Orchardists and home gardeners know that ample blossoms on their fruit trees are no guarantee of a bountiful crop. Success in fruit trees, as elsewere in the garden, depends on good pollination. Unfortunately, the dramatic reduction in the numbers of beneficial insects due to modern-day pesticide use has had a detrimental effect on fruit-crop production. And pollinating honeybees have their own troubles, not to mention that depending on them also requires taking on the responsibility of keeping a colony. So how do we solve the problem?
The answer may lie with the blue orchard bee, a.k.a. orchard mason bee (Osmia lignaria propinqua). Native to North America, they are prolific pollinators, reaching a whopping 95 per cent of the blossoms in their range (honeybees only reach five per cent). Only 250 females are required per acre to pollinate apples. In fact, just one blue orchard bee is capable of pollinating over 2,000 apple blossoms a day!
The blue orchard bee is found on both sides of the Rocky Mountains, but is especially suited to the wet, cool spring of the Pacific Northwest, emerging earlier in the season than the honeybee. In addition to its pollinating prowess, it does not sting and is immune to the deadly varroa and tracheal mites that threaten bees. Not entirely bee-like in its appearance, this dark metallic-blue insect looks much like a common housefly. It’s about nine to 16 millimetres in length; females are slightly larger than the males.
Life Cycle and Nesting
Called a solitary bee because it does not live in a colony or have queens or drones, the blue orchard bee makes a nest in pre-existing holes in wood and in the company of other blue orchard bees nesting nearby. As the temperature warms to 14°C, sometimes as early as March, the bees will be observed at work filling their nests.
The life of a male blue orchard bee is over soon after mating, while the female lives for about six weeks before she dies and the cycle begins again. Females make several cells in each hole, separating them with mud, and then finally plug up the nest holes with more mud [Fig. 2]. Female eggs are laid towards the bottom of the nest, with the males being deposited near the front of the hole. There are usually two males for every female, and an average of four to six cells per nest – but this can range from between one and 12 in a nest hole. The female provisions each cell with enough pollen and nectar to rear one larva. A few days after the laying of the egg, the larva hatches and feeds on the pollen and nectar for about 10 days, until it spins a cocoon and pupates. As summer fades, the bee transforms into an adult and waits until the following spring before chewing its way out of the cell.
Since these bees are native to B.C., there is usually no need to obtain them elsewhere, although they are available for purchase. If the conditions are right, they will happily move into the nests you provide for them – they may be all around without your knowing it! Blue orchard bees often nest between the shingles on house walls, and in hollow stems or woodborer holes.
Building a Bee House
Nesting blocks for the bees can be built easily at home using 10-by-15-centimetre mill ends of untreated pine, fir or hemlock. The bees prefer holes no deeper than 10 to 15 centimetres, with a diameter of seven millimetres. Research studies indicate this depth of hole produces the ideal female-to-male ratio for optimal pollination of the surrounding area. Holes are drilled to within 1.25 centimetres of the back of the block, as shown in the illustration (below). Do not drill all the way through, since predators may get inside. The holes should be drilled 1.9 centimetres apart. A slanting roof will give some protection from rain.
Alternatively, nests can be assembled by bundling paper soda straws folded once in the centre, then stabilizing them with masking tape and stuffing them neatly into a tin can or a container such as a milk carton. Overhead protection should be provided.
The blue orchard bee dislikes direct sunlight, shade, wind and rain, so proper positioning is important. Nests should be mounted on a building for stability, facing south or southeast. Do not place them on a swaying tree. Solitary posts are also not very successful locations. When selecting your spot, remember that the bees concentrate their search for pollen to within 30 metres of their nesting site.
Nesting blocks can be removed in September or October for safe storage over winter in an unheated building. They should not be moved when there is activity in the nest. They are also sometimes stored in a humidified refrigerator or insulated with bales of hay or straw. I choose to leave the nests in the garden.
If the nests have been over-wintered in protection, they should be set outside in their original locations when the temperature warms. If the bees nest successfully, their numbers should multiply by five each year. If more nesting blocks are added over a larger area, the pollination coverage will also expand.
Since the object of using the blue orchard bee is to pollinate fruit trees, one may want to prolong the nesting period to maximize bee yields by growing other members of the rose family, which flower later than fruit trees. Also appealing to the blue orchard bee, Chinese winter honeysuckle (Lonicera fragrantissima) offers enticing before-fruit-flower bloom and Tatarian honeysuckle (Lonicera tatarica) provides after-fruit-flower bloom. Honeysuckle provides for many other creatures as well, as hummingbirds feed on the nectar, and robins and other birds devour the juicy berries. Lily-of-the-valley bush (Pieris japonica), willow (Salix) and American cranberry bush (Viburnum) provide shrub flowers for blue orchard bees, while the ground-hugging strawberry (Fragaria) and the long-flowering cranesbill (Geranium) offer perennial flowers.
An otherwise unwanted crop of dandelions (Taraxacum officinale) can be mown while fruit trees are in bloom but when that pollination is over can be allowed to flower to help sustain the late blue orchard bees. Equally undesirable in the garden is broom (Cytisus scoparius), however blue orchard bees thrive on it. The mustard family (Brassica) contains grand all-purpose plants – take kale, for example, of which we eat the leaves; the flowers are relished by the blue orchard bees and the seeds sustain finches during winter. Godetia (Clarkia), baby-blue-eyes (Nemophila menziesii), sage (Salvia carduacea), dusty miller (Senecio), white clover (Trifolium repens) and vetch (Vicia californicum) are all plants that will keep the busy blue orchard bee at work in your garden.
Audrey Ostrom is a Master Gardener with VanDusen Botanical Garden and an experienced keeper of blue orchard bees. Thanks to Marina Princz and Dr. Richard Stace-Smith for their assistance with this article.