Blue Orchard Mason Bee (Osmia lignaria)

By Beatriz Moisset and Vicki Wojcik, Pollinator Partnership

Osmia lignaria.
Blue orchard mason bee (Osmia lignaria).

Osmia lignaria.
Blue orchard mason bee (Osmia lignaria).

There are a number of bees, called mason bees, that are very good at pollinating fruit trees, so much so that they are also known as orchard bees. They are related to other orchard pollinators like the leaf cutter bees featured in previous Pollinator of the Month highlights (see Leaf Cutting Bees, Megachile spp.). Mason bees (members of the genus Osmia) and leaf cutter bees (members of the genus Megachile) are similar in many ways: they carry pollen on their bellies rather than on their hind legs and they nest in holes. When building their nests, mason bees do not use cut leaves they way that leaf cutters do; mason bees use clay to make partitions and to seal the entrance. This unique mud-building behavior leads to their common designation as masons.

Honeybees are very important to commercial agriculture, but native bees like the blue orchard bees are better and more efficient pollinators of native crops. There are 140 species of Osmia in North America. They are all known for visiting fruit trees, such as apples, plums, pears, almonds, and peaches. The blue orchard bee or Osmia lignaria, is prized for its efficiency pollinating fruit trees and is one of the few native pollinators that is managed in agriculture.

Blue orchard bees are about the same size as a honeybee but there are a few key points that help you tell them apart. Blue orchard bees are a dark metallic blue, not striped brown and orange like the honeybee. If you pay attention to where they carry their pollen you can also easily tell apart masons and other leaf cutters from honeybees – honeybees carry round balls of pollen on their hind legs.

Osmia lignaria.
Nesting blue orchard mason bees (Osmia lignaria).

Osmia lignaria.
Blue orchard mason bee (Osmia lignaria).

Masons are solitary like most native bees. This means that each one tends to its own brood, instead of having a queen and worker bees. However, they seem to like the company of others of their kind and happily build their nests next to each other. They also readily accept the hollow tubes provided by the orchard grower for this purpose. This proves to be very beneficial to the fruit tree grower because it makes it easy to manage this valuable orchard helper.

Not only commercial fruit growers, but home gardeners too the opportunity to have some orchard bees in their own gardens by placing hand-made or store-bought bee houses or bee blocks in their yards.

The blue orchard bee season is early spring. Once they emerge they promptly mate, search for empty holes that are the right size and shape and go to work stocking their nests. The favorite food for their brood is fruit tree pollen plus some of their nectar. Females collect this food, bring it to their nests, and knead it into a ball, mixing it with nectar and their own saliva. Once they have a food store that is big enough, they lay an egg on top of this mass and seal-off the chamber or cell. Afterward, they build a little mud wall and start gathering food for the subsequent cell. They work this way until there are five to eight cells each with food and one egg. Then, they seal the entrance to the hole with a thicker mud wall. The larvae grow and, by the end of summer, metamorphose into pupae and later on into adults, which remain safe and sound inside the nest until the next spring. The new generation emerges the next spring usually in perfect timing with the blooming peach or apple trees.

A quick fact – the first brood cells that the orchard bee makes (those that are furthest back) will develop into female bees, while the ones closer to the entrance of the nest will become males. Scientists believe that bees do this for one of two reasons. Males need to emerge first so that they wait for new females during mating season – putting them closer to the entrance helps them emerge first. Bees also suffer nest predation, and the brood closer to the entrance would be predated first. Females are much more important to the reproduction of a species than males are. Putting the males as a barrier increases the survival and fitness of the species.

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