Solitary Bees
Carpenter bee on eryngium.
I didn’t really know a lot about solitary bees until last winter when I created a presentation on Pennsylvania pollinators. I gathered my photos and then looked for information on the insects I had photographed. I noticed the European wool carder bee for the first time in the summer of 2023 when I found one gathering fibers from a rose campion plant. So far, with the help of BugGuide.net, I have been able to identify and document many different species of solitary bee on the property. They are usually fast and active. I’ve learned that you really have to look for them closely and then be prepared to wait … and wait some more until they are still long enough to photgraph!
Nearly all of the bee species in Pennsylvania are solitary. Female solitary bees construct nests and collect food for their young on their own. The most common of Pennsylvania’s solitary bees at Willowdale Farm are the carpenter bee (large: subfamily Xylocopinae and small: genus Ceratina); the European wool carder, mason, and leaf-cutting bees (family Megachilidae); and the mining bee (family Andrenidae). Of these, I see the large carpenter bees the most frequently.
I see many sweat bees (family Halictidae), but I didn’t include them here. Although species in this family can be solitary, they can also be communal, semi-social, or primitively eusocial, which means they have less ridged social structures than fully eusocial bees (e.g., honey bees). Sociality can also vary based on the climate, season, and the location of the nest. Eventually, they will have their own page.
CARPENTER BEE, Xylocopa virginica
The carpenter bees are mostly solitary, but they tend to be gregarious, which means that groups of different sizes form under various conditions and may be referred to as aggregations, associations, clumps, or other terms.
Most members of this subfamily construct nests in plant stems, trees, other wood, or frame buildings. People often mistakenly think that the bees are eating the wood when instead they are excavating tunnels for shelter and as chambers in which to rear their young. They feed on nectar and pollen.
It can be difficult to differentiate species of carpenter bees from each other, but you can tell carpenter bees from bumble bees by looking at their abdomens: most carpenter bees have shiny abdomens, whereas bumble bees have abdomens that are completely covered with dense hair.
The Xylocopa virginica or Eastern carpenter bee is the only large carpenter bee found in Pennsylvania, which has 118 species in the family Apidae.
Click on photo to open gallery.
SMALL CARPENTER BEE, genus Ceratina
This genus contains more than 300 species in 23 subgenera. Small carpenter bees are black with a green or blue iridescence. They prefer to nest in hollowed-out stems of plants, such as raspberry or sumac bushes, and are easily distinguished from the large carpenter bee simply by size. The small carpenter bees are less than 8 mm (about one-third of an inch). They excavate nests with their mandibles in the pith of broken or burned plant twigs and stems. Females overwinter as adults in partially or completely excavated stems. In the spring, this resting place (hibernaculum) is modified into a brood nest by further excavation.
In this species, multiple females may occasionally be found in a single nest in which daughters or sisters form very small, weakly eusocial colonies. Some species in the genus are parthenogenetic, ie, females can produce female eggs without mating or fertilization.
Click on photo to open gallery.
Click for photos of the giant resin bee, Megachile sculpturalis, on Verbena bonariensis. The much smaller male can be further distinguished from the female by a line of golden hairs that resemble a mustache directly above the mandibles. Also, the abdomens of the males are less pointed than those of the female.
MEGACHILIDAE
Megachilidae is a family of solitary bees with ~630 species in North America. Most are native, and a few were introduced accidentally and intentionally. Globally the number of species exceeds 4,000, and 81 species have been recorded in Pennsylvania.
Most bees carry pollen on their legs in one of two ways: broom (scopa) vs. basket (corbicula). However, the megachilid bees are different: the underside of the abdomen is particularly hairy and is used for this purpose. If you see a black bee about the size of a honeybee with a yellow belly, it is probably a leaf cutter.
The common names of the nonparasitic species come from the materials they use to build nests:
- Carder bees use plant fibers or animal hairs.
- Leaf-cutting bees cut disks from leaves and petals (and sometimes plastic).
- Mason bees use mud. Click here to visit my blog on mason bees.
- Resin bees use plant resin. See slider gallery to the left.
Other species are kleptoparasites often referred to as “cuckoo bees” that feed on pollen collected by other megachilid bees. Parasitic species do not possess scopae. See photos below of the sharptail bee, an example of a Megachilid cuckoo bee.
Click on photo to open gallery.
EUROPEAN WOOL CARDER BEE, Anthidium manicatum
The name “carder” is derived from behavior in which the female scrapes hair from leaves such as lamb’s ear or the rose campion shown here to use as nesting material — males fiercely guard areas of these plants for potential mates. They carry this hair bundled beneath their bodies. The carder bees do not cut leaves or petals as is typical for megachilids.
- One of the few bee species in which the male is larger than the female.
- This species has been called the most widely distributed unmanaged bee in the world because of its expansive native and non-native range. It was accidentally introduced into the United States from Europe sometime before 1963, when it was discovered in New York. It has since spread from the northeastern United States and southeastern Canada to California, where it was first collected in 2007. These bees tend to occupy ready-made nesting sites, which allows them to easily spread to new locations.
← From left to right, these images show how a female wool carder bee finds a source of plant fiber (rose campion here; lamb’s ear is also commonly used), pulls the fiber off the plant with her mandibles, balls it under her abdomen, and flies with it back to her nest. It’s commonly thought that the females fly with the fibers held beneath them, but I saw this one fly off with the fibers in her mandibles.
