Blister beetles cause problems

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By Dennis Morgeson


Recently I have had several samples of blister beetles brought into the office. They have been on a rampage in a few tomato patches recently.  Blister beetles are about three quarters of an inch to an inch and a quarter long. They have narrow bodies, broad heads, and antennae that are about one-third the length of the entire body.  There are three types or colors in Kentucky, jet black, striped with orange and black, and margined (which were brought in the office) that are black and have a thin gray stripe around the wing covers.

Blister beetles generally aren’t a problem in most years, but they do favor dry, hot weather and where there was an abundance of grasshoppers the previous year. The larval stage of blister beetles develops on grasshopper egg pods in the soil.

Adult beetles feed on leaves in the tops of plants and are especially attracted to flowers, where they feed on nectar and pollen. They tend to gather in large, concentrated groups or clusters in fields and gardens and can do a lot of damage in a short period of time. 

Blister beetles get their name from a chemical or poison in their bodies called cantharidin, which is comparable in toxicity to cyanide and strychnine.  This chemical is very stable and carries over in dead beetles. Livestock are susceptible to poisoning by blister beetles if they are present in hay, even if the beetles have been dead for a long time, with horses being the most susceptible. Blister beetles can also cause problems for humans if the beetles are mashed on the skin. They can cause, as you would suspect, a blister! This would vary from person to person and allergic reactions, etc.

Cantharidin is produced by male blister beetles and is passed on to the female during mating.  Striped blister beetle has more cantharidin than the black and margined. It is estimated that it would take about 25 male striped blister beetles to kill a 275 pound horse and up to 1100 of the less toxic blister beetles to do the same.

Blister beetle control in the vegetable garden is pretty easy and with a chemical that many homeowners already have, carbaryl, which is the active ingredient in the brand name Sevin. Be sure to spray when bees are not active, which is very early in the morning or very late in the evening. Carbaryl is extremely toxic to bees. Make sure to read the label very closely so that you wait the appropriate number of days before harvesting produce after it has been sprayed. For more information on blister beetle, call the Washington County Extension Office at 859-336-7741. 

Happy gardening!