Blister Beetle

What it does

Adult blister beetles often feed in large, mobile swarms, stripping leaves, flowers, and pods before moving on. When handled or crushed, they exude cantharidin, a caustic defensive chemical that raises blisters on skin. Despite the adult damage, the larvae are predators that consume grasshopper egg pods in the soil.

Where it lives

Adults are found in vegetable gardens, alfalfa and hay fields, and on flowering weeds across the US, often most numerous in years following heavy grasshopper populations. They favor warm, sunny conditions and gather on legumes, tomatoes, potatoes, and amaranth.

How it breeds

Blister beetles undergo complex development called hypermetamorphosis. Females lay egg clusters in soil, and the active first-stage larva hunts down grasshopper egg pods, then molts through several less-mobile grub-like stages before pupating. There is generally one generation per year, with adults emerging in summer.

What it eats

Adults chew foliage, blossoms, and developing pods of many plants, especially legumes (alfalfa, beans), tomatoes, potatoes, eggplant, and amaranth. Larvae are beneficial predators that feed on grasshopper eggs in the soil.

Identification

Adults are 10-25 mm long, narrow and soft-bodied, with a distinctly narrow segment behind the head that gives a 'neck' look. Species vary from solid black or gray to boldly striped yellow-and-black. The soft, cylindrical body and long legs distinguish them from hard-shelled leaf beetles.

Signs of an infestation

• Clusters of long-necked beetles stripping foliage and blossoms
• Sudden, patchy defoliation of legumes, tomatoes, or potatoes
• Chewed flowers and developing pods
• Groups of beetles that move together from plant to plant
• Skin blistering after crushing a beetle against the skin

Prevention tips

• Wear gloves and handpick beetles into soapy water rather than crushing them
• Use row covers on vulnerable vegetables during peak beetle season
• Control broadleaf weeds that draw feeding swarms
• Scout hay and alfalfa carefully, since beetles baled into hay endanger horses
• Tolerate low numbers, since larvae help suppress grasshoppers

Professional solutions

Handpicking with gloves and row covers manage most garden situations. If swarms repeatedly damage vegetables, insecticidal options exist, but read the label and avoid crushing beetles onto skin. Horse owners and hay producers should contact their Cooperative Extension office, because cantharidin-contaminated hay is a serious veterinary hazard.