Bee Hotels - why they’re important & what it takes to get a five-star rating

Part 1 of our two-part overview on Bee Hotels and how to separate the good from the bad. 

Stroll down the aisles of practically any garden centre and you’ll find them - wood-cobbled contraptions framed in the shape of houses, their small facades stuffed with everything from bamboo tubes to pine cones locked within metal mesh chambers to shallow blocks of wood riddled with holes. 

Had you encountered these miniature neighborhoods of strange homes lining the store shelves a decade earlier, you might have scratched your head in confusion. But a growing awareness and interest in solitary bees and their impressive pollinating powers has fueled a desire to help them, and thus the “bee hotel” (also known as a bee condo or bee house) was born. 

Today, bee hotels are a mainstay product in the wildlife care section of most garden centres, and their features, sizes, material choices and prices are as diverse as the solitary bees they’re meant to attract. Unfortunately, in the current frenzy to help solitary bees, many if not most of the hotels being mass-produced and sold are not very effective. And some are just downright harmful. 

A typical bee hotel.  Unfortunately, many features on these products are often useless or can even be harmful to solitary bees. 

As The Old Saying Goes … Buyer Beware

As consumers, most of us have a tendency to trust that the well-intended products being offered to us are also well-designed and well-tested. But this is simply not the case with bee hotels. And to make matters worse, both people and bees might not know an issue exists until it's too late. 

So, to understand what makes a good - and bad - bee hotel, it’s up to us consumers to know what features to look for and why they can matter. Perhaps the best way to start is by understanding a little bit about solitary bees and what they need from a bee hotel. 

Why (And Where) Bee Hotels Are Now Needed

In the eons before we started hanging bee hotels for them, cavity nesting bees were (and still are) happily building nests in holes in dead wood and hollow plant stems and reeds. Unfortunately, when land is cleared to make room for a new road, shopping mall or housing development, dead trees and hollow plant stems disappear. But thanks to our love of flowering plants, small vegetable gardens, and the determined hardiness of “weed” species like dandelions and clover, the suburban landscape still offers solitary bees a food source. 

Add this up and the issue is clear - solitary bees now face a suburban-dominated world where there’s often lots of food but few places to call home. And to make matters worse, solitary bees are not long distance travelers. In fact, most solitary bees won’t travel more than a few hundred yards from their nests in their entire flying life. 

Given the situation, it becomes clear that this is where a good bee hotel can really help. 
It’s also where a bad bee hotel can really screw things up. 

No, It’s Not Really A Hotel

Despite the catchy name, a bee hotel isn’t meant for traveling bees that need a place to rest for a few nights. A bee hotel is in fact a bee nursery. Starting in early spring, most solitary bees emerge from their nests and almost immediately seek out a partner with whom to mate (and no, they don’t seek out a bee hotel for this either, they prefer the ground or a piece of vegetation). After mating, male solitary bees tend to die quickly, having fulfilled their primary role in this world. The females are left to spend the rest of their remaining four to six weeks fulfilling their purpose - finding a suitable place to nest, stocking it with nectar and pollen, and laying eggs that will develop into larvae and ultimately repeat the process next spring. 

It’s the “finding a suitable place to nest” part where the bee hotels come into play. 

However, considering the fact that bee hotels are a relatively recent addition to the garden and solitary bees have been around for millions of years, you might be wondering why we’re even hanging these things up in the first place. To answer that, it’s important to understand what bees are looking for in a nest and why we can (and often should) lend a helping hand. 
For those of you that prefer the cliff-note version of this information, we’ve highlighted important facts that directly relate to choosing a bee hotel in red. 

Bee Hotels Aren’t For Everyone

You might be surprised to hear this, but most solitary bees have no interest (nor a reason to visit) a bee hotel. That’s because solitary bees fall into two basic groups - cavity nesting bees and ground nesting bees. 

