Current Issue

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

Mason wasps: Monobia and Pseudodynerus


A quick glance could lead one to mistake black and white Monobia for a bald-faced hornet.


In previous episodes of Bug of the Week we enjoyed learning about mason bees, important early season pollinators of many native plants. Several years ago we also met dastardly parasitic leucospid wasps that raid mason bee nests and kill bee larvae within their galleries. Last week an eagle-eyed Master Gardener made an inquiry regarding black and white wasps poking around her mason bee colony. She wondered about their identity and intent. Were these another type of deviant ruffian out to eat mason bee babies and pillage their pollen cakes?

You may recall photographs of logs drilled full of holes to accommodate industrious mason bees in my mason bee colony. However, mason bees are not the only members of the bee and wasp clan that evolved to take advantage of vacant galleries in wood. The hollow chambers excavated by larvae of wood-boring bees like carpenter bees, and beetles like round headed borers are used by several species of mason wasps as homes to raise their brood. As adults, mason wasps provide the important ecosystem service of pollination as they seek nectar and pollen as food sources.


A little gentle poking causes Monobia quadridens to come out of her gallery and groom her antennae before returning to her work.

Unlike the larvae of mason bees that consume pollen cakes supplied by their mothers, many mason wasps consume living but paralyzed caterpillars. Prior to the hunt for caterpillar prey, the female mason wasp deposits her egg in the chamber where caterpillars will be stored. She then hunts for prey on flowers and foliage. Like potter wasps we met in a previous episode of Bug of the Week, female mason wasps use a potent venom to paralyze their pray. Sometimes as many as 19 caterpillars are captured, paralyzed, and used to provision the cell where an egg awaits. Once a sufficient number of prey have been captured, the chamber is sealed with a plug of mud or sand particles.

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. So, when you see these magnificent black and white wasps hovering around mason bee colonies, fear not, your mason bees are not under attack. These black and white beauties are likely highly beneficial mason wasps looking for an empty apartment in which to raise their brood.


This pretty Pseudodynerus seems to ponder how difficult it might be to break through a mason bee’s plug.


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