Sweet times for sweat bees: Sweat bees, Halictus ligatus and Agapostemon virescens

Echinacea, a.k.a. cone flower, renowned for its medicinal qualities as well as its beauty, is blooming at full throttle this week in gardens here in the DMV. Just after sunrise, the fragrance of cone flowers is delightful not only to humans, but also to a raft of pollinators. Bumble bees are usually the first arrivals in the morning, but shortly thereafter industrious bees, members of the halictid clan, arrive to collect nectar and pollen for youngsters in their colonies. The name “sweat bee” is a little goofy and somewhat misleading. These bees don’t sweat, but like sweat bees we visited at Bug of the Week some years ago, some halictids alight on humans and imbibe salt-rich perspiration. Guess this is why they are called sweat bees.

Unlike mason bees and plasterer bees we met in previous episodes, which are solitary with every female caring for her own young, halictid bees vary in their social structure. Some species like Agapostemon virescens adopt the solitary life style, while others like Halictus ligatus are eusocial (truly social) with queens producing non-reproductive daughters known as worker bees, tasked with foraging for nectar and pollen and tending the brood of their mothers. Colonies are founded in spring by females, survivors of winter’s ravages. The sweat bees we’re visiting this week adore cone flowers as a source of food, and both build nests in soil. Founding queens of Halictus ligatus gather pollen and lay eggs that hatch into daughters, worker bees, destined to help with caring for the young and gathering food for the colony. Some of these daughters will eventually become reproductively active and produce daughters and sons of their own. A fascinating study of Halictus ligatus by Miriam H. Richards and Laurence Packer discovered that when weather conditions were favorable, worker bees survived at greater rates and grew larger. Some of these large workers produced their own offspring. However, under less favorable conditions of temperature and food availability, workers were smaller and produced fewer offspring of their own, submitting to larger, more aggressive queens that produced the majority of offspring. Shifting conditions of temperature and food availability govern the social dynamics of halictid bees like Halitcus ligatus.

Cone flowers are a gold mine for many kinds of pollinators. Watch as female halictid bees, a.k.a. sweat bees, gather and store huge loads of pollen in pollen baskets, called corbiculae, on their hind legs. The smaller bee in the first two clips is Halictus ligatus or its sibling species, Halictus poeyi, and the last clip is the gorgeous Agapostemon virescens.  

Beautiful Agapostemon virescens, sometimes called “little green bees”, favor loamy soils with sparse vegetation as prime real estate to build their nests. Individual nests may contain more than 100 brood cells and, while more than one female has been observed in a single nest, this species is considered to be solitary. Once they find favorable plots of land, many Agapostemon virescens queens may move in and form large aggregations of nests. To enjoy these delightful native pollinators, consider letting part of your lush lawn go a little thin in a sunny spot to provide nest sites for nurseries of these bees. Both species of sweat bees and many others of their clan are generalist pollinators and a diversity of flowering plants provide food. However, on this week of Independence Day celebrations, Echinacea and other members of the Asteraceae are dynamite attractors of these beauties to your garden.  

Acknowledgements

Bug of the Week thanks Dr. Paula Shrewsbury for planting cone flowers that served as the backdrop for these delightful bees. Great references consulted for this episode include “Bees of Northwestern America: Agapostemon (Hymenoptera:Halictidae)” by  Radclyffe B. Roberts, “The Socioecology of Body Size Variation in the Primitively Eusocial Sweat Bee, Halictus ligatus (Hymenoptera: Halictidae)” by Miriam H. Richards and Laurence Packer, “The Insect Societies” by Edward O. Wilson, “The bees of the world” by Charles D. Michener, and “Bees, wasps, and ants” by Eric Grissell.  Special thanks to Sam Droege for identifying the halictids featured in this episode.      

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