If you see a carpet of beetles in your back yard, you might be looking at blister beetles. In front of our high tunnel planting in early July we saw a carpet of what looked like lightening bugs right outside the door. Even though the soil elsewhere was dry, the high tunnel is irrigated and the soil around it was moist. The carpet of beetles that emerged from the ground turned out to be striped blister beetles.
Blister beetles can be striped or solid and vary by species in shape, size (3/8 to 1 inch long) and color (solid gray to black or with paler wing margins, metallic, yellowish-striped or spotted). Most have long, cylindrical, narrow bodies that have heads that are wider than the first thoracic segment so they look like they have a neck.
Species of blister beetles have chewing mouthparts and devour leaves, flowers, buds and fruit from many vegetable and ornamental plants such as tomatoes, potatoes, beans, cabbage, beet, eggplant, corn, peas, onions and hostas. We noticed the day after we observed the carpet of striped blister beetles by the high tunnel, they disappeared. When the beetles emerge in swarms in late June or July they can do a lot of damage. If you don’t catch them at that point, they may scatter and not be as much a problem in the garden – of course this does not console you if they have eaten all of the leaves from your tomatoes.
Care should be taken to not handle these beetles. Their bodies contain a toxin, cantharadin, which can cause blisters to form on the skin. Make sure to wear gloves if you pick them off of plants. Beetles are difficult to control with pesticides. From what we have observed with the striped blister beetles, the damage is quickly done and the culprits dissipate afterward.
Blister beetles are also a concern to forage crop producers. Swarms that feed on leaves of legumes, like alfalfa may contaminate feed. Animals, particularly horses, ingesting beetle-contaminated feed become extremely ill and may die.
No bug is all bad, however, and blister beetles are no exception. The blister beetles overwinter as grubs in the soil and emerge to feed and lay eggs. The larval stages of some species are beneficial since they feed on grasshopper eggs. So look on the bright side. Your tomatoes may have been hit hard by blister beetles, but at least you won’t have as many grasshoppers to worry about!