This solitary, ground-nesting species is oligolectic (specialist) on Malvacaea and Convulvaceae plants and can be commonly found on Hibiscus flowers. It is also called the "mallow bee" because of the mallow family or Malvacaea. Marshmallow used to be made from the root of a plant Althea officinalis with the same common name--marshmallow. In gardens and farms containing okra, which is in the Malvaceae family (the flower looks a lot like hibiscus), this bee can be found foraging for okra pollen and nectar. In this way this specialist bee has adapted to the expansion of human beings into their habitats--okra, like most crops, is not native to this continent and is most likely originally from Africa, but is from the same plant family as its other host plants. The flowers of the mallow plants (Malvaceae) family are herkagamous, meaning that the male and female parts are spatially separated in a hermophroditic flower, which encourages cross pollination instead of self pollination. Selfing is a great back-up method for plant reproduction in case pollinators are not around to help move pollen around but some plants have evolved various methods like this to make outcrossing more likely than selfing, thus enabling more genetic diversity and a healthier. more resilient population.
The Hibiscus bee looks superficially like a small bumblebee but upon closer inspection has a very different morphology (body form) and does not have corbicula/pollen baskets like bumblebees. Instead they have dense thick black hairs on their hind tibia for carrying pollen loads back to their nests. Under a microscope it has a beautiful velvety appearance --the softest looking bee I've probably ever seen. The abdomen is pretty short and rounded and has a larger head and thorax. This American bee has a relatively wide range throughout the central and eastern US. I only collected 1 specimen of this species in my PhD research even though most of my work was done in Charleston, SC where you are almost always a stone's throw away from a salt marsh.
Specialist bees like this one are likely the most vulnerable to climate change, habitat destruction and other threats to wildlife. Since they rely on a smaller range of plant species as their food source, if that one food source goes away or is affected by climate change in some way that changes its relationship with the bee (i.e. change in bloom period, change in elevation/geographic range) the consequences are severe for this bee. Compare that to generalist species which have a wider breadth of choices in terms of plants to forage on (honey bees are the "ultimate generalist")--they will have an easier time adapting to a changing environment. This is one of the reasons it's important to focus on planting flowering species that are needed by specialist species, in addition to the flowers that attract a wide range of species.
There aren't many accounts of this species' natural history but there is one great paper* from Richard Rust from 1980 (and several others from Michener and others from 1940's and before) that details their behavior and nest locations. He found that this solitary species nests underground in hard-packed soils close to coastal salt marshes where Hibiscus palustris grows en masse. The females make water foraging trips and use water in nest construction, using their mandibles to shape the mud. They make one or two-celled nests around 50-75 mm below the soil surface. They have an amazing ability to "skate" on the surface of the water and use their proboscis to take up water. He cited severe impacts of spraying Dibrom (an organophosphate still used today) for mosquito control over the marshes in Delaware on the populations of this bee (reduction from hundreds of active nests in early august 1975 to only 35 nests in late august).
While females have a strong innate fidelity to their host plants and thus will only be seen foraging on Malvaceae or Convulvaceae plants, the males can be seen foraging for nectar on other plants. I observed males foraging on purple coneflower Echinacea purpurea and cup plant Silphium perfoliatum. The males will hang around near hibiscus flowers in order to mate with females.
*Rust, R. (1980). The Biology of Ptilothrix bombiformis (Hymenoptera: Anthophoridae). Journal of the Kansas Entomological Society, 53(2), 427-436.