About 70% of solitary bee species are ground nesters, meaning they dig or find a hole in the soil to lay their eggs. And no, not even a beautiful new bee hotel hanging nearby is going to change their minds. However, if you do wish to help this larger and equally important group of bees (and we know you do if you’re reading this) then consider leaving them a small area of bare, undisturbed soil (even a square meter is good) that is free of mulch and rocks, and also make sure to keep your garden pesticide and herbicide-free. Adding a south-facing berm can also attract ground nesting bees, most of which love a nice morning sun warm-up before heading out to the flowers.

Cavity Nesters…Your Hotel Guests

The smaller group of solitary bees that are cavity nesters - and thus potential bee hotel guests - still contains about 6,000 species worldwide. No matter where you live, the chances are extremely high that you have a large variety of cavity-nesting solitary bee species close by.

Most cavity-nesting solitary bee species range in size from 6mm long on the small side to 15mm long on the large side, which is important to know because a solitary bee’s size - especially its body’s diameter - will determine the size of the cavity it chooses for nesting. In Europe and North America, nesting cavities with a diameter of 6mm to 8mm will be a good fit for most of the solitary bees that will be interested in a bee hotel, with some species interested in cavities as small as 4mm and some as large as 10mm. Bees will never use a cavity that’s too small, but they may choose a cavity that is slightly larger than desired if a better option isn’t available. 

As you might expect, larger-bodied bees like mason bees require wider (8mm) cavities or tubes, while smaller-bodied bees like summer leafcutter bees require narrower (6mm) cavities or tubes.

The Perfect Cavity

When choosing a nesting space, whether within dead wood, a plant stem, or a bee hotel tube, female solitary bees will seek out a cavity with these features:

• consistent in diameter
• deep enough to offer protection from weather & temperature extremes
• strong enough to offer protection from predators such as birds
• free of wing-damaging debris or sharp edges
• closed on the far (or back) end

Egg-Laying 101: Ladies First

As we make our way toward better understanding what makes for a good bee hotel, it’s time to discuss a critically important aspect of solitary bee nesting - how a female bee lays her eggs.

Once a female solitary bee has mated and chosen a nest cavity, she will then collect nectar and pollen from flowers, roll it into a sticky food blob, and place it in the back of the nesting cavity before depositing a single egg. With the egg and its food source placed, she walls off the chamber with mud, resin or plant fibre (depending on the bee species) and repeats the entire process again and again until the cavity (or bee hotel tube) is full.  

It’s an impressive feat of strength and engineering by these little bees. And what makes it even more impressive is the fact that this egg laying process, chamber by chamber, isn’t random. 

Female bees carry (and lay) both fertilized and unfertilized eggs. Just as with hive bees such as honey bees, the fertilized eggs develop into female bees and the unfertilized eggs develop into male bees.

When a female solitary bee is laying its eggs in the nest cavity, it begins by laying fertilized/female eggs (in the back) before switching to the unfertilized/male eggs as the chamber fills and she moves closer to the front. 

Why do female solitary bees do this? First and perhaps most obvious is the fact that male solitary bees emerge from the nest a few weeks before the females, so they require the closest exit from the narrow chamber (or tube) opening. As the more “expendable” sex, males need to be ready to mate as soon as the females emerge, because there’s simply no time to waste for the females when their mature lives are measured in weeks. 

Another reason for placing the female eggs in the back of the nest chamber is to provide them with better protection. The deeper the female eggs are within a natural cavity such as dead wood or a plant stem, the more insulated they are from the elements - including high and low temperatures - which can kill the developing larvae. Yet another reason is the limited space within the cavity. Fertilized female eggs require more development (they are larger and stronger than the males) and thus need more food and space to grow. This results in a larger nesting chamber compared to their male counterparts, so placing females in the back allows the nesting female to better judge how to fit her eggs within the available space. 

This is important to understand, because if a female solitary bee chooses a nesting cavity that is too shallow, her instinct to lay male eggs a certain distance from the entrance will override everything else, and the result will be very few female eggs - and a disproportionately large  number of male eggs - being laid. A repetition of this trend can be disastrous for the local solitary bee population in just a few years. 

Why would the female bee pick a shallow cavity to begin with?  Again, with limited housing options and very limited time to choose a nesting site, solitary bees have to make decisions on what’s available to them. For them, a shallow nesting cavity is better than no cavity at all.

