Whether this will be your first vegetable gardening season or your 50th, your success with many crops will depend on the timely presence of pollinators. Cucurbits, including cucumbers, squash and melons, are pollinated by bees carrying pollen from male flowers to female flowers. Tomatoes, eggplants and peppers are also bee-pollinated. It is hard to imagine a successful gardening season without these industrious insects at work in the garden.
Non-native honeybees are in decline. I could count on one hand the number of honeybees that visited Marjorie’s Garden in each of the past several years. The job of pollination in our garden has been handed off to native bees, including several species of solitary bee and the industrious bumblebees that are ever-present through the spring and summer.
Gardeners need to do everything possible to ensure the yearlong presence of native pollinators, and March is a great time to get started.
Constructing garden nest boxes for solitary bees
Female solitary bees lay their eggs in hollow plant stems, such as raspberry canes, or in the holes made by other insects in dead trees and fence posts. Because the population of solitary bees is limited by lack of natural nesting sites, constructed nest boxes placed around the garden can increase the number of these gentle bees at work there.
Solitary bees start searching for nest sites in May. A single female may lay up to 35 eggs with each tunnel of the box containing up to 16 egg cells. Within each cell, the female bee lays a single egg on a loaf-shaped provision of nectar and pollen, then seals the cell with a thin partition of masticated plant material or mud.
This is tireless work. For the Maine blueberry bee, it takes up to 20 trips to blueberry flowers to complete one cell.
The eggs hatch into larvae (grubs) that feed on the nectar-pollen provision. They then go through a nonfeeding pupal stage. By late fall, they have become adult bees that remain dormant through the winter, emerging the following spring.
Solitary bee nest boxes are easy to construct:
• 2-by-6-inch pine or spruce board cut into 5-to-6-inch lengths
• Metal ¾-inch strapping tape (also called plumber’s tape) cut into 3-inch pieces
• Screws for attaching the plumber’s tape to the back of the box
• Nails to attach boxes to stakes
• Drill with 5/16-inch and 7/16-inch drill bits
1. Drill 12-14 holes, evenly spaced, into the 2-inch side of each board. Make half of the holes 5/16-inch in diameter, the other half 7/16-inch, a mix of sizes that will provide nesting sites for more than one species of solitary bee. Each hole should be about 5 inches deep. Adult female bees will not nest in tunnels that are open at both ends, so be careful not to drill completely through the block.
2. Screw the piece of plumber’s tape onto the back of the block so that the tape extends two inches above the top of the block. Take care not to screw through a tunnel.
3. Mount the bee nest 3 feet from the ground on either a stake, tree trunk or fence post, angling the front of the box slightly downward to avoid flooding by rain. Bees prefer boxes that face the morning sun. If you like, you can add a slightly overhanging plywood roof to keep out the rain.
And what about those hardworking bumblebees, what can the gardener do to encourage their presence?
Bumblebees are colony-building bees, although the population of the underground colony is much smaller than the honeybee hive. The queen bumblebee is left alone through the winter, hibernating in an old mouse hole that she has selected as next year’s colony site. She awakens in the early spring filled with fertilized eggs, hungry for pollen and nectar. She needs to build her strength before starting a new colony.
To provide suitable nesting sites for bumblebees, leave a portion of your garden wild and unkept. For example, in Marjorie’s Garden, an old stump rots on the edge of the perennial bed. Its skeletal roots, tunneling through the soil, have opened up numerous cavities suitable for a bumblebee nest. In the summer, when the cat mint blooms in the bed, the flower spikes sway under the weight of bumblebees foraging just a few feet from their underground nest.
And on those early spring days when the hungry queen has just emerged from winter’s sleep, the dandelions at the garden’s edge provide the much-needed pollen.
Providing nesting sites for native bees is but the first step in attracting pollinators to your garden. You must also provide yearlong sources of pollen and nectar. In addition to letting the dandelions bloom, the gardener can plant an insectary, a collection of flowering plants that provide nectar and pollen from spring through fall. Next week’s column will be devoted to this task.
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