Blister Beetles Can Be a Problem

David Hillock- Kay County OSU Extension

Blister Beetles (Epicauta pennsylvanica) can be a problem this time of year. These rather large native beetles (adults range from 1/2 to 1 inch long) have a characteristic narrow, elongate, soft body with a head wider than the pronotum. The flexible wing covers are rounded over the abdomen. Colors vary with species and range from black to gray to brown, some with conspicuous orange stripes or other patterns.

Their name derives from the fact that adults produce a toxin (cantharadin) that can cause blisters to form on the skin. But gardeners are more concerned about the plants whose leaves the beetles are stripping. The adult beetles feed on many different garden plants, particularly legumes, eating foliage and flowers. The adults generally appear in large groups or swarms and can be quite a problem in the garden.

Before we decide to kill off these bothersome beetles, we might consider the fact that the larvae of these beetles are predaceous and feed on grasshopper eggs. But if that is not enough to convince you to spare them, then let’s look at a few management options.

Hand-picking the beetles can be rather unpleasant because the toxin in the beetles’ bodies can irritate the skin. It is better to shake them into a pan or bucket of soapy water. Blister beetles often drop to the ground and play dead when disturbed, so you may have to pick up a few from the ground. Be sure to wear a pair of gloves to keep the cantharadin off your skin.

Some gardeners have grown calendula as a trap crop or left a few pigweeds (Amaranthus species) growing in the garden. Both these plants are highly attractive to blister beetles. If you prefer a chemical approach, there are a few insecticides labeled for control of blister beetles including those with carbaryl (Sevin), malathion and pyrethrin as the active ingredient. Multiple applications may be needed as adult blister beetles are migratory and others may visit the garden.