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U.S. Forest Service

Rose-mallow Bee (Ptilothrix bombiformis)

By Melissa Simpson, U.S. Forest Service Ecologist

In contrast to the social lifestyles of honeybees and bumblebees, most bees are solitary, meaning a single female excavates a nest, lays her egg(s), and collects pollen and nectar provisions for her larvae without any cooperation from other bees. Solitary bee nests are often found in aggregations, but a single female occupies each nest.

Like other herbivorous insects, the 18,000 known bee species show great variation in their diet breadth. Generalist bees, including the well-studied honeybees and bumblebees, collect pollen from a wide range of unrelated host plants. Specialist bees are picky-eaters, feeding on a particular genus or species, or even a few closely-related plants. All of the social taxa of bees are generalists, whereas solitary bees can be generalists or specialists.

Ptilothrix bombiformis (Hymenoptera: Apoidea) is a solitary, ground-nesting, specialist bee belonging to the tribe Emphorini. All emphorine species for which host-plant affinities have been studied, including Ptilothrix species, are specialists, typically collecting pollen from a single host genus or several related genera. Ptilothrix bombiformis is a Hibiscus specialist.

Ptilothrix bombiformis is a robust bee that resembles, and could be mistaken for, a bumblebee. Its head and mesosoma are covered with a mix of short, dense black and light-yellow hairs. In addition to the hair on the head and mesosoma, the presence of short, light-yellow hairs on the basal metasomal tergum (first segment), distinguish this bee from other look-a-likes. It has long, gangly, hairy legs. Females’ hind legs are covered with very dense, plumose hairs because she carries pollen in the hairs to transport it to her nest.

It nests in hard-packed roadways and levees in close proximity to water sources. Female Ptilothrix can “walk” on water! They alight on the water’s surface to collect water to moisten the hard-packed soil while excavating burrows. The entrance to the nest is surrounded by a turret, which looks like a mud chimney. Female Ptilothrix construct and provision several nests during one season. Nests are vertical and usually one or two-celled and each cell contain an egg and a pollen provision. The female lays her egg on the pollen provision and the larvae eats its way around the pollen mass as it grows and develops.

When feeding is complete, the larva deposits a layer of fecal material that covers the entire interior of the cell, and then covers itself in a cocoon. The layer of feces appears as a layer of pollen exines without recognizable fecal pellets, which is a unique emphorine characteristic.

Mating behavior involves male bees resting in Hibiscus flowers, waiting for foraging females, and males will fly from blossom to blossom in search of a female. Males will chase-away or fight with other males that try to land on their flower.

Distribution of Ptilothrix bombiformis. Distribution of Ptilothrix bombiformis. Courtesy Discover Life.

Male resting on Hibiscus flower. Male resting on Hibiscus flower. Photo by Melissa Simpson.

Male drinking Hibiscus nectar. Male drinking Hibiscus nectar. Photo by Melissa Simpson.

Close up of nest. Close up of nest with turret. Photo by Melissa Simpson.

Close up of rose-mallow bee. Close up of specimen. Photo by Melissa Simpson.

Close up view of rose-mallow bee from above. Yellow hairs on head, mesosoma, and first segment of metasoma. Photo by Melissa Simpson.

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