The vast majority of the world’s approximately 20,000 bee species are solitary. Nearly 90% of all known bee species lead an independent life, meaning they do not live in colonies or produce honey, unlike social insects such as honeybees and bumblebees.
A solitary bee’s life cycle is characterized by the absence of a queen, worker castes, or any shared division of labor. The female bee performs every necessary task alone, including nest construction, foraging for food, and egg-laying. Once the female has provisioned the nest and laid her eggs, she does not provide parental care for the developing young. This lack of a shared nest defense mechanism means solitary bees are generally non-aggressive and rarely sting unless physically handled.
Nesting and Provisioning
The female solitary bee focuses on the creation and provisioning of her individual nest. She first finds or excavates a suitable tunnel in bare soil, hollow plant stems, or pre-existing holes in wood. She then divides this tunnel into a series of separate compartments known as brood cells.
For each cell, the female collects a mixture of pollen and nectar to form “bee bread,” which is the sole source of sustenance for the larva once it hatches. After placing the bee bread and a single egg into a cell, the female seals it off. The materials used for sealing and partitioning these cells are species-specific and can include mud, pebbles, tree resin, leaf pieces, or plant hairs.
The bee strategically lays fertilized eggs (females) in the rear of the tunnel, and unfertilized eggs (males) closer to the entrance. This arrangement ensures the males emerge first, ready to mate when the females appear in the following season.
Key Pollination Role
Solitary bees are often considered superior pollinators for many plants compared to the generalized honeybee. Honeybees moisten pollen and pack it neatly into specialized structures called pollen baskets (corbiculae) on their hind legs. In contrast, many solitary bees, such as mason and leafcutter bees, carry pollen loosely on dense hairs (the scopa) located on the underside of their abdomen.
This dry, external carrying method means pollen easily falls off as the bee moves between flowers, maximizing the amount transferred. For example, one female red mason bee is estimated to perform the pollination work of over 100 worker honeybees.
Some solitary bees also engage in buzz pollination, a specialized technique impossible for honeybees. This involves the bee grasping the flower and rapidly vibrating its flight muscles, which dislodges pollen from flowers with tubular anthers, such as tomatoes, blueberries, and cranberries. Solitary bees typically forage much closer to their nests, often traveling only a few hundred yards, making them highly effective for localized crop pollination and native plant ecosystems.
Common Solitary Bee Groups
Solitary bees are broadly categorized into three common groups based on their nesting habits.
Miner Bees
Miner bees (genus Andrena) make up the largest group, excavating their nests in bare or sparsely vegetated soil. These ground-nesting bees typically prefer sunny, well-drained locations to create their tunnels and brood cells.
Mason Bees
Mason bees (genus Osmia) are cavity-nesting bees that utilize pre-existing holes, such as hollow reeds, plant stems, or tubes in wood. They are named for their habit of constructing partitions and seals between their brood cells using mud or clay. The Blue Orchard Bee (Osmia lignaria) and the Japanese Hornfaced Bee (Osmia cornifrons) are well-known examples often managed for fruit tree pollination.
Leafcutter Bees
Leafcutter bees (genus Megachile) are also cavity nesters, recognized by their unique method of using precisely cut pieces of leaves or flower petals to line and cap their brood cells. They use their mandibles to snip out circles or ovals, which they carry back to their nest to construct leaf-wrapped chambers. The Alfalfa Leafcutter Bee (Megachile rotundata) is widely utilized in agriculture for pollinating seed crops.