Blister beetles are so-called because they contain a chemical in their hemolymph, or blood, called cantharidin. If rubbed or pressed the beetles release cantharidin as a defense mechanism, which causes blistering on human skin. The Black Blister Beetle (Epicauta pennsylvanica) belongs to a genus of beetles that are highly toxic to horses — a few beetles fed in a single feeding of alfalfa can be lethal.
As larvae, most blister beetles are predators, often invading wild bee colonies and consuming bee larvae, as well as nectar and pollen. Black Blister Beetle larvae and other blister beetles in the genus Epicauta prey on the eggs of grasshoppers. Female Black Blister Beetles lay clusters of eggs in the soil in late summer. The small, active larvae that hatch from these eggs crawl over the soil surface entering cracks in search of grasshopper egg pods which are deposited in the ground. After finding an egg mass, the blister beetle larvae become immobile and spend the rest of their developmental time as legless grubs. The following summer they transform into the pupal stage and soon emerge in the adult stage. This is why blister beetle numbers increase dramatically following high grasshopper populations.
Once they mature into adulthood, Black Blister Beetles feed on plants (phytophagus) and are commonly found on flowers, especially those in the Aster/Composite family. They are said to be there for the nectar and pollen, but the Black Blister Beetle pictured denuded several aster blossoms of all petals during the time it was observed.
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