Sometimes we receive calls about wasps eating outdoor wooden furniture or even structures. While some wasps like paper wasps and yellowjackets will scrape wood with their strong mouthparts to make their paper nests, they are not technically eating the wood but rather using it as building material. Other wasps will nest in holes in wood, including nail holes, drill holes, or emergence holes made by wood boring beetles. For example, powderpost beetle larvae feed and live in wood until they pupate into adults and emerge out of their wooden food source to mate. These emergence holes can be future nesting sites for a host of solitary wasps called potter and mason wasps.
Potter and mason wasps are in the insect family Vespidae, which contains many of the most common wasps we see, but they are in their own subfamily, Eumeninae. They are different than mud dauber wasps, which are in different wasp families (Sphecidae or Crabronidae) and generally tend to be larger and slenderer. Also, mud daubers often make exterior mud nests, with some looking like organ pipes, where they lay their eggs and provision them with spiders. Potter and mason wasps typically build little mud pots in crevices or cavities of objects where they lay their eggs and provision them with caterpillars or other insect larvae.
The wasps in the attached video and photos are a species in the genus Symmorphus and are some of the smallest potter wasps in our area. The larger female is about 3/8 inch in length while the smaller male is about a ¼ inch in length. They prefer to nest in beetle holes only about 1/8 inch in diameter. The female will go deep into old beetle holes where she makes a series of chambers with a wall of mud between each chamber. Starting from the back and moving toward the entrance, the female will lay an egg in a chamber and then hunt tiny leaf beetle larvae and caterpillars. She will sting her captured prey, immobilizing it but not killing it. She will place her bounty in with one of her eggs and then seal her chamber pot. She will repeat this until she has laid all her eggs in that gallery. She will then cap off the entrance hole with additional mud to give her offspring privacy and protection to develop. After all this hard work, there are even tinier wasps that will parasitize her eggs if they can find the Symmorphus egg chambers so the female will mud over some dummy, vacant holes to throw the parasitic wasps off the hunt. Clever.
Video of Symmorphus potter wasp retreating from an old beetle emergence hole in an outdoor wooden bench. Though she is only 3/8 of an inch long she is too wide to turn around in the 1/8 inch diameter hole and must back out after she has done her work deep in the gallery.
Video credit: Eric Benson, ©2020, Clemson University.
In most situations, potter and mason wasps are beneficial. Remember, potter and mason wasps do not eat wood. They prey on other insects that eat plants, some of which can be garden pests. If they are in wooden outdoor furniture and you don’t want them there, try to clean the item and then seal any holes with wood putty or polyurethane, removing the nesting sites. If you see them in a natural setting, do nothing but watch these fascinating wasps do their work. As with many other insects, they are a wonder.