How to Manage Pests
UC Pest Management Guidelines
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 do not cause widespread feeding damage to alfalfa, however, they contain a chemical, cantharidin, that is toxic to livestock. Cantharidin is contained in the hemolymph (blood) of the beetles and may 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. 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 serious problem in alfalfa in the northern United States, the Midwest, and the south for many years, but until recently have not been a problem in California.
Alfalfa contaminated with blister beetles in the southern Owens Valley has been linked to the death of several dairy cows. At this point, it is not known if blister beetles are widespread or confined to the Owens Valley. Likewise, it is not known if the problem is likely to spread and hence become a common occurrence in California alfalfa. In the meantime, growers and PCAs are advised to be on the lookout for blister beetles and to contact their farm advisor for advice if these insects are found.
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 bloom. If beetles are found, remove the conditioner wheels from the swather in order to prevent crushing beetles. Also, these beetles are 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.
UC IPM Pest Management Guidelines: Alfalfa