Agapostemon virescens: this green metallic sweat bee loves your garden

by Chloé Markovits and Nicholas Dorian

Ask a kid to draw a bee, and they will make it big and fuzzy, with black-and-yellow stripes. But did you know that not all bees look like that? One of the most common bees in eastern North America is slender, shiny, and green!

The bi-colored striped sweat bee (Agapostemon virescens) is a stunning green bee that you can find in your garden all year long. It is so beloved that it’s the official bee of Toronto. It lives mostly in the Midwestern and Northeastern United States (but with sightings from coast to coast). To help you spot one, let’s review its life cycle, flower preferences, and appearance. For a quick guide, reference our A. virescens field ID tips.

A. virescens are active for most of the growing season—typically from mid-May through late October—consisting of two generations of females. In mid-May, mated females emerge from hibernation and build nests underground. They use their natal nest or dig a new hole in sparsely vegetated soils. Mulched garden beds, sandy paths, patchy lawns are suitable nesting habitat.

Agapostemon virescens live in underground communal nests.
Agapostemon virescens live in underground communal nests. Females take turns guarding the nests from unwanted intruders like parasitic flies and cuckoo bees. PC: Nicholas Dorian

Unlike the social societies of honey bees or bumble bees, Agapostemon nest in communal societies of egalitarian females. Like apartment complexes in the human world, many females (up to 30!) will live in a single nest, but each will take care of her own offspring independently. A nest has one entrance (a hole at the surface of the ground) which is used by multiple females.

Ground-nesting bees are at risk from predators that aim to steal the nest contents. But Agapostemon has figured out a defense strategy: while some females are out foraging, at least one always stays back to guard the nest. Females take turns guarding the nest, and the presence of a guard reduces the chance of a visit from an unwanted intruder.

While out foraging, A. virescens females gather pollen and nectar. Females can be found on a wide variety of plants—from roses, to strawberries, to sunflowers—but in general have a penchant for plants in the Asteraceae family. In particular, summer-active females love collecting pollen from purple coneflower (Echinacea purpurea), cup-plant (Silphium perfoliatum), false sunflower (Heliopsis helianthoides), and lance-leaf coreopsis (Coreopsis lanceolata).

Agapostemon virescens loves asters like cup-plant.
Agapostemon virescens loves asters! Cup-plant (Silphium perfoliatum) is one of their favorite plants. PC: Nicholas Dorian

In late-July to early-August, a second generation of A. virescens emerges. This generation consists of both males and females. Males cloud around flowers waiting for the chance to mate. They are not welcome back in their natal nests, so males find places to sleep outside. Sometimes, males will sleep together in aggregate in sheltered nooks and crannies– bee slumber parties!

Fall-active females sip nectar from flowers in order to survive winter, but do not build nests or lay eggs. Look for fall-active A. virescens on asters like New England aster (Symphyotrichum novae-angliae), goldenrods (Solidago spp.), and sneezeweed (Helenium autumnale). When temperatures drop, mated females enter hibernation in their natal nests until spring, and males die.

A. virescens females can be easily identified on flowers or with photographs taken with a phone camera. Look for a metallic green thorax and black-and-white striped abdomen.  Males are trickier to distinguish from males of other Agapostemon species but can be identified by their metallic green thorax, black-and-yellow abdomen, and the lack of dark streak on their hind femur.

Agapostemon virescens females can be easily identified via the combination of a metallic green thorax and a black-and-white abdomen.
Female Agapostemon virescens can be easily identified via the combination of a metallic green thorax and a black-and-white abdomen. PC: Nicholas Dorian

If you’re lucky enough to spot one, take a moment to appreciate its fascinating life cycle. Is a male darting around in search of a mate? Or a female working hard to collect pollen for her offspring? Is she hovering close to the ground? That might mean you’ve found her nest! Despite its abundance, there is still much to be learned about this bee. What soils do they like for nesting? How far do they forage? How long do females live after provisioning nests in early summer? Why do females prefer aster pollen over all others? TPI scientists are hard at work addressing some of these questions.

Container gardening for pollinators

In the city, outdoor gardening space can be hard to find. Backyards are replaced by balconies, but this doesn’t mean you can’t still garden for pollinators. You just have to get creative about it. This National Pollinator Week, TPI has tips and tricks for how you can support urban pollinators with container gardening whatever your outdoor space looks like (even if it’s just a windowsill!).