Solitary Bee Predators & Pests

Unfortunately, solitary bees have plenty of natural enemies out to get them, including birds and small mammals that will happily snatch them from their cavities if discovered. And the problems don't just exist outside. Inside the nest cavity, solitary bees have to worry about pollen, fungal infections like chalkbrood, and even smaller parasitic bees and wasps intent on taking over their nesting chambers and cannibalizing their young. 

A horned mason bee (Osmia cornuta) infested with kleptoparasitic ectoparasitic mites.

The best way to deal with all of these pests is by choosing a nesting cavity or bee hotel that is well made to deter these problems, and we’ll talk about this more next.

So What Makes a 5-Star Bee Hotel? 

With a basic understanding of solitary bees now covered, it’s much easier to look at solitary bee hotels and recognize what to look for and what to avoid.  

And to make it even easier, we’ve created our own Star Rating System to help you through the process. By giving bee hotels a star for each of these important features, you can quickly separate the good from the bad. 

Here’s our Five-Star Solitary Bee Hotel Rating System: 

★ - Excellent Cavities 
★ - Excellent Depth
★ - Excellent Materials
★ - Excellent Protection
★ - Easy to Clean & Inspect

Here’s a more detailed summary of each star rating and what it takes to get the star:

★ Excellent Cavities

What this means: A bee hotel with excellent cavities comes with a variety of tube sizes that are well matched to the size needs of several of your local cavity nesting solitary bees.

What to look for: 
tubes ranging from 6mm to 10mm in diameter
two or more diameters mixed together
consistent width for the length of the tubes
smooth, flat entrance holes

What to avoid: 
tubes smaller than 4mm or larger than 10mm in diameter
a single tube diameter
crushed of bent tubes
sharp edges or obstructive features

 ★ Excellent Depth

What this means: A bee hotel with excellent depth offers better protection from weather,  temperature, and pests - especially for the all-important female eggs.

What to look for: 

• tubes with at least 10cm of depth, with 15cm being ideal
  TIP: A great rule to remember - the larger the diameter, the deeper the cavity should be

What to avoid:

• tubes shorter than 10cm
  

★ Excellent Materials

What this means: A bee hotel with excellent materials uses weather-resistant or weather-proof materials for the outer barrier, and natural, chemical-free materials that are mold-resistant for the inner tubes.

What materials to look for: 
   Outer barriers or walls:

• natural untreated wood
• recycled inert resin such as HDPE

   Nesting cavities/tubes:

• natural reeds that can be opened
• paper tubes that can be opened

What to avoid: 
   Outer barriers or walls:

• treated woods or metals
• virgin or non-recyclable plastics 

   Nesting cavities/tubes:

• bamboo or heavy reeds that cannot be opened
• wood blocks or tubes that cannot be opened

★ Excellent Protection

What this means: A bee hotel with excellent protection comes with features that help it minimize the risk of both predators & pests as well as weather and temperature extremes

What features to look for:

• well-made and sturdy
• mounts securely to a wall
• offers little or no room for predatory birds or animals to perch
• deep enough to hold and shelter the nesting tubes

What to avoid:

• flimsy or easily broken
• doesn’t mount firmly to wall
  

★ Easy To Clean & Inspect

What this means: A bee hotel that’s easy to clean and inspect is important for the long-term health of your local solitary bees, especially when it comes to preventing the spread of a fungus or mites that aren’t easily seen from the outside. 

What to look for:

• A design that’s easy to remove (or remove the nesting tubes from) the wall and place in a cool protected place
• Nesting tubes that can be easily opened (in the fall) to carefully inspect cocoons and ensure there is no mite or fungal disease among the brood
  
  

Up Next - Reviewing & Rating Bee Hotels 

Whew! Yep, we’ve covered a lot of ground, but it’s vitally important to make sure we get the word out on both bees and bee hotels. Solitary bees are critically important to our environments - which makes them critically important to us.

In Part 2 of our deep-dive coverage of bee hotels, we're going to look at a few examples of bee hotels and evaluate their features. Stay tuned!