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Blister Beetles: Common Inhabitants of Alfalfa and Soybean Fields

August 2, 2002
Last week, Robert Bellm, crop systems Extension educator at the Edwardsville Extension Center, reported finding blister beetles in alfalfa fields. Robert was collecting approximately one blister beetle for every 20 sweeps he made. At this point in the season, it's common to find these soft-bodied beetles in both alfalfa and soybean fields. Although the adult blister beetle is considered an occasional pest, blister beetle larvae prey on grasshopper eggs and are viewed as beneficial. Because they feed on grasshopper eggs, they tend to occur in greater numbers following years of high grasshopper populations.

They are soft, slender beetles about 5/8 to 1 inch long and may be solid black, gray, black with a gray border stripe, or brown with yellow stripes. Illinois has more than 20 species of blister beetles, but the most common ones infesting soybeans and alfalfa are the gray, margined, and striped blister beetles. Adult beetles feed on soybean foliage and leave only the main veins behind.

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.

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.

1. 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.

2. 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.

3. 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:

a. 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.

b. Do not feed this hay to horses.

c. Consider the application of an in secticide. Sevin XLR Plus and Warrior are labeled products for blister beetles in alfalfa. Carefully read the label for preharvest restriction guidelines and other instructions. Warrior is a restricted-use insecticide and may be applied only by certified applicators.

d. 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 dead beetles.

e. 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

Striped blister beetle.

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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