Blister Beetles

Blister beetles are narrow and elongate and the covering over the wings is soft and flexible. They may be solid colored (black or gray) or striped (usually orange or yellow and black) and are among the largest beetles likely to be found in a sweep net sample in alfalfa.

Blister beetles have an unusual and complex life cycle. Females deposit clusters of eggs in depressions in the soil and the newly hatched larvae (called triungulin) seek out subterranean grasshopper egg pods or eggs of ground-nesting bees to complete development. The triungulin of some species of blister beetles "hitch a ride" back to the hive with adult bees to feed on bee eggs. The larvae pass through three more growth stages, with each becoming more sedentary, and eventually change to pseudopupae, which is the overwintering stage. In the spring, they enter the pupal stage from which adults emerge. Adults survive summer and deposit eggs to complete the cycle.

Blister beetles do not cause widespread feeding damage to alfalfa; however, they contain a chemical, cantharidin, which is toxic to livestock. Cantharidin is contained in the hemolymph (blood) of the beetles, and can contaminate forage directly, when beetles killed during harvest are incorporated into baled hay, or indirectly, by transfer of the hemolymph from crushed beetles onto forage. Horses are particularly susceptible to the toxic effects of cantharidin. Consuming as few as six beetles can kill a horse.

As the name implies, handling these insects may result in blisters, similar to a burn, on the hands or fingers. Blister beetles have been a problem in alfalfa in the northern United States, the Midwest, and the south for many years, but are an occasional problem in California.

There are no known predators or parasites that effectively control blister beetles. Blister beetles are attracted to blooming alfalfa. Therefore, to reduce the incidence of blister beetles in alfalfa, cut hay before peak bloom. Alfalfa fields near natural areas or rangelands may have higher levels of blister beetles due to the availability of grasshopper egg pods or ground-nesting bees in these undisturbed areas. Blister beetles may not be present all summer (each species has a peak period of activity), thus samples and observations may be helpful to determine activity patterns in particular areas. Research has not been conducted to determine this.

These beetles are also found on the edge of the field or congregated in groups within the field. Skip such areas when cutting or pick up the bales for these areas separately and isolate them from the rest of the field. No treatment thresholds have been established for blister beetles and insecticide applications generally are not needed.