Originally published May 25, 2024 in the Yakima-Herald Republic
by Don Flyckt
The orchards, gardens, and natural areas of the Yakima Valley depend on pollinators to flourish. Managed hives of non-native honeybees pollinate the orchards. After blossom drop, these hives are moved to the next commercial crop. When the managed hives leave, our gardens and natural areas must rely on native pollinators.
Some of the most important native pollinators are the often unnoticed and seldom appreciated tunnel-nesting bees: the mason bee and the leaf-cutter bee. They are members of the Megachilidae family of mostly solitary bees. The United States is home to 130 mason and 140 leaf-cutter species.
Mason bees emerge about the same time the apple blossoms open and disappear by early June. In late May, at about the same time the mason bee disappears, the leaf-cutter bee emerges and will remain active throughout the summer.
Mason bees are slightly smaller and have a rounder body than a honeybee. They have a fuzzy hair-like covering and are dark in color, ranging from metallic blue or green to black. Because of their appearance they are sometimes mistaken for a black fly.
The leaf-cutter bee is smaller and has a narrower body than a mason bee. They are light beige in color, have a fuzzy belly, and are very quiet flyers.
130 times more effective than a honeybee, mason bees are amazing pollinators. Between 250 and 500 female blue orchard mason bees can pollinate an acre of orchard, a task requiring a honeybee colony of 20,000 bees.
Tunnel-nesting bees have the unique ability to carry pollen on the underside of their abdomens, rather than on their legs, as honeybees do. They do not produce honey and lack the venom found in honeybee stingers.
Shortly after emerging in the spring, the female tunnel-nesting bee selects a new tunnel and marks it with her scent. This is where she will relocate her nest. In a single day the female bee will complete 15-25 foraging trips, prepare a mixture of pollen and nectar, lay a single egg, and then seal the tunnel cell. After laying about 34 eggs, she will die.
Tunnel-nesting bees face numerous threats including habitat loss, climate change, pesticides, diseases, and parasites. The most effective way for gardeners to help is to provide a healthy and diverse habitat that includes flowers that bloom from early spring into late fall. In my own garden, I’ve attempted to implement these recommendations, yet each year I observe fewer native tunnel-nesting bees.
This spring I’ve decided to take a more aggressive approach to help my tunnel-nesting friends. I will build them a hotel. This comes with the responsibility to do it in a manner that does not do more harm than good. A poorly maintained hotel could become infested with parasites and diseases.
Commercial mason bee nests are designed to meet the nesting preferences of a single bee species, with a single tunnel diameter and depth. The tunnel diameter determines the species that will adopt the nest. The tunnel depth impacts the sex ratio of the next generation of tunnel-nesting bees.
Tunnel-nesting bees prefer a tunnel diameter that matches their body size. Increments as small as 1/64” determine which species adopts the tunnel. Tunnel diameters from 7/32” to 3/8” are adequate for most species. Tunnels should be drilled perpendicular to the wood grain on 1” centers and at least ¾” from the side of the wood.
Tunnel-nesting females have the ability to determine the sex of the egg. Male eggs are laid closest to the tunnel entrance. If the tunnel is too shallow the next generation will have an abundance of males. A good tunnel length is between 3” and 8”, with a 6” depth meeting the needs of many species.
My bee hotel is constructed with preservative-free dimensional lumber. The tunnels are drilled in scrap lumber, tree branches, and reed nesting tubes. Anecdotal evidence suggests tunnel-nesting bees prefer dark nests. A light charring of the hotel with a propane torch provides the finishing touch and meets that need.
The hotel is attached to a post 5’ above the ground, with the nest entrances facing southeast. The location is within 30’ of flowers and has a nearby source of water and mud.
Before the first fall frost, I will remove the nests and store them in my unheated barn. Next spring before blossom season, I will place the nests in a wooden box with a ½” hole called an emergence box. Adult bees will emerge from the nests and exit through the hole. Last year’s nests in the hotel will be removed and burned, and the hotel will be ready for new occupants.
It’s my hope that I will see completed mason bee nest at the end of apple blossom season, and completed leaf-cutter nests by the end of summer. The final reward will be seeing mature bees emerge to pollinate my trees and garden next spring.