Raising Mason Bees: Do’s and Don’ts
Mason Bees Have Naturally Occurring Pests, Parasites, and Predators That Can Sicken or Kill Them
Reading Time: 5 minutes
Raising mason bees is as simple as buying or making suitable housing and placing it where it will be discovered by the bees that already live in your area. If you don’t buy mason bees, starting is a bit slower, but the results are worth the wait.
Three years ago, I ordered some leafcutter bees from a local company and allowed them to emerge inside a mesh container. To my surprise, only 30% yielded leafcutters and the others had been consumed by chalkbrood disease.
Recently, a friend did a similar experiment with mason bees. He had a better emergence rate, but fully 20% of the live cocoons contained parasitic wasps instead of mason bees.
If you start by erecting your mason bee housing in a good location, you will get a few bees the first year — ones that randomly discover your wonderful condo! During the second year, the females that emerge will each fill several tubes with cocoons, and by the third year you are likely to be overrun. These are the very best bees, locally adapted and likely disease free.
What is Suitable Housing?
To provide the very best housing for mason bees, it helps to understand why things go wrong and then try to avoid those situations.
Just like honey bees, mason bees have naturally occurring pests, parasites, and predators that can sicken or kill them. In natural environments, most animals occur somewhat randomly. For example, some bees may nest in a rotting log, some choose dead berry canes, and some are happy with old beetle borrows. Because the distance between each nest may be considerable, the chance of pestilence passing from one nest to another is small. Similarly, a predator who consumes one nest is unlikely to find all the other nests.
But in artificial nesting, we tend to put all the individuals close together. Just like a feedlot or a chicken factory, once a disease affects one individual, it can spread quickly with nothing to stop it. For that reason, afflictions that appear occasionally in nature, become overwhelming problems in artificial high-density settings.
In addition, nests in the wild are not regularly re-used. The stumps and berry canes rot, the holes in the ground wash away, the beetle burrows may be picked apart by birds. When those nests disappear, so do the pathogens or parasites that lived there. What all this means is that mason bee housing should be variable and constantly renewed.
Problems with Raising Mason Bees
Unlike the varroa mites that plague honey bees, pollen mites (Chaetodactylus krombeini) do not feed on bees or spread disease. Instead, they feed on the pollen and nectar stored for the bee larvae, thus starving the bee to death. They latch onto adult bees as they pass through the nest in order to hitch a ride to another nesting cavity. Sometimes, an adult bee may carry so many mites that flying becomes difficult or impossible.
Pollen mites build up over time, so one of the best control measures is rotating housing every two or three years. By simply discarding the old nests and providing new, you can get rid of most of the mites.
Because mason bees will nest in the very tube they emerged from, steps must be taken to keep the bees from reusing old tubes or cavities. One common method is called an emergence box. Because masons don’t like to enter a darkened area to find their nesting tube, you can put cocoons, tubes, or an entire condo inside a box with a single exit hole that faces the sun. Near to the emergence box, within about six feet, you place your new nests. The bees emerge, mate, and then nest in the sun-exposed tubes.
You may hear of some mason bee keeper who scrub the cocoons with sand or soak them in bleach. This controversial practice is not at all natural, and in my opinion it should be avoided. If you regularly rotate your tubes or nesting blocks, you should never have to resort to scrubbing cocoons. Remember, too, that even clean cocoons can still harbor parasitic wasps.
Mold can become a problem when moisture is not wicked away from the nest. Remember that mason bees live for 10 months inside the cavity, so any material that prevents water from leaving the nest should be avoided. Plastic straws, for example, should never be used. Some people have had similar problems with bamboo, although bamboo performs well in some environments. You will need to experiment within your local climate to see what works best. I have found paper straws to work well, in addition to the hollow stems of lovage, elderberry, and teasel.
Parasitic wasps, especially in the genus Monodontomerus, are lethal to mason bees. These wasps, which can be mistaken for gnats or fruit flies, can insert their eggs right through the side of a nesting tube and into a developing bee. Once the wasps hatch, the larvae eat the mason bee from in the inside. The adult wasps then leave the nest, mate, and hover around waiting for a chance to lay more eggs.
Luckily, the wasps become active just as orchard mason bees are finishing their season, so it is easy to remove the housing and store it in a place that is safe from predatory wasps. I usually put the tubes in a fine mesh bag and store them in a cool, dry place until spring.
Birds, especially woodpeckers, can be a problem in some areas. The easiest way to deter them is to put wire mesh or poultry netting around the mason bee condo in such a way that the birds can’t reach through the holes.
Biodiversity and Bee Health
Another way to slow disease transmission and maintain a biodiverse selection of pollinators is to provide a wide selection of hole sizes. When I drill holes, I randomly make 1/16, 1/8, 3/16, 1/4, 5/16, and 3/8-inch holes in each block and space the blocks far apart from each other. That way, only a few tubes of each species live close together in each block.
Many different species, including masons, leafcutters, and small resin bees, will occupy the holes. Since each species has its own life cycle and nesting habits, the accumulation of predators and pathogens is greatly reduced.
The problems with mason bees vary with their location. What control measures have worked best for you?