Mellow mallow bees, Ptilothrix bombiformis

In 1966 during the construction of the planned city of Columbia, MD, a 27-acre reservoir named Lake Kittamaqundi was created from several unnamed tributaries of the Little Patuxent River. In the intervening decades, along the banks of Kittamaqundi patches of marshmallow, Althaea officinalis, put down roots and now show off their dazzling floral displays, five large petals dressed in shades of white, lavender, pink, and purple. From June into early autumn, marshmallows and other members of the hibiscus clan are visited by Ptilothrix bombiformis, an apid bee that specializes on members of the mallow clan. An encounter with this charming bee began on a sunny morning while walking along a trail that circumnavigates the lake. My eagle-eyed companion noticed several small bees darting in and out of turreted pencil-sized holes in the hard clay soil on the bank of the lake. These cute bees, Ptilothrix bombiformis, are unique in that they are one of only two species in the genus Ptilothrix found in the US.  Ptilothrix bombiformis occurs mostly in the eastern half of the US and its sister species occurs in Arizona and New Mexico.

A Ptilothrix bee begins to build her nest by wetting hard mud and digging with jaws and legs. Off she goes to get more water for softening the soil. She regurgitates water to moisten the soil and work it into a turret for her nest. Beneath the earth, water is mixed with soil and mud pellets are removed to enlarge the gallery. Then off she goes to visit mallow blossoms for nectar and pollen. She returns to her nest with provisions to sustain her brood as they grow and develop underground.

These industrious bees construct nests in soil. To excavate galleries in hard-packed earth, females land on the surface of the lake, imbibe water, fly back to the nest site, and regurgitate the water to moisten and loosen the soil. During nest construction the area around each gallery is littered with an array of small mud pellets deposited by the bee as she removes soil to construct the subterranean nursery for her young. Watching these clever bees roll mud balls with their hind legs out of the gallery is highly entertaining. In a nearby patch of marshmallows, mothers gather nectar and pollen to provision their nest with pollen cakes for their young. After providing sufficient food for their brood, the female bee seals the gallery with a plug of mud to prevent parasites and predators from entering the nursery and devouring her youngsters. Ptilothrix bombiformis has taken advantage of human-made features such as the aforementioned shores of Lake Kittamaqundi and roadways passing through marshlands as favorable habitats to construct their nests. Ornamental members of the mallow clan, including Rose-of-Sharon found in residential landscapes are used as sources of nectar and pollen. These behaviors provide opportunities to meet these bees. With some luck and a little nature-focused attention, you may catch a glimpse of these mellow mallow bees in the blossom of a hibiscus or busily tending their pellet-strewn nest sites along the banks of a lake or trails through a marsh.

Acknowledgements

Once again, we thank Sam Droege for generously taking time to identify Ptilothrix bombiformis and share his wisdom about these beauties. Information about the bees featured this week came from Joseph S. Wilson and Olivia Messinger Carril’s amazing book “The Bees in Your Backyard”, and “The Biology of Ptilothrix bombiformis (Hymenoptera: Anthophoridae)” by Richard W. Rust. Many thanks to Professor Shrewsbury for spotting the nest site of Ptilothrix bombiformis and providing video for this episode.

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