1. Choose perennial native plants. Apart from a few species with deep taproots, many native plants will thrive in containers. Since they’re perennial, your plants will come back year after year, and support pollinators without any extra cost. Here are some good options that TPI has had success with:

  • swamp milkweed (Asclepias incarnata)
  • anise hyssop (Agastache foeniculum)
  • lance-leaf coreopsis (Coreopsis lanceolata)
  • smooth aster (Symphyotrichum laeve)
  • great blue lobelia (Lobelia siphilitica)
  • cardinal flower (Lobelia cardinalis)
  • blazing star (Liatris spicata)
  • virginia mountain mint (Pycnanthemum virginianum)
  • purple coneflower (Echinaecea purpurea)
Blazing star (Liatris spicata), aka gayfeather, is a favorite of pollinators and easily grown in a container! PC: Flickr, CC.

2. Combine plants with complementary bloom times to have the greatest impact. For your first container, try picking three plants: one that blooms in early summer, one in mid-summer, and one in fall.

3. Deep containers are your friend. Choose containers at least 16” deep to allow your native plants to build strong root systems and thrive for years to come.

4. If you only have space for shallow planters (<12” deep), annual or biennial native plants are a great option. These plants don’t invest in a deep root system and can survive in shallower soils. Black-eyed susan (Rudbeckia hirta), spotted horsemint (Monarda punctata), and partridge pea (Chamaecrista fasciculata) are three good choices.

5. Flowering herbs are also a great (and tasty) option for small spaces. TPI previously waxed the benefits of letting your herbs for pollinators, and the same applies here. Dill, lavender, thyme, mint, and cilantro are all popular herbs that do well in containers and favorites of pollinators.

Container herb gardens can be a win-win! Flowering mint feeds all sorts of pollinators, like this metallic green sweat bee (Agapostemon sp.), and you can enjoy mint leaves in sun tea and watermelon coolers! PC: Nick Dorian.

6. Water often. Soil in containers dries out faster than those growing below ground, and as your plants become root bound, they will need water more frequently. As always, water the soil rather than the leaves.

7. Don’t fertilize. Native plants are adapted to soils that are low in nutrients, and adding fertilizer will result in many big leaves and not many flowers. Amend store-bought potting mix with perlite and sand to create a well draining medium for your plants. Leave the manure and kelp fertilizer for your veggies!

8. Protect your container plants over the winter with 3-4” of leaf mulch, by moving your planter to a less exposed area, and potentially covering with a tarp. The key is to keep the soil warm enough so the roots don’t freeze through.

The right way to leave stems for native bees

About 30% of New England’s native bees build nests above ground. Besides bee hotels (many of which have their own issues), a great way to support these above-ground nesting bees is to leave dead plant stems standing in gardens. Bees will lay and provision offspring in these hollow or pithy stems. TPI members are often asked by gardeners, “when is the best time to cut down stems?” The answer is at least two years (ideally never), which is longer than you might think. Let’s review bee and plant biology to understand why.

Year 1: Plant stems are growing. Native plants like joe-pye weed, elderberry, wild bergamot, mountain mint, and swamp milkweed produce hollow or pithy (e.g. soft, spongy tissue) suitable for nesting bees. Bees won’t nest in these actively growing stems. At the end of the growing season (December through March), cut the stems back to between 6-18” tall. Use sharp tools to ensure a clean cut. By cutting back the stems, you have created homes for next year’s bees.

Joe-pye weed (Eutrochium purpureum) stems were cut back in December 2020. The stems are hollow and will provide homes for twig-nesting bees during the 2021 growing season. TPI will leave them stems standing until the end of the 2022 growing season to ensure that all bee offspring have emerged.

Year 2: Bees active during this year will nest in the stems you left standing. They will lay eggs in the stem and provision each egg with a nutritious ball of pollen and nectar. Inside the stem, bees will develop from eggs into larvae and adults that hibernate through winter. Bees won’t emerge from stems until next growing season. Remember to cut back the new, green stems produced this year for next year’s bees.

Leafcutter bees (Megachile sp.) will live in your garden if you provide undisturbed stems for them to nest in.

Year 3: In spring of year 3, stems produced in year 1 still contain bees; stems produced in year 2 do not contain bees. Leave both generations of stems standing throughout the year. Spring-active bees will emerge from year 1 stems by June, whereas fall-active species might not emerge from year 1 stems until August or early September. During this time, new bees will nest in year 2 stems, so leave them standing!

While this may seem like an awfully long time to leave stubble in a garden, it is the only way to ensure that native bees find safe, undisturbed places to nest. Posting signage in your garden to inform visitors about how gardens can be managed to balance aesthetic and ecological goals can be helpful.