Scorching temperatures arrived this week in the Washington metropolitan region. With the heat, homeostatic cooling functions of humans, in the form of sweat, have been working overtime. Last week while sitting outdoors after a hike in the forest, my sweaty leg was visited by a gorgeous small green bee known as a sweat bee. Sweat bees belong to a family of bees called Halictidae. We visited other bees including honeybees, bumble bees, carpenter bees, mason bees, and plasterer bees in previous episodes. Unlike social bees such as honeybees and bumble bees where queens are attended by legions of workers who are their daughters, Augochlora pura bees conduct a “you and me against the world” association with their offspring. They are known as solitary bees. After a brief interlude with a mate, female sweat bees construct a gallery in wood or soil depending on the species. In the case of Augochlora pura , galleries made by other insects in rotting wood are favored locations to build a home. Within these galleries tiny cells are constructed and provisioned with balls of pollen and nectar, soon to be food for the sweat bee’s offspring. The female sweat bee lays eggs on the pollen cakes and her larvae consume this nutritious food. Once larval development is complete, pupae form within the cells and emerge later as adults. With the pressing need to gather food for her young and acquire an energy source herself, it is easy to see why Augochlora pura is so often found on blossoms of many flowering annual and perennial plants in landscapes and gardens. More than 40 different species of plants have been recorded as food.
On a hot summer’s day, watch as the sweat bee laps at invisible salty deposits on my skin. The bee’s “tongue” is the jointed appendage beneath its head.
But what is this business of lapping up sweat from humans and other mammals? In both insects and humans, electrolytes like sodium play important roles in nerve and muscle function and a variety of other life processes. To maintain homeostasis, concentrations of electrolytes are closely regulated in the blood of insects and people. In a fascinating article, Dr. Edward Barrows clearly demonstrated that filter papers moistened with a solution of NaCl were highly attractive to Augochlora pura. On hot days when men sweat and women glisten, halictid bees will stop by human salt licks to lap up the abundant salt contained in sweat. Many people fear bees, and rightly so for those with allergies to their stings. However, unlike honeybees and other social stinging insects that depend on coordinated aggressive attacks to defend their hives or nests, solitary bees like Augochlora pura are docile and reluctant to sting even when licking sweat from a human. Why? Social bees and wasps have legions of workers to support the hive and defend the queen and brood inside. The death of soldiers while protecting the colony is an acceptable loss. Solitary bees lack the luxury of sociality. If they risk death by attacking an aggressor and lose, their death terminates the opportunity to reproduce to the fullest. To test the mettle of my solitary bee visitor, I interrupted its sweaty feast with my finger. It took several minutes of prodding, poking, and restraining before the bee delivered a tiny sting to my thigh. To demonstrate that this was merely a warning, the bee quickly resumed imbibing sweat from my skin. When sweat bees stop by for a drink, try not to panic. You might even enjoy observing them for a moment before you gently shoo them away.
A more traditional encounter with a sweat bee unfolds as the bee searches for nectar in blossoms of butterfly weed.
Bug of the Week thanks Nan, Larry, and Sandy for providing the inspiration for this episode. The interesting articles "Nesting Habits and Life Cycle of a Sweat Bee, Augochlora pura (Hymenoptera: Halictidae” by Karl Stockhammer, “Featured Creatures - Sweat bees, halictid bees” by Katie Buckley, Catherine Zettel Nalen, and Jamie Ellis, and “Aggregation behavior and response to sodium chloride in females of a solitary bee, Augochlora pura (Hymenoptera: Halictidae)” by Edward M. Barrows were consulted to prepare this episode.