Wisconsin Native Solitary Bee Nesting

Excerpted from: Dancing With Bees - A Journey Back To Nature, by Brigit Strawbridge Howard

The ways in which solitary bees excavate, provision, seal and protect their nests varies greatly.

While ground-nesting species have evolved the physiological means and strength to excavate their own nests, hence the use of the name 'mining bee', most of the cavity-nesting species tend to take advantage of whatever holes, tubes or other cavities are available. A few of the 'cavity-nesters', including carpenter bees, have become adept at burrowing into dead wood and hollow plant stems. There are some 'cavity-nesters' that dig into mortar or clay.

Most cavity-nesters do not care whether their nest is made from natural or man-made materials. They will happily nest in a bee nesting box or in cavities such as garden hose pipes, wind chimes, keyholes and watering can spouts.

  Sex And The Single Bee

Whatever a solitary bee's preferred nesting site, be it an aerial cavity or a burrow underneath the ground, their life cycles follow roughly the same pattern.

With one or two exceptions, male solitary bees emerge from their natal nests before the females. They spend a week or two hanging around the nest area or patrolling flowering plants that they know females will forage on. They are awaiting the female bees to emerge.

When the females emerge, mating takes place and male bees play no further part in the life cycle. You can think of female solitary bees as 'single moms', as they take sole responsibility for providing food and shelter for their young.

  Ground-Nesting Bees

Unlike the aerial opportunists who take advantage of existing tunnels and cavities, ground-nesting solitary bees have to construct their nests from scratch before they can begin to think of laying eggs.

  Tunneling

The construction process begins by burrowing down into the earth using first her mouthparts, then her legs and body to create the main tunnel. After she has dug this tunnel, she will usually dig a number of lateral tunnels, each branching out from the main one. At the end of each of these lateral tunnels she fashions a small egg chamber large enough to accommodate a fully-developed adult bee.

And this is where she gets clever. It is obvious that any nest constructed beneath the ground is going to be vulnerable to flooding. At the very least, it is likely to become damp. However, ground-nesting bees solve potential water problems by smearing the sides of the nest chambers, which are seldom made at the lowest point of the tunnel, with anti-fungal secretions. Depending on the species, these secretions are produced from her salivary or abdominal glands.

  Tunnel Differences

No two species of ground-nesting bees are the same. Just at their body size, appearance, flight times and food preferences differ, so too their nest architecture. Some dig shallow burrows just 1 to 5 centimeters beneath the ground, but others burrow much deeper. In addition, the depth of the burrow depends in part on the soil type. For example, bees that nest in very dry soil tend to dig deeper tunnels. There are bees in North America that create tunnels that extend 2.5 meters!

  Tunnel Design

Structurally, ground-nesting bee nests follow a fairly simple design, with a main tunnel running vertically down from the ground, or horizontally beneath the surface, and then branching off in one or two directions. The branches have a series of evenly-spaced nest chambers . Others resemble labyrinths.

Whatever the design, once she is satisfied that the nesting chamber is water-proofed, the female solitary bee sets about provisioning the nest with pollen which is sometimes mixed with a little nectar. She collects the pollen and nectar over many foraging trips back and forth to the nest. Most solitary bees are not too fussy about which flowers they choose for nectar but they can be very selective when it comes to choosing flowers for pollen.

  Preparing For Her Brood

Once she has sufficient food for her offspring, the female solitary bee lays a single egg inside each chamber. She seals the chamber with whatever building materials are found. More often than not this will be soil or sand. She repeats the process again and again until she had laid up to 20 eggs. By the time she has finished laying all her eggs, she will have lived for about 8 weeks. Exhausted, and her job done, she dies.

Inside the nest the eggs hatch into larvae which feed upon the provisions left by their mother. The next stage is pupation during which the larvae undergo complete metamorphosis and emerge at the end as adult bees. Many ground-nesting bee species remain in their chambers, hibernating until the next spring.

Ground-nesting solitary bees that emerge later in the spring or summer tend to overwinter as full-grown larvae and pupate only shortly before emerging.

  Aerial or Cavity-Nesting Bees

The life cycle of cavity-nesting bees is pretty much the same as that of ground-nesting bees. However, as these bees make their nests in cavities above the ground, they do not have access inside their nests to excavated soil or sand. To seal the individual egg cells and block the main entrance, they must forage materials to protect their nest from weather and raiders.

Some species collect mud, sand, resin, tiny pebbles or chewed-up leaf pulp from the surrounding area for this purpose (hence the name 'mason bees'). Others us cut leaves or flower petals (hence the name 'leaf-cutter bees'). You can sometimes identify the species by simply looking at the materials used for constructing their nests.

  Come Fall, 'Save The Stems'

Many hollow or pithy plant stems and branches provide excellent places for cavity-nesting bees to call home over Wisconsin's winter time. During the winter, many of them hide in the hollow stems of bee balm or ornamental grasses.

The desire to clean up your garden after flowers fade and die is understandable. Shriveled brown leaves, blossoms and stems are not as appealing as the bright flowers that dominate summer and fall. However, they are beneficial to overwintering pollinators.