Pollen Wasps

pollen wasp on Penstemon haydenii.
Pollen wasp on Penstemon haydenii. Photo by Bonnie Heidel, Wyoming Natural heritage Program.

By Vince Tepidino
USDA ARS (retired), Bee Biology & Systematics Lab, Logan, Utah

Behold, a "pollen wasp" (Masarinae), superficially yellow jacket-like, but with an identity problem. Almost all wasps are flesh-eaters, but masarines are more like Ferdinand the Bull, who was more interested in smelling flowers than in goring matadors. Pollen wasps forsake stinging, eating, and feeding other insects to their offspring, for plying flowers. However, they differ from Ferdinand in harvesting pollen and nectar rather than simply inhaling sweet odors.

Masarines share their vegetarian habit and other interesting characteristics with bees, their distant relatives in the order Hymenoptera. Like most bees, masarines are solitary - there is no colony or evidence of social behavior - each female acts independently to nest and forage for her offspring. Masarines have long mouthparts, common (though far from universal) in bees, that enable them to reach the nectar of some flowers with deep corolla tubes, like beardtongues (Penstemon). They collect nectar and pollen in an internal crop, as do some bee species, not in a hairy external apparatus on the hind legs or under the abdomen, as do most bees.

Pseudomasaris vespoides nest on the side of a gray rock.
Pseudomasaris vespoides nest. Photo by USDA Bee Lab, ARS, Logan Utah.

Like some other bees (and wasps), North American masarines make hard nests of mud attached to rocks, ledges, and sometimes twigs (see picture on the left). Typically, nests are arranged as multiple parallel cells. Each cell contains an egg and a single pollen-nectar loaf, sealed with a mud plug. In the nest shown in the image, five individuals completed development and successfully exited, while one apparently died before emerging.

Masarines occur on all continents but Antarctica, although they are abundant in relatively few places. They are especially diverse in the fynbos and karoo biomes of southern Africa, but not in North America where only 14 members of a single western genus, Pseudomasaris, occur. The clubbed antennae of Pseudomasaris are a distinctive characteristic enabling one to separate them from our more common wasps.

Pseudomasaris vespoides on Penstemon cyananthus.
Pseudomasaris vespoides visiting Penstemon cyananthus. Photo by Teresa Prendusi.

As far as we know, all species of Pseudomasaris collect pollen only from a small group of plants. Most are specialized on members of the Water Leaf family (Hydrophyllaceae) like Phacelia and Eriodictyon. The species shown in the picture on the right, Pseudomasaris vespoides, is a beardtongue specialist, and an important pollinator of many species. It is seen in the picture (above) in Wyoming visiting the flowers of Penstemon haydenii, a species listed under the Endangered Species Act. Pseudomasaris vespoides, beardtongue heroine, visits and pollinates many other rare beardtongues, including P. debilis, P. grahamii, P. harringtonii and P. penlandii. When foraging from species such as P. speciosus, whose flowers possess a row of short, stiff, strong spines along the line of anther dehiscence, a loud rasping or clicking sound can be heard, as the dorsal thorax rubs against the anthers.

Those interested in learning more about these fascinating insects should consult "The Pollen Wasps" by Sarah K. Gess.