Blister Beetles: One Group of the Insects of Summer

The sound of cicadas and katydids whining in the woods or velvet ants and blister beetles running around on the ground are signs of summer in Kentucky. However, our relationship with blister beetles is complicated. Depending on the situation, they can be beneficial predators of pests, pests in their own right of alfalfa and specialty crops, or used as a source for medicines.

There are several species of blister beetles that are common in Kentucky and are widely recognized as potentially deadly contaminates to livestock when crushed beetles are in alfalfa hay. Cantharidin is a bitter chemical that is toxic to several organ systems in livestock, particularly the digestive system. Cantharidin is a stable toxin that remains in the dead beetles after being crushed during the harvest of alfalfa. As few as 5 to 10 of these beetles can be fatal to horses when ingested because of the cantharidin in their hemolymph (insect blood). While horses are very susceptible, other livestock consuming blister beetles may become sick or die.

Although cantharidin can cause skin blistering with people, it also has medical uses and can be prescribed for the removal of warts and for a few other conditions.

Life Cycle

Blister beetles have an interesting life cycle that begins as eggs laid in crevices of soil in late summer.  The eggs hatch in the fall and, depending on the species, the active larvae search for clusters of grasshopper eggs to consume or possibly hitch a ride with a solitary bee to infiltrate its nest and consume its larvae. As grasshopper egg predators, they are considered beneficial. The adults emerge from the ground the following year in late spring. The adults feed on the leaves and blooms of alfalfa but can also be pests of tomatoes, peppers, and melons where they also feed on foliage and flowers (Figure 1). With alfalfa, the adults usually emerge after first cutting.

Figure 1. The margined blister beetle, Epicauta funebriscan, be a pest of tomatoes, peppers, and melons. (Photo: Ric Bessin, UK)

Figure 2. Oil beetles, Meloe sp., are slow, flightless blister beetles that can feed on solitary bee larvae. (Photo: Ric Bessin, UK)

Hay Contamination

Blister beetle contamination of hay is often due to beetles being crushed during harvest. Beetles crushed by crimper rollers become incorporated into the bales. However, if beetles escape being crushed at cutting, most will leave the field as the hay cures. As blister beetles tend to be found in small clusters in the field, often only a small number of bales are contaminated.


Avoid using crimpers during cutting to reduce blister beetle contamination of alfalfa hay. Also avoid driving over freshly cut alfalfa as this too can crush beetles in the cut hay. It is best to scout fields prior to cutting hay during periods when blister beetles are active. Often, only portions of the field need to be sprayed. Pay particular attention to field margins as blister beetles often do not move far into fields.

Short-residual insecticides before cutting are used to reduce blister beetle levels.  See ENT-17, Insecticide Recommendations for Alfalfa, Clover, and Pastures – 2022, for a list of recommended insecticides.

By Ric Bessin, Entomology Extension Specialist

Posted in Forages
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