During a warm spell back in late March, adult Mason Bees began emerging from nest sites to begin mating. During recent days, we photographed the fertile females as they continued the process that will produce the next generation of these fruit-friendly pollinators.
Mason Bees visiting a box stocked with blocks of wood wherein holes have been drilled to provide nesting cavities for the egg-laying females.Mason Bee populations using man-made nesting structures in our region include two non-native species. Both show horn-like structures on the face, denoted here by the white hairline. To pollinate orchard crops, Osmia cornifrons, the Horn-faced Bee, was intentionally introduced to the eastern United States from northern Asia in 1978. Another early season pollinator, Osmia taurus, was accidentally introduced to North America from eastern Asia. O. taurus was first reported in Maryland and West Virginia in 2002 and had reached Pennsylvania by 2006-2008.Inside a nest cavity, a female Mason Bee will deposit pollen balls and an egg within each cell she creates using mud partitions. Cell by cell she fills the cavity with eggs and pollen. When the cavity is full, a thicker mud plug is constructed to seal off the entrance. After hatching, each larval bee will remain inside a cocoon within its individual cell consuming the enclosed pollen ball and maturing. After passing the winter in the nest, the new generation of adults will emerge in the spring…outermost cell occupants first.When reusing a nest cavity from a previous year, a female Mason Bee will excavate old partitions and other debris before starting her own nest. This female is removing saw dust and is possibly enlarging the hole on this new block of wood that we drilled just a few weeks ago.The majority of our Mason Bees prefer 1/4-inch holes, but some use the smaller 3/16-inch ones. Holes up to 3/8 of an inch in diameter are readily accepted by both Osmia cornifrons and taurus.A look at several female Mason Bees backed into their nest cavities to deposit eggs and pollen. Note the load of pollen adhering to the abdomen of the head-first bee at the bottom of the image and the yellow residue around the entrance to the cavity above it.Before a female Mason Bee has a chance to seal up a cell or to plug up the entrance to the cavity, a Drosophila fly (left) or other nest raider may show up to plunder a portion of the stash of pollen.Once filled with cells, each containing an egg and a pollen cache, the mother bee seals the nest cavity with a wall of mud.A bee seals up a 3/16-inch cavity with a mud barricade. Adult female Mason Bees may select smaller than usual holes to deposit eggs that will produce their smaller male young.Upon finishing the masonry job here, this female will move on to yet another cavity to deposit the balance of her fertilized eggs.One week later, the Mason Bees’ work is done.The adults are then soon to perish. But meanwhile, the next generation begins hatching and growing within the protection of the nest. We’ll see them early next spring.
