Oklahoma State University Stillwater, OK
Oklahoma Alfalfa Oklahoma Cooperative Extension Service Oklahoma Agricultural Experiment Station

Blister Beetles
Life Cycle and Damage. Several species of blister beetles are common throughout eastern and southern areas of the U.S., and in no way is the problem with blister beetles in alfalfa restricted to Oklahoma. The species found in alfalfa range from uniform black or gray in coloration to a striped pattern of alternating brown and orange colors. It is the striped blister beetle that most commonly forms large aggregates or "swarms" in alfalfa fields. The blister beetles found in alfalfa complete one generation per year, which begins during late summer as females lay eggs in cracks and cavities at the soil surface. Eggs hatch and tiny, long-legged larvae called "triungulins" crawl over the soil surface in search of clusters (pods) of grasshopper eggs, which are also laid within 1-2 inches of the soil surface. Upon finding a site where grasshopper eggs have been laid, each triungulin tunnels into the egg "pod" and begins to feed. The larva continues to grow and develop while consuming the grasshopper eggs, then overwinters in the soil and emerges as an adult beetle the following May and June. These beetles feed on foliage and blooms of many plants including alfalfa. However, it is not their feeding activity that gives them pest status, but the fact that the beetles contain a chemical called "cantharidin", a blistering agent that is highly toxic and may cause illness or death in livestock, particularly horses, when consumed in forage. Cantharidin is a highly stable chemical that remains active even within the dried remains of beetles. If the beetles are killed during alfalfa harvesting, as would occur when a crimper/conditioner is used for swathing, they may be baled into hay and inadvertently fed to livestock. In the case of the striped blister beetle that frequently forms "swarms" in alfalfa fields, remains of many beetles may be found in baled hay if they are killed at the time of cutting either by crimping hay or by wheel traffic over windrows. If they are not killed when alfalfa is cut, the beetles leave the windrows as hay dries before baling and the threat of cantharidin toxicity is eliminated. All cases of mortality in horses resulting from cantharidin toxicosis that have been confirmed by Oklahoma State University, College of Veterinary Medicine, have involved the striped blister beetle. Cantharidin toxicosis causes ulceration of sensitive skin areas (lip, nose, and mouth) and mucous membranes (esophagus and stomach) Severity of symptoms of toxicosis is dependent primarily upon the numbers of beetles consumed. Cantharidin is highly damaging to tissues of the digestive and urinary tracts. If large amounts of cantharidin are ingested, death may occur within 6 hours. When smaller amounts are consumed, horses may display symptoms of colic (pawing and stretching). Due to reduced levels of electrolytes (calcium and magnesium) in the blood stream, animals may exhibit stiffness or an exaggerated "goose-stepping" gait. Small amounts of darkened urine (blood in urine) may be voided resulting from damage to the kidneys and bladder. Horses displaying these symptoms should be promptly examined by a veterinarian, and hay being fed should be examined for the presence of blister beetle remains. Sampling and Control. There is no way to "guarantee" that alfalfa hay harvested in the southern U.S. is completely free of blister beetle contamination. However, several precautions can be taken to greatly reduce the chances that hay will be contaminated: Use hay harvested before mid-May or after early September as forage for horses. Chances of blister beetles being present at harvest is greatly reduced at these times. Scout fields beginning in border areas to look for "swarms" of blister beetles. Often they will be found within 50-100 yards of the field edge, but can occur anywhere in fields. If swarms are found, a short residual insecticide should be applied before cutting. Do not use a crimper when cutting hay intended for horses, especially during mid-summer when beetles are found most commonly. Avoid driving over windrows Cut alfalfa in the bud stage; the presence of blooms is attractive to blister beetles and increases the likelihood of infestations. When small amounts of hay are being handled as when feeding small, square bales, examine hay carefully as it is fed to detect the presence of blister beetle bodies or body parts. As there is no way to completely eliminate the possibility of blister beetles being in alfalfa, the prudent approach for management is to take all possible precautions to reduce the likelihood that they are present, particularly when marketing hay to horse owners. Additional information on blister beetles in alfalfa can be obtained from OSU Extension Facts No. F-2072 and Oklahoma Alfalfa.
Phil Mulder, Extension Entomologist  Department of Entomology and Plant Pathology  Oklahoma State University

 

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