It’s both fun and easy to make nesting blocks for mason bees, a highly useful pollinator for orchards and gardens. Although they don’t produce honey, there are several advantages to keeping mason bees. They are gentle, with stings no stronger than a mosquito bite. They are very efficient pollinators; only a few hundred are needed per acre to pollinate an orchard. Mason bees are remarkably easy to keep, having few pest or disease problems and minimal management needs. Last, they are very industrious and fun to observe in action.
Mason bees are one of many types of small solitary bee species that aid greatly in pollination. There are two primary species of mason bees used for pollination: the native blue orchard bee (Osmia lignaria) and the hornfaced bee (Osmia corniforns) from Japan. Unlike honeybees, mason bees don’t live in hives with queens and workers. Each female bee mates and makes her own nest of mud, stores pollen and nectar, and lays her eggs. Although they don’t live in colonies, they do prefer to nest close to other mason bees.
Mason bees are only active for four to six weeks in the spring, making them ideal pollinators for orchards. Unlike honeybees, male mason bees also contribute to pollination. In Japan, hornfaced bees have been used in commercial orchards for over sixty years and are rapidly replacing honey bees for this task. After their busy spring season, mason bees spend the rest of the year in their nests, first as larvae and then as dormant adults through the winter.
BUILDING MASON BEE HOMES
In nature, mason bees mostly nest in hollow reeds, bamboo, and holes made by other insects in wood. Making homes for them only requires mimicking these conditions. Research has shown that holes 5/16″ wide and 4 to 10 inches deep are ideal, although some variance from this will work to a lesser degree. Mason bee homes can be made in a number of different styles with varying degrees of involvement and management required.
ATTRACTING NATIVE BEES
Wild mason bees and other native pollinators are everywhere. Bringing wild mason bees into your landscape can be as simple as drilling holes in existing stumps, logs, or posts. In the case of urban areas not close to a sufficiently “natural” landscape, it might be necessary to put out drilled blocks in such an area and then bring the nested blocks back to your own landscape the following year.
CONSTRUCTING NESTING BLOCKS
Managing mason bees more actively and effectively is also quite easy. There are many suppliers of pre-made nesting blocks and tubes, but making your own is cheap, simple, and fun for all ages. Here’s a how-to video from a teenager and one from an elderly man. You can basically use any piece of scrap wood of at least 4″ thickness, avoiding pressure treated or aromatic woods like cedar. Lay out a series of holes approximately 1″ apart on a side that will allow for holes at least 4″ deep. If you don’t intend to use liners (see below), drill the holes with a 5/16″ bit and avoid drilling all the way through the wood. If the blocks are to be put in an exposed location, you will want to add a piece of shingle or other material as a roof to protect from rain.
BLOCKS WITH PAPER LINERS
The use of paper liners in the holes allows the mason beekeeper to more effectively control pests and diseases. Slight modifications are required for this style of nesting block. To accommodate the liners, it’s necessary to drill the holes slightly wider, with a 3/8″ bit, and drill all the way through the wood. Paper or cardboard liners are commercially available, but again it’s cheap and easy to make your own. Cooking parchment works best, although wax paper may also work. You’ll need a rolling rod at least a couple inches longer than the holes of your block. A 1/4″ metal rod or dowel works fine. The parchment should be cut in to sections 4 to 5 inches wide and about 3/4″ longer than the holes. Tightly roll a section on to the rolling rod and then push it into one of the holes. When you release the parchment, it should unroll and expand to fill the hole. Repeat with all the holes, lining up the ends of the liners flush with one edge of the bock and sticking out on the other side. Bend these exposed ends down with a sharp crease (they’ll be used to remove the liners from the block at the end of the season). Seal the block end with duct tape or plywood cut to size and nailed into place.
SETTING OUT YOUR NESTING BLOCKS
Early spring is the time to set out your bee blocks. Mason bees begin to emerge at around the same time as crocuses and forsythias bloom. Nesting blocks need to stay dry, so they are best placed under eaves, decks, or other protected spots. Where there are no convenient structures, bee shelters can be created using garbage cans, dog houses, or any other item that will give some weather protection. Bee blocks should always face south or east, receiving some morning or early afternoon sun, and be placed so that the holes are horizontal. Mason bees also require a source of clay or mud to build their nests. Dig a small pit around a foot deep and make sure it’s open and accessible throughout the active season.
STORING YOUR MASON BEES
In late spring the adult bees begin to slow down and die off. Although you can leave the blocks out year round, you’ll have much better survival rate if you move them inside as soon as the active season is over. They should be kept in a dry, protected space; a basement or unheated outbuilding work well. Temperatures should ideally stay between 50 and 90 degrees Fahrenheit through November. Some colder temperatures (but not much below freezing) are required over the winter in order to break dormancy in the Spring. With paper liners, the bees can be removed and put in the fridge in a box with a moist sponge or towel to maintain humidity.
PESTS & PROBLEM SOLVING
The primary pest that affects mason bees is a tiny parasitic wasp (Monodontomerus obscurus). The wasps begin to attack mason bee nests just as the active season is ending, so it’s important to remove your nesting materials promptly in late May to minimize damage. It’s also a good idea to change to new nesting blocks or tubes every couple years and either sanitize or destroy the old materials. This is easy with paper liners: simply remove the dormant bees each winter, give them a 1% bleach bath, let them dry overnight, and then store them in a box in the fridge until spring. In blocks where the bees can’t easily be removed, place them out in the spring in garbage bags with a small hole to fly out. The bag will confuse the bees when they return and they’ll choose to nest in the new blocks you set out instead.
Birds can sometimes cause problems for mason bees. You can protect the nesting boxes by covering with chicken wire with holes at least 2″ wide. Bird-scaring tape can also help. If necessary, bees in storage can be protected from mice and other pests by storing in a garbage can, although some holes for ventilation are required.
Here’s an online resource with very detailed information about keeping blue orchard bees, but I prefer this more concise description for hornfaced bees. If my description was confusing, here’s some more details about homemade paper liners. To purchase dormant bees and commercially made nesting materials, check out www.beediverse.com or www.pollinatorparadise.com. In southeast Pennsylvania, the Back Yard Fruit Growers sell empty tubes and dormant bees at their events. Thanks to Darren Gordon for sharing his time and knowledge about keeping mason bees. Happy pollinating!