000 to 3000 eggs during the life span of 20 to 50 days.

As is typical with blister beetles, the larva initially has long legs and is quite mobile. The legs become reduced in size, however, as the larva develops, and instars 6 and 7 (if present) have small head capsules and do not feed. Newly hatched larvae are whitish in color but soon turn reddish brown with dark brown bands on the thorax and abdomen. The first instar must disperse from the egg mass and locate grasshopper eggs on which to feed. Once locating a grasshopper egg pod, the larva feeds and molts at short intervals.

The mean duration of the first five instars, all of which feed on grasshopper eggs, is 3.4, 1.2, 1.1, 1.6, and 11.3 days, respectively, at warm temperatures. Overwintering also may occur in the fifth instar, followed by pupation in the spring. Alternatively, larvae may display a sixth and seventh instar before pupation, with the sixth instar persisting for about 230 days and the seventh for six to 14 days. Seventh instars are found from April through July at about the same time other individuals are pupating or emerging as adults.

The pupal stage is found in the soil. The pupa greatly resembles the adult in form, though the wings and legs are tightly drawn to the ventral surface. Initially the pupa is whitish in color but becomes darker with maturity. Duration of the pupal stage is nine to 13 days. Pupae are found in May-August.

The adults measure 9 to 17 mm in length and are black and yellow in color. The color pattern varies geographically, but normally two black spots occur dorsally on the head, two black stripes occur dorsally on the thorax, and the elytra each bear two or three black stripes. Northern populations often bear only two black stripes on the elytra whereas southern populations usually bear three black stripes.

The southern coastal plain race, which has three stripes on each elytron, is termed the lemniscate race. Like most other blister beetles, the adult is elongate and slender in form, the thorax is narrower than the head and abdomen, and the legs and antennae are moderately long. The body bears numerous small punctures, and is densely clothed with short hairs. The elytra are long, covering the abdomen but separated or divergent at the tips. The hind wings are transparent.

The adults are most active during the morning and late afternoon, seeking shelter from the sun at mid-day. In particularly hot, arid climates they remain inactive during the day, confining activity to the evening hours. They are easily disturbed, dropping readily from the plant and hiding or scurrying away if disturbed. The preovipositional interval of striped blister beetle is about 20 days, with a 10 day interval between production of egg masses.

Damage : The interest in blister beetles is not due to potential plant damage, but rather the potential injury to horses or less commonly to cattle and sheep if they ingest blister beetles with harvested forage. The bodies of blister beetles contain a lipid soluble blistering agent called cantharidin. Cantharidin causes blisters on skin tissue upon contact.

Horses are particularly susceptible to blister beetle poisoning. Part or all of a horse’s digestive tract can be severely irritated, leading to secondary infections and bleeding. Cantharidin is absorbed and excreted through the kidneys, thus irritation of the kidneys, ureter, urinary bladder and urethra could be followed by secondary infections and bleeding. The substance also lowers serum calcium levels and causes damage to heart muscle tissue.

A cantharidin analysis was conducted on several species of blister beetles commonly found in Colorado and differences were found in amounts among species and sexes. Males produce cantharidin and pass it to females at mating. The minimum lethal dose of cantharidin is about 1 mg/Kg body weight of a horse. The lethal dose for cattle may be as low as 0.5 mg/Kg body weight. Consequently, a few beetles with a high cantharidin level would kill a small horse, but quite a few with a low level would be required to kill a larger horse. The Colorado researchers estimate that it would require 1,700 black blister beetles to kill an 825-pound horse, but only 120 three-striped blister beetles. However, only 40 three-striped blister beetles would kill a 275-pound colt. As little as 4-6 grams of dried beetles can be fatal to a horse.

Signs and symptoms of cantharidin poisoning in a horse may include blisters and ulcers in the mouth, gastritis, esophagitis, edema of the submucosa of the intestine, colic, diarrhea and blood and mucous in the stool. Other signs include frequent attempts to urinate but voiding of little urine and blood in the urine.

The lowered blood serum calcium levels may cause body tremors and a breathing pattern characterized by a periodic jerking contraction of the diaphragm associated with the heartbeat. Poisoned horses may place the muzzle in water without drinking, have an increased temperature, increased pulse and breathing rate, be dehydrated, depressed and in shock. Oral and gastrointestinal ulcerations may be observed in cattle and sheep. If cantharidin poisoning is suspected, consult a veterinarian.

Treatment There is no specific antidote for cantharidin. Treatment is directed toward supportive care. The gastrointestinal tract needs to be decontaminated by activated charcoal and mineral oil. Fluid therapy and bicarbonate need to be used to alleviate shock and acidosis. Calcium also may be needed.

Management: During haying, adults can be crushed and can contaminate the hay, particularly when cutting and crimping occurs in the same operation. When hay is cut and allowed to dry in windrows, adult beetles will usually disperse prior to baling. Alfalfa should be inspected for blister beetles just prior to cutting. The plant canopy should be inspected, concentrating on the young foliage, flower buds, and opened flowers.

In Wyoming, blister beetles do not appear to congregate in large numbers in alfalfa, but sporadic occurrences do occur. Blister beetles are much more problematic in the Great Plains than in the intermountain region. If hay is at risk for blister beetle contamination, and particularly when it is sold for horse feed, insecticide use is justified.

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