Kim Taylor/Bruce Coleman Inc.
Most mason bees are smaller than honey bees, but some are about the same size as honey bees or slightly larger. They have stout bodies, and many species are metallic green or bluish in color. Mason bees are common in the western United States, especially in forested regions, but they are also found in many other parts of the northern hemisphere. About 140 species of mason bees are found in North America out of about 200 species worldwide. These bees have a sting but do not attack defensively unless handled.
The orchard mason bee, or blue orchard bee, is a metallic blue-black species about 13 mm (0.5 in) long. This bee, native to North America, specializes in collecting pollen from the flowers of fruit trees. In some parts of the United States, the bees are cultivated to pollinate orchard crops, especially apples. This bee nests in holes in wood and the females prefer to make nests close to each other in aggregations. These traits are used to concentrate enough bees in an area for commercial pollination. Blocks of wood with holes drilled in them attract nesting bees. These nest blocks are hung from trees or are placed in shelters for protection from the weather.
Orchard mason bees mate in the spring. The females then begin to collect pollen and lay eggs. Larval bees feed for several weeks inside their closed cells. They pupate in late summer and spend the autumn and winter as adults inside their pupal cocoons in the nest. They emerge from the cocoons in the spring, coinciding with flowering of many orchard crops. The new generation of bees then begins the cycle over again.
Orchard mason bees are very effective pollinators. Two or three females can pollinate the equivalent of a mature apple tree in one season. They fly in cool or rainy weather and can supplement or replace honey bees as commercial pollinators in some situations.
Other mason bees are also used for pollination. Another North American species, the blue blueberry bee, is used as a pollinator for blueberry plants. The Japanese hornfaced bee is native to Japan and has been used for apple pollination there for more than 50 years. One female can pollinate over 2000 apple flowers per day. The Spanish hornfaced bee is used similarly in Spain for pollinating the flowers of almond trees.
Scientific classification: Mason bees comprise the genus Osmia in the leafcutter bee family Megachilidae, order Hymenoptera. The orchard mason bee is Osmia lignaria, the Japanese hornfaced bee is Osmia cornuta, and the Spanish hornfaced bee is Osmia cornifrons; the blue blueberry bee is Osmia ribifloris.