Blister Beetles in Alfalfa

Crop Injury: Multiple species of blister beetles (Epicauta spp.) commonly feed on alfalfa foliage, including the ​​striped, black, margined, and ashgray blister beetles. However, the primary risk of blister beetle infestation in alfalfa is livestock poisoning due to insect consumption in harvested hay. Blister beetles contain a toxin known as cantharidin, which causes injury to animal tissue on contact. Only the striped blister beetle is injurious to horses. 

Pest Description: Blister beetles are large, narrow insects that can be up to 1 inch long. They have a narrow neck and broad head with long, skinny legs. Blister beetle antennae are approximately ⅓ the length of their body. Adult forewings may be all black, all gray, black with gray margins, or striped black and orange. Large numbers of beetles may congregate in localized areas of a field. 

Life Cycle: Blister beetle larvae hatch from eggs laid in the soil. Blister beetle larvae are predaceous and Epicauta species feed on grasshopper eggs. Larvae overwinter in the soil and pupate in the spring. Adults emerge beginning in June. 

Scouting: Check fields on a weekly basis starting at the second alfalfa harvest. Different species of blister beetle are active at different times of the year and presence of a particular species varies by region. Blister beetle risk increases when grasshopper abundance is greater, an occurrence favored by drought conditions.  

Management: Insecticide application will not remove beetles from an alfalfa field, and dead blister beetles may end up in harvested hay. Use of a self-propelled swather to cut hay, with the crimping mechanism removed and on a wide-set wheel base, can reduce blister beetle contamination risk. Hay from the first cutting, prior to emergence of blister beetles, should be used to feed horses. 

Prepared and reviewed by the Alfalfa Pest Management Working Group.