A Powerful Pollinator

Last fall I planted snowdrops (Galanthus spp.) and crocus (Crocus spp.) to have very early blooms available for the first pollinators that hatch in the spring. Even though these are not native, they do provide early nectar and pollen sources for native solitary bees. This would be before native plants such as Pussy Willow (Salix discolor) and Spicebush (Lindera benzoin), as well as spring ephemerals like Bloodroot (Sanguinaria canadensis) and Dutchman’s Breeches (Dicentra cucullaria), begin to flower. I am trying to gradually lose the lawn and to plant at least 70% native plants, as recommended by entomologist Douglas Tallamy in his book Bringing Nature Home (Timber Press, 2007).

Those spring bulbs I planted are in full flower now. I count at least three different species of bees hovering around  them. There is one standout in full force: a mason bee (Osmia spp.). Many of the species in this genus are polylectic, meaning they gather nectar and pollen from the flowers of different plant species. However, there are a few specialist mason bees that only like certain flowers. The Beardtongue Mason Bee (Osmia distincta) only visits the flowers of beardtongues (Penstemon spp.), whereas Osmia virga frequents blueberries (Vaccinium spp.).

A close-up of the Blue Orchard Bee (Osmia lignaria), which is actually quite small. This species is less common now due to competition from other introduced mason bees as well as other environmental issues. Photo by the Bee Inventory and Monitoring Lab, Sam Droege.

There are about 20 species of mason bees in Connecticut. One of the most common is the Blue Orchard Bee (Osmia lignaria). Like many native bee species, however, it seems to be on the decline. Researchers from the Connecticut Agricultural Experiment Station have noticed that two non-native mason bees, the Japanese Horned-face Bee (Osmia cornifrons) and the Taurus Mason Bee (Osmia taurus), are displacing the Blue Orchard Bee. The Japanese Horned-face Bee was released in the Northeast to pollinate orchards. The Taurus Mason Bee, a very similar species, showed up soon after.

The Japanese Horn-face Bee (Osmia cornifrons) was introduced for fruit tree pollination and is displacing the Blue Orchard Bee. Photo by Beatriz Moisset, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0&gt;, via Wikimedia Commons.

Scientists studying mason bees in the U.S. Mid-Atlantic states used pan traps to find out how common the two exotic and six native Osmia species have been over the span of 15 years, from 2003 to 2017. All the native species showed substantial annual declines ranging from 76% to 91% since 2003. The two exotic species fared much better—O. cornifrons was stable and O. taurus increased 800% since 2003. Scientists think these exotic species are outcompeting native species for nest sites and food sources. They may have also introduced parasites to native species.

Unlike Eastern Carpenter Bees (Xylocopa virginica), which as you probably know drill holes in wood, mason bees depend instead on holes and tunnels made by other insects and by birds. About 30% of our native bees are tunnel nesters.

Mason bees are a little smaller than honeybees. They are not aggressive. The male doesn’t sting and the female will only sting if handled roughly. Adult females collect pollen and nectar to feed their young. A female will pack this pollen and nectar into “bee bread,” deposit it into a chamber, and lay an egg next to it. There are five to eight of these brood cells in each tunnel. She lays an unfertilized egg, which will be a male bee, at the end of the chamber closest to the outside. When the eggs hatch, the larvae feed on the pollen and nectar stored in the nest. After 10 days, each spins a cocoon and becomes a pupa in its cell. By the end of summer the bees transform into adults, a stage called an imago, but remain in their cocoons throughout the winter.

A close-up of mason bee “cocoons” in the spring, with one hatching .” This the Red Mason Bee (Osmia bicornis), a European species.

Males hatch first in the spring. They wait outside the tunnels and mate with females when they hatch. Mating is their sole purpose. Female mason bees live about one month, males much less.

Mason bees are incredibly effective pollinators. Unlike honeybees, which aren’t up and about early in the morning, mason bees are out both early and late. Apple farmers know how efficient and effective these pollinators are and have created bee boxes for them. You can too and now is the time to put them up. Like bird boxes, bee boxes are mostly for your enjoyment. They are not a necessity for the bees, since they can find tunnels in the wild.

Now is the time to install a mason bee house. This type of house has cardboard tubes that can be removed after the bees emerge next spring. Other boxes are wood blocks with holes drilled in them. Photo by the author.

You can make your own mason bee box by drilling holes in a block of wood. The holes should be between a quarter inch (6 mm) and three-eighths inch (9.5 mm) in diameter and 6 inches (15 cm) deep. Keep the holes closed at one end. You can also buy a mason bee nest box at specialty stores like the Fat Robin Wild Bird and Nature Shop in Hamden, Connecticut. Another way to create your own box is by cutting off the lower stems of invasive Phragmites grass (Phragmites australis), tie them in a bundle, and plug the hole on one side with plastic wood filler or something similar.

The best location for a bee nest box is in a dry, protected place with an eastern or southeastern exposure. In the fall you can use a paper straw liner, inserted into each hole, to retrieve the cocoons to “wash” your bees. This will get rid of most of their mites and diseases. Clean the cocoons between October and December, because the adults are fully formed by then.

First, soak the cocoons in cool water to soften and remove mud. Be sure to use water no warmer than 50 °F (10 °C), so you don’t “wake up” the bees. Using a sieve, gently roll and move cocoons through the water. Discard any debris. Next, soak the cocoons, for no more than 10 minutes, in cool water with a 0.05% bleach solution (1 tablespoon bleach per 1 gallon water, or 15 milliliters of bleach in 3.75 liters of water) to kill bacteria, fungi, and most mites. Rinse well under cool water to remove all traces of bleach. Dry on a clean paper towel for one hour. Sort and discard damaged, diseased, or parasitized cocoons. Put the cleaned, air-dried cocoons in a small container with air holes and store this in your refrigerator for the winter. When you see bees flying around your garden in the spring, open the box outside in a sheltered location. See the comprehensive, free online book called How to Manage the Blue Orchard Bee by Jordi Bosch and William P. Kemp (Sustainable Agriculture Network, 2001) for more information.

Many of our pollinators are undergoing population declines. You can help mason bees by providing native trees, shrubs, and perennials in your yard. Try to have a variety of plants in bloom from early to mid-spring, when the bees are active. Other plants these bees will also visit are Redbud (Cercis canadensis), oaks (Quercus spp.), Black Willow (Salix nigra), Beach Plum (Prunus maritima), and shadbush (Amelanchier spp.), and many more. If you have to cut down a tree, consider leaving 10 to 20 feet (3 to 6 meters) of the trunk that can become nesting sites and tunnels for mason bees. We can all do our part.

Published by Jim Sirch

Jim Sirch is the author of Beyond Your Back Door, a weekly blog about nature in your neighborhood. He is also Education Coordinator for the Yale Peabody Museum, a UConn Master Gardener and board member of his local land trust. As a trained naturalist, he brings a deep understanding of geology, plants and wildlife and how they interact within a particular ecosystem. He holds a B.S in Forestry from West Virginia University, a B.S. from Miami University in Science Education; and an M.S. in Environmental Studies Administration from Antioch University. He is also the 2014 Sigmund Abeles Award recipient from the Connecticut Science Teachers and Supervisors Association for outstanding science teaching and professional development.

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