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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Humans help wasps, wasps help humans: Four-toothed mason wasp, Monobia quadridens


Pollination is one important ecosystem service provided by mason wasps like Monobia. Note the broad white band at the front of the abdomen.


White bands at the end of the tail are features of the baldfaced hornet seen here, but are not found on the four-toothed mason wasp that wears a white band at the front end of its abdomen.

Really, who likes washing windows? I always greet this semi-annual ritual with a large dose of foot-dragging and a large bucket of soapy water. Last week as I toiled on a picture window, I spied what I first thought was a small baldfaced hornet dodging into one of the small drain holes drilled through the lower edge of the vinyl window frame. After lingering for a few moments, I watched this bold black and white beauty reappear and dash away only to return and reenter the hole a few moments later. The four-toothed mason wasp, Monobia quadridens, is sometimes mistaken for the more common baldfaced hornet we met earlier this season. At close range, with a little courageous observation, these two are rather easy to tell apart. Monobia has a broad white band encircling the front end of its abdomen and lacks white markings on its tail. Baldfaced hornets lack the white band at the front of the abdomen and have several tail segments banded with white. Boom! What’s more, as we learned previously, baldfaced hornets are a chummy bunch of sisters, mom, and occasionally brothers living in a paper nest, while mason wasps are solitary wasps darting in and out of holes.

Millions and millions of years ago, before the advent of vinyl windows with weep-holes, mason wasps evolved to take advantage of naturally occurring voids such as the hollow stems of plants, or holes made in wood by wood-boring beetles, or the galleries made by carpenter bees. As adults, mason wasps are often seen visiting flowers where they provide the important ecosystem service of pollination as they seek nectar to power the hunt for prey and pollen as a protein source to be transformed into eggs. After finding a suitable gallery, which could be the drain hole in a window frame, the female mason wasp deposits an egg within the gallery. In a remarkable display of gender control, the female wasp is able to lay either a male or a female egg. Due to the shorter developmental time of the male offspring, male eggs are usually placed near the opening of the gallery and female eggs are placed deeper within. The female wasp then exits the gallery to hunt small caterpillars, many of which are garden pests. Upon encountering a caterpillar, the female mason wasp uses a potent venom to paralyze the prey. Sometimes as many as 19 caterpillars are captured, paralyzed, and used to provision the cell where an egg awaits. Ah, but these caterpillars are not truly dead, only mostly dead: paralyzed, fresh meat that will be consumed alive once the egg hatches and the wasp larva drops onto the hapless caterpillar. Once a sufficient number of prey have been captured, the chamber is sealed with a plug of mud or sand particles, hence the name mason wasp.


Human-made holes in vinyl window frames create nesting sites for gallery dwellers like mason wasps, as do holes drilled in firewood to provide nesting sites for mason bees. Watch as a mason wasp checks out a potential nest site recently vacated by a mason bee.

If you see these magnificent black and white wasps entering drain holes on your window frames, fear not, your home is not under attack. You have provided a nesting site for these black and white beauties. In return, they will pollinate your plants and help eliminate pests in your garden and on your trees and shrubs. Humans helping wasps, wasps helping humans.

By the way, mason wasps are not the only benefactors of the small drain holes found in vinyl window frames. In a previous episode we met cricket hunting Isodontia wasps that also utilize drain holes and window frames for nest sites.


The wonderful reference “Trap nesting wasps and bees: Life histories, nests, and associates” by Karl Krombein was used to prepare this episode.