photo by: Shutterstock

Blister beetles are named for a chemical contained in their bodies that causes blisters on sensitive human skin.

When the leaves of tomato plants and other vegetables are stripped very quickly in late summer, the most likely culprit is an infestation of blister beetles. Blister beetles are about one-half inch long with narrow, cylindrical bodies. They may be gray, black or striped depending on the species.

Blister beetles are named for a chemical contained in their bodies that causes blisters on sensitive human skin. The chemical is released when the insect is pressed, rubbed or squished. Avoid handling blister beetles with bare hands to prevent contact with this chemical. Blisters are not usually reported to be painful and only last a short time, but they are uncomfortable.

Blister beetles are voracious feeders and a small number of adults can decimate plants in a short period of time. Their favorites in the vegetable garden are tomatoes, peppers, eggplants, potatoes and leafy greens. They will sometimes feed on other crops and on ornamental plants as well.

Control of blister beetles in the garden requires prompt attention. As soon as plants show signs of feeding, look under leaves and amongst the remaining foliage to try to find the beetles. If plants are stripped but beetles cannot be found on the plants, they have already moved on and control measures are unnecessary.

If beetles are found, there are a few options for control. If there are only a few beetles present, wear gloves and pick them off by hand into a jar of soapy water. Sometimes they can be knocked or shaken from the plant into the jar to avoid handling. They are very aware of their surroundings and may even drop to the ground and play dead if disturbed, only to get up and start feeding again later.

If there are too many beetles to pick off, there are several products labeled for control. For those looking for a biological pesticide, look for products labeled for blister beetles that contain the active ingredient spinosad. For those who choose to use conventional pesticides, look for products labeled for blister beetles that can be used in vegetable gardens. Pay attention to harvest intervals if you plan to harvest from the plants being treated.

There is only one generation of blister beetles per year in Kansas. Females lay their eggs in the soil beneath plants they are feeding on though, so presence of blister beetles this year increases the likelihood they will return.

Gardeners who consistently have problems with blister beetles from year-to-year may wish to plant a trap crop. A trap crop is one that is grown because it is very desirable to certain insects. When the plants become heavily infested, they can be removed and destroyed, thus destroying most of the undesirable insects with them.

For blister beetles, calendulas are an ideal trap crop. Plant a few in the perimeter of the garden and check them frequently in late summer to detect the beetles quickly after they arrive. Pigweed also works as a trap crop for blister beetles.

There are a couple of good things about blister beetles. First, the chemical contained in their bodies is cantharidin, which is used in the treatment of warts. These insects are the only source of it. Second, the larvae of many species of blister beetles feed on grasshopper eggs, thus reducing the number of grasshoppers.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.