Native Bee Biology
There is an astonishing diversity of native bees across the USA. About 4,000 species have been identified and catalogued, ranging in length from less than one eighth of an inch to more than one inch. They vary in color from dark brown or black to metallic green or blue, and may have stripes of red, white, orange, or yellow. Many common names reflect the way they build nests: plasterer bees, leafcutter bees, mason bees, wool carder bees, digger bees, and carpenter bees.
Others are named after particular traits, such as cuckoo bees that lay eggs in the nests of other bee species (like the cuckoo bird), sweat bees that like to drink salty perspiration, or bumble bees, who got their name from the loud humming noise they make while flying. Since most don’t fit the stereotyped image of a bee (black-and-yellow-striped, living in a hive, and apt to sting) they are easily overlooked. Out of sight, out of mind they gently get on with foraging, and in doing so provide the vital ecosystem service of pollination.
Check out this great video on native pollinators, featuring Xerces’ Pollinator Program Director, Mace Vaughan: watch the video here at America’s Heartland website
The life cycle of a solitary bee consists of four stages: egg, larva, pupa and adult. Solitary adult bees provision their young with a pollen-ball, illustrated above.
Bees can be divided into two groups by their lifestyles: solitary or social. Despite the fact that the stereotypical image is of a bee living in a hive, only a few species of bees are social. Social bees share a nest, and divide the work of building the nest, caring for the offspring, and foraging for pollen and nectar. The principal social bees are the honey bee (not native to the U.S.) and the bumble bees (about forty-five species in the U.S.).
In contrast, the vast majority of bees, nearly four thousand species in the U.S., are solitary nesting. They tend to create and provision a nest on their own, without cooperation with other bees. Although they often will nest together in great numbers when a good nesting area is found, the bees are only sharing a good nesting site (like people wanting to live in a beautiful, lakeside location!) and not cooperating.
Solitary bees generally live for about a year, although we normally only see the active, adult stage of its life, which usually lasts for only three or four weeks. These creatures spent the previous eleven months growing through the egg, larva, and pupa stage in the brood cell or nest.
During its brief adult life, the bee is focused on successful reproduction. The male bee will hang around nesting areas or foraging sites hoping to mate with a female. The female bee of most species will mate only once–she stores the sperm and releases it when needed–and then spend her time creating and provisioning a nest in which to lay her eggs. Just like honey bees, female native bees have amazing engineering skills, and go to extraordinary lengths to construct a secure nest. In natural conditions, solitary bees will nest in all sorts of places. A few species construct domed nests out of mud, plant resins, saps, or gums along with tiny pebbles on the surface of rocks or trees. Others will even use empty snail shells. Many bees use abandoned beetle burrows or other tunnels in snags (i.e., dead or dying standing trees) or excavate their nests within the soft central pith of stems and twigs. Most species, however, nest in the ground, digging a tunnel in bare or partially vegetated, well-drained soil. Sadly, a human desire for tidiness often results in the planting or covering of bare soil, and the removal of snags and other suitable nesting places.
Each bee nest usually has several separate brood cells in which the female will lay her eggs. The number of cells varies according to the species. Some nests may have only a single cell, but most have more, often ten or more and, occasionally, in excess of sixty. These cells are often in a single line filling the hole or burrow, but some species dig complex, multi-chambered tunnels. Many species line the cell to protect the developing bee. Sometimes these cells are lined with a polymer-like secretion, but often the interior is just made smooth by the bee. The bees that make cells in lines typically nest in holes in plants or trees, and use a cap of plant materials or soil to close each cell and separate one cell from the next. For example, leafcutter bees neatly trim leaf pieces from broad-leaved plants and use them to line their brood cells, cutting different sizes and shapes for different parts of the cells. Mason bees typically use mud or leaf pulp to seal their nests.
Before she closes each cell, the bee must provision it with food for her offspring. She mixes together the nectar and pollen she collected to form a loaf of “bee bread,” which she places inside each brood cell. She then lays an egg in the cell, usually on the loaf, and caps the cell. When she has completed and capped all the cells in her nest, the bee will seal the nest entrance and leave. After the eggs hatch each larva feeds on the bee bread inside its cell until it changes into a pupa. After a period of inactivity, it will finally emerge as an adult and begin the cycle again.
