by Lowell D. Breeden and Robert J. Bauernfeind
Kansas State University Cooperative Extension Service
Insects in the family Meloidae are commonly referred to as blister or oil beetles. Different species vary in size and color, but most are easily recognized by their elongate, narrow, cylindrical and soft bodies. When viewed from above, blister beetles have a “necked appearance” which is due to a constriction of the back of the head capsule and its attachment to the narrowed anterior end of the thorax. While adult blister beetles generally feed on plant foliage, the larvae of some common species are associated with, and feed on grasshopper eggs.
Adult blister beetles are seen after they have emerged from the soil (in early to mid-summer) and have begun their foraging activities. Blister beetles have a complex life cycle which is characterized by several different immature forms. Their life cycle is illustrated in the following description for the striped blister beetle, Epicauta vittata, a species closely related to the common 3-striped blister beetle found in Kansas alfalfa fields. Striped blister beetles, as is true with other common species, have only one generation per year.
During the summer, clusters containing up to 100 eggs are deposited in the ground. Within two weeks, the eggs hatch, and the tiny larvae make their way through the soil searching for food, primarily grasshopper eggpods (these triungulin larvae have legs designed for active movement). In about a week, this form gives rise to second stage larvae and within a month every insect will have gone through three more stages, each of which is more sedentary in its habits. By the time they are ready for overwintering, the larvae (now called Pseudopupae) have lost their legs, and have developed a thickened skin enabling them to withstand adverse weather conditions. They remain in this stage for over seven months until moisture and temperature conditions in the late spring allow for the development of the final immature larval stage. Shortly thereafter, they enter the final immature stage (pupal). In an additional two weeks, new adults emerge (June and July), begin feeding, and lay eggs for the next generation of blister beetles
Blister beetles occasionally cause localized areas of damage within soybean and alfalfa fields. However, the significance of damage to these crops is questionable. This is because the gregarious nature of the more commonly occurring three-striped, clematis (gray), spotted and/or black blister beetles limits the area attacked (beetles feed and then move about or exit the field) and because soybean and alfalfa plants can compensate for substantial foliar losses.
It is in this area that blister beetles may become a major concern. The bodies of blister beetles contain a substance called cantharadin. This chemical is an irritant capable of causing the formation of blisters upon those body tissues exposed to the chemical. Livestock may come into contact with blister beetles via the consumption of alfalfa hay containing dead beetles.
Horses are especially susceptible to blister beetle poisoning. Prior to the incorporation of crimpers, crushers, and conditioners on swathing equipment, blister beetles had ample time to walk out of windrowed hay prior to its drying and being baled. Now, however, many of the beetles are crushed or killed in the swathing process and thus are likely to be included in hay bales (the cantharadin remains intact despite the fact that the beetles are dead. Also drying and heating do not effectively degrade the chemical).
Portions of, or the entire horse digestive tract could be severely irritated. Secondary infections and bleeding may occur.
Cantharadin is absorbed and excreted through the kidneys, thus irritation of the kidney, ureter, urinary bladder, and urethra could be followed by secondary infections and bleeding.
For unknown reasons, calcium levels in horses may be drastically lowered and heart muscle tissues destroyed.
There are some signs and symptoms of the poisoning to watch for. Gastrointestinal associated signs and/ or symptoms may include blisters and ulcerations in the area of the mouth, colic, diarrhea, and blood and/ or discarded intestinal tract muscosal linings in the stool. Signs associated with disorders of the urinary system include frequent attempts to urinate, the voiding of only small amounts of urine, possibly pain while urinating and blood in the urine. Lowered calcium levels (hypocalcemia) may result in body tremors and a peculiar calcium-deficient associated breathing pattern characterized by a periodic jerking contraction of the diaphragm synchronous with the heart beat (synchronous diaphragmatic flutter).
Other signs or complications associated with any of the aforementioned disorder(s) may include the placing of muzzles in water without drinking, congested mucal membranes, elevated temperatures, increased pulse and breathing rates, dehydration, depression and shock.
There is no precise way of determining how many blister beetles it would take to kill a horse. Obviously the size and general overall health status of an individual horse would somewhat determine the number of beetles necessary to adversely affect that horse. Also, the amount of cantharadin per beetle would vary depending on the size of the species of beetle ingested, and even the cantharadin content within individual beetles of the same species may vary.
The number of blister beetles ingested in relation to the size of the horse seemingly would regulate the time interval between ingestion and sign-symptom expression of poisoning. It has been reported that sudden death may occur without the appearance of any signs or symptoms of poisoning. In other instances, signs have appeared 4 to 6 hours after infestation of the beetles.
Fortunately horses have a low pain threshold which would work to their advantage if inflicted with beetle poisoning. They would most likely demonstrate their discomfort in some observable manner. If one suspects poisoning, inspection of the hay and hay feeder may reveal the presence of dead beetles. All old hay should be removed and replaced with new hay that has been inspected for the absence of blister beetles. Horses should be provided with fresh water and also allowed freedom of movement to prevent colic injury. A VETERINARIAN SHOULD BE CONSULTED.
Treatment may depend upon the severity of the case. Digestive tract protectants might be administered for the soothing of irritated linings. Mineral oils might be given to dilute the oil-soluble cantharadin. Laxatives may aid in the elimination of the toxin from the animal. Intravenous medications might be used to restore fluids and vital components lost through urine and diarrhea.
Good care and a long period of time might result in an animal’s recovery, the extent of which could vary from partial to complete.