Oklahoma Alfalfa
Oklahoma Cooperative Extension Service
Oklahoma Agricultural Experiment Station

Blister Beetles

Blister beetles are slender, soft-bodied, black, gray, brown, spotted or striped beetles from one- half to three-quarters of an inch long. The adults feed on foliage, while immature forms are often beneficial because they feed on grasshopper eggs. Blister beetles are important in alfalfa production, not so much for their feeding but because they contain cantharidin, a secretion that is toxic to horses consuming hay in which dead beetles are found. Recent research indicates that cantharidin can also decrease the digestibility of fibrous feeds by ruminants (e.g., cattle and sheep). Cantharidin is secreted from leg and body joints of the insect and is toxic even in dried pulverized bodies of dead beetles within bales of hay. Blister beetles are gregarious and often congregate in large swarms within alfalfa fields. These swarms move frequently and are typically comprised of one or two species of small striped beetles. Dead beetles may be found in baled hay because crimpers on harvest equipment crush congregating adults. 

No one has quantified the value of decreased digestibility of ruminant animals that consume one or 30 beetles. Published research results have estimated the number of beetles required for a lethal dose of cantharidin to horses. A summary of some of this information is provided in Table 9. 
 

Scouting Procedure 
 
Fields should be checked for concentrations of blister beetles just prior to harvest, particularly if forage is to be fed to horses. A sweep net can be used to estimate beetle numbers; however, adequate samples should be taken to insure detection of low numbers. Particular attention should be directed within 15 feet of the field edge. However, do not limit sampling to these areas. Several factors may determine possibilities of infestation. These include: past history of blister beetles, heavy grasshopper populations the previous season, age of the stand, and time of year. 

Threshold Levels and Management 
 
It should be stressed that blister beetles are not just an Oklahoma problem, but are found throughout the major alfalfa growing regions of the U.S. Several measures can be taken to avoid the presence of blister beetles in baled alfalfa hay. These measures include: cutting hay without conditioning (if additional drying time is available), routine spraying just prior to harvest to kill beetles, use of modified cutters that allow for straddling cut hay in the field so that beetles are not crushed in windrows, not cutting alfalfa any later than the 10 percent bloom stage since flowers attract foraging adults. Horse owners may help themselves by buying only first cutting alfalfa, since beetles are not normally present in Oklahoma until May. If feeding small amounts of baled alfalfa, examine each flake for presence of dead blister beetles. For a more detailed discussion on blister beetle life cycle and control see OSU Extension Facts F-2072.