To get the food back to the nest, most bees carry nectar in their crop (a special sac-like chamber in their digestive tract), but how they carry the pollen depends upon the species. Most solitary bees have an area of stiff hairs, called a pollen brush or scopa, into which pollen grains are pushed. These hairs are located either on the underside of the abdomen or along the hind legs. In a few species the scopa extend to reach the sides of the abdomen or the rear of the thorax. A few species don’t have scopa and carry pollen in their crop, probably alternating between pollen and nectar on separate foraging trips.
Solitary bees can be divided into two loose groups according to their foraging habits:
- Generalists are bees that gather nectar and pollen from a wide range of flower types and species. Often these are the more resilient species, able to survive in degraded environments with weedy or non-native plants.
- Specialists, on the other hand, rely on a single plant species or a closely related group of plants for nectar and pollen, and are more susceptible to suffer from landscape or habitat changes. The life cycle of these bees species are often closely tied to their host plant, and the adults will often emerge from their brood cells just when the plant is flowering.
Bumble bees are the only bees native to the US that are truly social. They live in colonies, share the work, and have multiple, overlapping generations throughout the spring, summer, and fall. However, unlike the non-native, European honey bees, the bumble bee colony is seasonal. At the end of the summer only the fertilized queens survive to hibernate through the winter. In the spring, she will found a new nest that eventually may grow to include dozens of individuals (occasionally a couple of hundred).
Bumble bees need a cavity in which to nest. The queens are opportunists, looking for any suitably sized cavity. Sometimes this is above ground, such as in hollow trees or walls, or under a tussock of grass, but they mostly nest underground. An abandoned rodent hole is a favorite, as this space is warm and already lined with fur. The queen creates the first few brood cells from wax, and then provisions them with pollen and nectar and lays eggs. It will take about a month for her to raise the first brood. When they emerge, these bees become workers, taking on the tasks of foraging and helping the queen tend the growing number of brood cells. The workers may live for a couple of months, by which time there will be more bumble bees to replace them. The queen will continue to lay eggs, so the colony will grow steadily through the summer. At the end of summer, new queens and drones will emerge and mate. As the cooler weather of fall arrives most of the bees, including the old queen, will die, leaving only the new, mated queens to overwinter.
They are generalist feeders, often the first bees active in late winter (February) and the last in fall (November). Since they are active for so many months, they must be able to forage on a wide range of plant species in a wide range of weather conditions to support a colony. Some individual bees in the colony, however, choose to forage exclusively on a single species or a limited range of related plant species, effectively becoming specialist foragers. When foraging, the female bumble bee carries pollen in a concave, hairless area surrounded by stiff hairs on her rear legs, known as the pollen basket or corbicula. This basket can be seen clearly when it is empty and, when full, the pollen ball pressed into it is obvious.
Bumble bees also differ from solitary bees when feeding their larvae. They provide food gradually, adding it to the brood cells as the larvae need it (“progressive provisioning”) rather than leaving all the food in the cell before laying the egg. In addition, bumble bees do make a small amount of honey, just enough to feed the larvae and themselves for a couple of days during bad weather.
A number of bee species do not make their own nest and, instead, lay their eggs in cells prepared by another species of solitary or bumble bee. These cuckoo bees are not true parasites as they do not feed on the host bee (although they do kill the larva in the cell so they can get all the food). They are called cleptoparasites, as they grow from egg to adult by feeding on the provisions stolen from the host species.
Typically, cuckoo bees that prey on solitary bees enter the nest to lay eggs while the host is out foraging. Cleptoparasites of bumble bees, however, have to enter an established colony full of workers. Sometimes they will fight to the death with the existing queen and sometimes they’ll hide in the nest until they take on the same smell as the host colony. Once accepted by the colony, the invading cuckoo bumble bee will take over the role of queen, laying eggs which the workers tend.