LEAF-CUTTING BEE
Leaf-cutting bees vary in size, but on average, they’re about the same size as a honey bee. Nonparasitic Megachilidae, including leaf-cutting and carder bees, typically divide their nests into cells:
- After finding a suitable spot (often near where she emerged), a female builds a first cell, stocks it, and ovoposits.
- Each cell receives a supply of food (pollen or a pollen/nectar mix) and an egg.
- She builds a wall with a leaf to separate the completed cell from the next one.
- The larva hatches from the egg and consumes the food supply. After molting a few times, it spins a cocoon and pupates.
The leaf-cutting bee below cut a piece of sundrop petal and then flew away with it.
Click on the photo to open the gallery.
Kleptoparasites, aka “cuckoo bees” in the family Megachilidae
Shown above are sharptails, leaf-cutting cuckoo bees, genus Coelioxys. Members of this genus of solitary kleptoparasitic bees in the family Megachilidae do not have pollen-carrying adaptations (scopae or corbiculae) since they do not provision nests. Instead, they lay their eggs primarily in the nests of other Megachilid bees but also in the nests of Osmia (mason bees) and Anthophora (wood digging bees) on their provisions of pollen. The larvae of Coelioxys species kill the host larvae with their strongly developed mandibles and feed on the host’s pollen provisions. They spin a cocoon at 11–16 days. Adults feed on nectar from a wide range of flowers and are shown here on calendula.
Andrena, MINING BEES
Pennsylvania has 100 species in the family Andrenidae, the mining bees. The photos to the right shows a bee in the genus Andrena, which includes more than 1,500 species. These solitary, mostly short-tongued bees are called “mining bees” because they excavate pencil-thin nests in the ground. They are among the first bees to emerge in the spring.
Andrena are small to medium sized hairy bees that are black or dull metallic blue or green. Most species have pale bands of hair on their abdomens, and the abdomen is long compared to other bee groups. Andrena have a variety of striking colorations, but species are difficult to differetiate.
The genus includes no parasitic or social species, but some nest communally or in aggregations. Many Andrena are host-plant specialists, visit flowers of only a single or a few closely related plants, and have physical and behavioral characteristics that specialize them for their pollen preferences. For example, species that focus on the plant family Asteraceae and have highly branched, fluffy scopal hairs to hold aster pollen.
Spring 2024 Update
I’ve found many mining bees since February. A few of them are shown to the right.
Click on photo to open gallery.
Especially interesting is the Dunning’s mining bee (Andrena dunningi). And it’s identifiable–many are identifiable only to genus. The large size, bright orange hairs, and dark wings make this a relatively distinctive spring mining bee. It’s notable for the combination of orange thorax and entirely black abdomen. One generation per year. Click on the image to open the slider gallery below.
Dunning’s mining bee on purple dead nettle and crab apple blossoms.
Three of the species I’ve documented are not as common: the Pseudoanthidium nanum, the orange-tipped wood digger bee, and the pugnacious bee.
Pseudoanthidium nanum
I’m able to find very little information on the Pseudoanthidium nanum, an exotic bee that has no common name. A member of the Megachilidae family, it is a newly established old-world bee that was originally discovered in the new world in New Jersey in 2008 and has spread to neighboring states. They frequent Centaurea and and are specialists on plants in the family Asteraceae. Little is known about their nesting behavior, but most species in the genus nest in pre-existing cavities such as stems, wood, galls, or crevices. They have been documented to use plant fibers to partition cells in nests in plant stems and oak galls. I recently found a checklist of bees in Pennsylvania that indicates the Pseudoanthidium nanum was not reported here before 2020.
Pseudoanthidium nanum on cornflower
Pseudoanthidium nanum on cornflower
Pseudoanthidium nanum
Pseudoanthidium nanum on New England aster
Pseudoanthidium nanum on cornflower
Pseudoanthidium nanum on cornflower
ORANGE-TIPPED WOOD DIGGER BEE, Anthophora terminalis
The orange-tipped wood-digger Bee (Anthophora terminalis, member of the Apidae family) has been referred to as “relatively unique and distinctive.” It is most often found in and around shrubby woodlands, where it hovers around flowers or rests on leaves a few feet from the ground.
- The Anthophora bees in general are fast and noisy and forage on a wide variety of flowers.
- The wood digger prefers deep, tubular flowers such as the verbena bonariensis shown here, which they access with long tongues.
- Unlike most species in the genus that nest in the ground, they build their nests in hollowed out pithy stems or in rotting wood, which is why they are called “wood digger.”
- Females have a distinctive orange tip to their abdomens, while males have extensive yellow on the lower portion of their faces.
Female orange-tipped wood digger bee, Anthophora terminalis, on verbena bonariensis.
Male organge-tipped wood digger bee on Solomon’s seal. Click to open gallery.
PUGNACIOUS BEE, Megachile pugnata
Probably the most common of the three is the pugnacious bee, Megachile pugnata, a leaf-cutting bee that is called pugnacious because of the fierceness with which the females guard their nests. The pugnacious leaf-cutter females can be distinguished by their large mandibles, which are longer than their eyes. Members of this species are known specialists of plants in the Asteraceae family.
