Walking through the garden after the big snow, the sun shining bright, the temperature in the mid-teens, I see few signs of life other than two crows in the top of a spruce tree, a mixed flock of pine siskins and chickadees at the porch feeders, a red squirrel perched atop a tall cedar stump. And then I notice the native bee nest box mounted about four feet off the ground on one of the grape arbor posts. It was easy to ignore during the garden season when it was surrounded by broad grape leaves, but now it is in plain site, facing east. I made it a few years ago from an old 2 x 4, drilling one of its narrow faces with 3 1/2-inch-deep holes, some 3/32-inch and others 3/8-inch in diameter. It is this side that now faces the low-angled sun. Several of the drilled cavities are sealed off at the entrance with a substance that looks like dried mud.
There is life behind each of those mud walls. Each sealed cavity contains a row of cells separated by walls of mud, and within each cell sleeps a pupating solitary bee, waiting for a thermal signal that it is time to emerge. Rather than construct elaborate colonies like those of the bumble bee, each solitary bee lives its life alone.
The young adult bees within the cells of the nest box are mason bees (Osmia spp.), perhaps orchard mason bees, O. lignaria, a species that seals its nest cavities with mud. Native to the Pacific Northwest, this metallic greenish-blue bee was introduced for crop pollination in New England states and elsewhere in this country and is now firmly entrenched in the native fauna.
The bees in the nest box are spending winter as adults, each snuggled within a cocoon and burning stored fat to stay warm, waiting for the daytime temperature to reach 57 ºF . Should the weather stay cold for too long, the bees could die of starvation. Alternatively, an early spring warm spell could trigger emergence of the bees at a time when pollen sources are scarce or when freezing weather could return, both potentially lethal conditions.
Male bees are the first to emerge in spring, about the time the apple trees begin to bloom. They stay close to the nest, waiting for the females to emerge. Mating occurs immediately and the female begins to forage, waiting for her ovaries to fully mature before seeking out a suitable nest.
Nest boxes, like the one in our garden, have taken the place of many old fence posts as preferred nesting sites. In addition to a suitable nesting cavity, a source of silty or clayey mud with the correct moisture content is also needed to build walls within the nest cavity. And so the female bee may inspect several potential nests before settling in. Once the decision is made, she performs an aerial dance, imprinting major visual features to find her nest when returning from foraging.
She starts her nest construction by collecting mud and building a back wall for the first partition. Working tirelessly from dawn to dusk, she forages for pollen from flowers that are near the nest, visiting approximately 2000 flowers over 25 trips, to provision the first cell. She nourishes herself with nectar from the same blossoms. Once she has packed enough pollen in the first cell, she backs into the hole and lays one egg directly on the lump of pollen, then collects more mud to seal off the cell. This new wall also serves as the back wall of the next cell. This process continues over several days until she has filled the nest cavity with capped cells.
Carrying sperm from the male, the female bee can determine the gender of each egg by fertilizing it or not. Unfertilized eggs develop into male bees, while fertilized eggs become females. Female eggs are laid at the back of the nest cavity, male eggs towards the front, adhering to a sex ratio of about three males to every one or two females.
After filling the nest cavity, the female plugs the entrance hole with a mud wall that is thicker than the internal partitions, a guard against the intrusion of a woodpecker looking for a meal, although it does not always work. She then looks for another suitable nest cavity to begin the process anew. She works her entire life in this manner, a span of four to eight weeks, filling an average of four nest cavities with about eight eggs per cavity. Over the course of her life, she visits approximately 60,000 blossoms, a testament to her productivity as a pollinator in the garden.
By early summer, each bee larva has consumed all of its pollen provisions and begins spinning a cocoon around itself, entering the pupal stage. The transformation from pupa to adult occurs within the cocoon and, as cold weather approaches, each bee enters a diapause, a state of suspended development, until spring arrives.
Diversity among solitary bees is mind-boggling. While about 30 percent (approximately 1,200 species) of North American solitary bee species build their nests in wood cavities, 70 percent (approximately 2,800 species) construct tunnel-like nests in the ground. These earthen nests range from single short tunnels to complex, branching tunnels. Many ground-nesting species, like the polyester bee (genus Colletes), secrete substances to construct water-proof linings for their egg cells.
Like mason bees, solitary leafcutter bees (Megachile spp.) also nest in wood cavities but use pieces of leaves or flower petals instead of mud to seal the ends of each cell. Using their mandibles, they cut the plant pieces into specific shapes and sizes to line the entire cell as well as sealing the ends.
Along with bumble bees, solitary bees are the primary pollinators in Marjorie’s Garden. Honeybees show up in mass only when the Shirley poppies are blooming and remain totally focused on them. Solitary bees, however, visit the blossoms of many different garden plants in the same day.
Solitary bees are among a gardener’s best friends. All the gardener has to do is provide them with year-round forage and a place to live. We accomplish the former task with patches of self-sowing calendulas, one of their favorite plants, on the edges of every vegetable garden bed, ensuring that they will have a summer-long source of pollen and nectar to fill gaps in the flowering of the vegetable crops. And we let the dandelions bloom around the garden, providing and early source of pollen. We put up nest boxes, attaching them to arbor posts and fence posts, always facing the rising sun.