You’ve probably seen them in a friend’s back garden, or a local park. Some are boxes or plastic containers stuffed with twigs and reeds. Others are formed by row after row of holes of various apertures, bored into wooden blocks. These strange little garden features don’t appear to serve any immediate purpose, but if the sun is shining, and the weather is warm – hopefully you’ll be treated to the buzzing of one of the inhabitants: a bee. These strange little structures are nest boxes, aka: bee hotels.
The public loves bees. And why shouldn’t we? Bees are unquestionably charismatic. The concept of a creature that meanders about from flower to flower, sipping nectar, and gathering pollen is the very definition of insect charisma. Meanwhile, the activity of bees provides us with some of the world’s greatest foods; Each almond, apple, pear, watermelon, and blueberry is made possible by the visitation of a pollinating insect (in most cases, a bee).
We love bees so much, that we go out of our way to make their populations flourish. We’re encouraged to use bee-friendly plants in our roundabouts, gardens, and parks. We protest and lobby against the use of pesticides toxic to bees, and as mentioned previously – we give bees homes.
Although bumblebees and honeybees might be the first kind of bees that come to mind, these new miniature developments we’ve been installing aren’t designed for them. They’re for solitary bees – which make up about 98% of global bee diversity. Rather than living in hives containing hundreds or thousands of individuals, working collectively for the common benefit of the colony – solitary bees use a totally different strategy.
Solitary bees live alone, many nesting underground. Many others live in the excavated pith of stems and twigs. Where the right cavity exists, the bee will create a brood ball – a densely packed sphere of pollen – where she will lay her eggs to develop into the next generation of solitary bees. By giving bees perfect nesting opportunities, we can give them the leg up by boosting populations. Maybe.
While bee-nesting boxes are becoming increasingly more common in our gardens, it’s worth noting that it’s not just solitary bees that use the boxes. Ants and solitary wasps have been known to colonise these structures. The high density of nesting sites also mean that bees are more likely to experience attack by parasitoids. Furthermore in some locations, native bees are often outcompeted at nest sites by introduced bee species – likely causing knock-on effects where higher competition for floral resources, and increased disease transmission could further damage native bee populations.
To address this lack of knowledge, a study from the University of York set out to quantify whether the availability of nest boxes helped increase populations of native bees. The team set several hypotheses. The first was that because introduced bees have more flexible habitat requirements, bee hotels would be more likely to be colonised by non-native species. Secondly, because solitary wasps are less finicky requirements for nesting materials – they would be more common in nest boxes. Finally, as explained by the enemy release hypothesis – native bees will be more highly parasitized than introduced species. This would be further exacerbated by the unnaturally high density of bees found in nesting blocks.
By placing 200 ‘bee-hotels’ along a gradient of urban intensification in Toronto, Ontario – the team tracked which insects were using the nest cavities over three summers. In the autumn, the tubes within the next box were split, and each brood cell was placed into an individual container from where the adult insects emerged. All emerging insects were then identified to species. This was no small feat – considering that 27,000 individuals were collected during the three years of this experiment.
The team found no significant differences between abundance of native and non-native bees across all sites. However, native bee abundance was significantly lower than the aggregation of competing groups – comprising only 27.6% of all insects reared. The team found that native bees were three times more likely to be parasitized than introduced bee species. Providing these nest habitats didn’t increase the populations of any bees (native or non-native) across years, however the abundance of non-native wasps increased throughout the study (which could have been due to any number of factors).
The team concluded that bee-hotels as they stand today might not be the best intervention to increase native bee populations. They warn, at their worst, bee hotels could serve as population sinks for predators and parasites. As the unnaturally high density of nesting sites within a bee hotel is likely to cause higher rates of parasitism, perhaps creating less aggregated sites could be the answer to supporting native bee populations. On the flip side, parasitoids can often be more rare than their hosts, and bee-hotels could play an important role in conserving these at-risk populations.
While it’s wonderfully encouraging that the public are voicing their concern about the decline of native bees, and are actively searching for solutions – it’s important that our steps to conserve their populations are helping rather than hindering. Many steps like planting nectar and pollen-rich plants, leaving woody browse in your garden, and reducing (or better yet abolishing) the use of pesticides in your are tried and true ways to keep the bees buzzing. While you’re at it, why not experiment a bit with different sorts of bee hotels?
As suggested by the researchers, more appropriately designed nesting sites could be the solution. Try producing greater numbers of smaller nesting sites, isolated holes in bits of wood, or leaving sandy bits of bare earth for mining bees to colonise. Try making openings available from 360-degrees, rather than in two dimensions to reduce the likelihood of a parasitoid attacking all the bees in your garden.
There are many ways to help native bees and other pollinating insects, but making them more susceptible to disease, parasitism, and predation by providing improperly designed trap-nests isn’t one of them. More research and increased responsibility by retailers of bee-hotels are required to reduce negative effects on bee populations, and to keep our gardens buzzing with life. But in the meantime, why not do a little bee-hotel architecture + experimentation yourself?
For a list of bee friendly garden plants, please check out these resources below.
North America: Xerces Society
Plants for UK and Mainland Europe – Royal Horticultural Society
MacIvor, J.S., Packer, L., 2015. “Bee hotels” as tools for native pollinator conservation: a premature verdict? PLoS One 10, e0122126. doi:10.1371/journal.pone.0122126