Blister beetles do not have a toxic bite or sting, but if you crush one of these pests you can wind up with small blisters on your skin several hours after contact due to the secretion of cantharidin (a toxic compound in their bodies).
Grey blister beetle (top left), Black bister beetle (top right)-photos United Agri Products
Blister beetles have very long thin cylinder-shaped bodies with large heads. They are about 2 cm long (from ½ to 1 inch in length) and relatively easy to identify because of the distinct “neck-like” section between the body and the head. They usually fly around in groups and there are many types of them. They are grey, black and striped with some species shiny black-blue, or black-green.
Blister beetles are found in most parts of the US and most parts of Canada. It is the adults that cause most of the damage. Fortunately however they don’t stay around an area too long so the best control is to let them run their course unless infestation is severe. They overwinter as semi-pupae in the soil and emerge in the spring and molt. Adult blister beetles emerge in the summer months.
Life cycle of blister beetle-NC State University
Diet & Damage
Blister beetles eat the leaves and flowers of most plants… peas, beans, potatoes, turnips, tomatoes, many other veggies and flowers. They can defoliate plants in no time. The larva have been known to eat grasshopper eggs so they are somewhat beneficial. In some cases where these beetles are severe on alfafa, farmers must take caution when cutting as their livestock can die from ingesting dead blister beetles. The condition that affects livestock after eating grains infested with these beetles is called Cantharidiasis
1) Hand Pick
Be sure to wear gloves or use tweezers as they can release a substance that causes blisters on the skin. Drop them in hot soapy water. Be sure not to touch them even when they are dead as they can still cause painful blisters on your skin.
2) Keep Weeds Low
Weed around edges of your garden attract blister beetles because of grasshopper eggs. Eliminating the weed problem will help control infestation.
* A word of warning, killing them is not a good option unless you dispose of the bodies. Their poison stays in their bodies long after they are dead.