Here are some reasons why cheap mason bee houses can end up hurting bee populations. I live in Pennsylvania (USA), so some of the issues might be specific to areas with high heat and humidity. And to the species that live here, of course. Check with your local authorities for more detailed advice.
1. Nesting blocks, tubes, reeds are not removable
Glued nesting materials is the number one reason why most commercial bee hotesl can become death traps. Because the nesting material cannot be removed and cleaned (or just thrown out), over time there will be an increase in the population of parasitic wasps, parasitic bees, parasitic flies, kleptoparasitic mites, fungi, bacteria, and viruses. And you typically won’t see these dangers because the solitary bees will still use the hotel (they can’t help themselves). But inside, out of view, their eggs, larvae, and pupae are probably dying at a much higher rate than when you first installed the house. And if you leave that house out for years and decades, that pest-ridden hotel might be doing more to increase the population of pests than solitary bees.
Ideally, empty out the house every year or two and give it a good cleaning. You can do this by removing all the filled nesting tubes at the end of the season (e.g., October) and storing them in an unheated garage or shed. Then, in the early spring put these nests inside a cardboard box that is equipped with a small exit hole — so bees can escape but can’t re-inhabit the nests. Finally, once everyone has emerged, clean the trays (brush, wash with bleach), re-drill holes (for blocks), and throw away reeds and paper tubes. While all this is happening, equip your (cleaned) house with fresh nesting trays, blocks, paper tubes, reeds, etc. All of this can’t happen, of course, if everything is glued in place.
If the above sounds like way too much maintenance, perhaps don’t buy a mason bee house. Get a garden gnome or a gazing ball.
2. Inadequate instructions
Even if you buy a good bee house and generally know things about the needs of solitary bees (they need a supply of mud or leaves, lots of native plants, etc.), you can still benefit from detailed instructions from somebody who thinks about solitary bees all day long. There are a lot of tricks. All of the cheapo houses lack such instructions. Most have no instructions at all, so you’d never even know about the all parasites and such that will likely move in if you leave it out for a few decades.
3. Nesting tubes open at back
If holes aren’t capped in some way at the back, parasites can easily gain entrance. Mason bees will plug the back with mud but it’s best to have a wall.
4. Reeds or holes with large splinters
Sharp edges inside the holes can snag and tear delicate wings.
5. Stems blocked at front
In the majority of cheapo houses that use bamboo, many of the sections are blocked off near the front by an impenetrable node. This is just another sign that the makers of the mason bee house could care less about bees. They just want your money.
6. Roof lacks overhang
Mass-produced mason bee houses are designed to be as compact as possible so they can be packed and shipped efficiently from factories in Asia. This usually means that they won’t have roofs that protect the nest entrances from water. That’s bad, especially in rainy climates. Moisture causes larvae to rot and also seems to cause kleptoparasitic pollen mites to flourish (that’s bad).
7. House attaches to tree with string or hook
Mason bees start their lives as eggs on top of a mound of pollen, and houses swinging in the wind could cause them to fall off or be crushed by moving pollen balls (when too dry they can shift). At least that’s the word on the street — I don’t have an observation window on my bee hotel so I can’t confirm this by hitting it and watching them fall off and die. Perhaps a more important reason to keep the house stable is that mason bees likely find it challenging to land on a moving nest. Again, they isn’t a literature documenting this problem but I’ve witnessed many mason bees (and related Megachilidae) that simply cannot stick landings on even non-moving nests. If you have a mason bee house that is design to hang, just get creative and attach it somehow so the bees have an easier time landing.
8. Nesting material is impermeable to water
Like anything that’s alive, mason bee eggs, larvae, and pupae produce water from respiration. When that moisture can’t escape the bees are likely more susceptible to bacterial and fungal pathogens. Plus the pollen rots. So avoid houses that use plastic straws or glass tubes. Some say (e.g., The Honeybee Conservancy) that bamboo is too impermeable but that might depend on where you live. If you are concerned about humidity, opt for paper/cardboard tubes (thick paper straws might work, too), hollow plant stems, or holes drilled in wood. Unfortunately, using paper and cardboard tubes might make the bees more susceptible to parasitic wasps that can oviposit (or chew) through thin-walled nesting tubes. There are trade-offs like all things in life.
9. Nests are too shallow
The house pictured at the top of the page is approximately 2″ deep, and that could lead to most of the progeny being males. In nature, mason bees lay fertilized eggs (which develop into females) at the backs of long nests, then pack the front with unfertilized eggs (which develop into males). This means that houses with short nests can end up being primarily nurseries for worthless guys (sorry, guys). If you want a house that can generate an ecologically useful proportion of females, buy houses with nesting tubes that are approximately 6″ (152 mm). Shorter tubes are OK for smaller bees, it seems. That said, bees will use even 1/2″-deep holes.
10. Too many nesting units
When thousands of mason bees are living in close proximity, parasitic insects and mites can easily find every nest. If your heart is set on a mason bee mega-complex, just be extra sure that all components are truly cleanable or disposable. It’s also critical to actually clean it, too, so don’t buy it if you’re the type that doesn’t like to do that sort of thing.
11. Honey bee shown on packaging
This is a minor concern, but it bugs me. A photograph of a honey bee on the label is a likely sign that the house is probably not up to code.
Flow chart for making buying decision
Here’s a summary flow chart to print and keep in your wallet so you’re prepared when you see something adorable. Or print and give to that shop owner who carries trap houses.
“OMG I own a death trap what should I do?”
If you have a cheapo mason bee house that saddens you, don’t do anything rash. Just leave it up and let your current residents do their thing. But at the end of the season, after all the holes are filled, put it in an unheated shed or garage for the winter (so that birds don’t feast on the pupae). In early spring, put the whole house inside of a cardboard box (here’s an image of durable emergence box). Poke a small hole in the top or side so that emerging bees see the light and escape through that hole, and then set it outside in a place where it will stay dry. After all the bees finish emerging (summer), throw out the house. Or, better, burn it and record a video of the fire. Then post the video online to bring needed awareness to low quality mason bee houses. If you have a good photograph of a burning mason bee house, I could use one right here.
“I want a cute bee house but not the commitment”
If you absolutely want the cute look of a bee hotel but don’t want all the maintenance fuss, here’s a trick: print this photograph onto a large piece of paper and mount onto a board. To make it last, laminate or apply a coat of clear varnish.
- “How to Make and Manage a Bee Hotel: Instructions that Really Work” by Marc Carlton
- “Tunnel nests for native bees: nest construction and management” by The Xerces Society
- “Beware! Is your ‘bee hotel’ a nursery for disease and pests?” by George Pilkington
- “Insect Hotels: A Refuge or a Fad?” by Jo-Lynn Teh-Weisenburger
- “Are Are bamboo tubes causing mason bee Armageddon?” by Rusty Burlew