Top left clockwise: Megachile erythropyga, Megachile aurifrons, Megachile erythropyga
Words and images by Kit Prendergast (BSc, PhD researcher at Curtin University)
It is no secret that bees are under threat. Across the globe, there have been signs of bee declines, with flow-on effects for pollination services, including for fruit and vegetables that we rely on. How can we help reverse this dire situation? Most people have focussed on the European honeybee Apis mellifera, yet this is just one species out of the 20,000 or so species of bees in the world, and moreover, European honeybees are actually doing quite well and are in no threat of becoming extinct. It’s the other species of bees which are really in in need of help. And thus, keeping your own honeybee hive is not going to do much for these other crucial pollinators. The other issue is that the majority of native bees have very different requirements for honeybees, in terms of both what they forage on, and what they need to reproduce. To survive and reproduce, bees not only need flowers, but they also need nesting habitat. Whereas honeybees can be supplied with hives to nest in, and in the wild, they nest in hollows in trees, the majority of native bees have very different nesting requirements. About 30% of all bee species in nature nest in pre-made cavities in wood created by wood-boring beetles. This is where bee hotels come in: by attempting to recreate such habitat features, bee hotels have been offered as a way to provide nesting habitat for these cavity-nesting bees.
What are bee hotels? Do they work?
Historically used by researchers to study native bees and their nesting biology, bee hotels, known by the relatively unsavourable name of “trap nests” in the scientific literature, are making a splash in being offered as a way to help save the bees. Put simply, bee hotels are any construction with apertures that are put out to encourage cavity-nesting bees to nest in them. Lots of people are getting behind the bee hotel bandwagon, but despite good intentions, the crucial question is, do these work? The answer is: it is highly dependent on the design of the bee hotel. Many pre-made structures are stuffed with pine-cones, wood-shavings, large open spaces, cavities that you could fit your thumb into, and with holes that are open ended or extremely shallow. Such designs are completely unattractive to native bees. Moreover, many are made with treated, imported wood, or painted. Whilst these bee hotels may look fancy and highly attractive to our human aesthetics, to native bees they are not. Such “bee hotels” may attract wasps, cockroaches, spiders and lizards – and there’s nothing wrong per se with providing habitat for these other critters – but if the goal is to provide nesting habitat for native bees, when you purchase such a hotel you’re likely be disappointed. The good news is that making a bee hotel for native bees is actually very simple, and they can be easily made with sustainable materials at a low cost.
There are two basic bee hotel designs:
1) wooden blocks with holes
2) hollow canes
These can be combined together as well. For the first design, a bee hotel can be created by getting a block of wood, preferably of a tree species that is native in origin to your area, and drilling holes in it. For the second design, hollow stems of bamboo or reeds can be bundled together and packed tightly in a large container with one end sealed, such as a large bamboo culm, or empty food tin e.g. instant coffee tin. The crucial factors are the length of the holes and their diameter. Generally, the longer the better, because this means that more cells can be laid. If the hole is too short, only a few cells can be laid by a female bee, plus it also increases the chance that there will be a sex bias in the offspring, and the entire nest may be parasitised by one of the many creatures that parasitise cavity-nesting bee nests. A general recommendation is at least 100mm deep. Secondly, diameter is very important. Anything over 12mm is highly unlikely to be nested in. Bees like to nest in holes that are slightly larger than their own body diameters, which means between 3mm-11mm, depending on the species. My research has found that 4mm and 7mm tends to be the most occupied. It’s best to include a range of hole diameters, so that your bee hotel can cater to a greater number of species.
Who uses bee hotels?
The main taxa that use bee hotels in Australia are species in the family Megachilidae, and species in the subfamily Hylaeinae, family Colletidae. Megachilids are important pollinators as females have scopae – the specialised hairs for collecting pollen – on the underside of their abdomen, and thus in an ideal location for transferring pollen between flowers. Some species of Megachile, such as those in the subgenus Eutricharaea, line their nests with discs of leaves or petals that the females cut off plants with their mandibles. Bee hotel nests occupied by megachilids are capped by leaf discs (Megachile (Eutricharaea) spp.), or mixtures of resin, sand, and masticated plant material (Megachile spp.). The introduced African Carder Bee Afranthidium repetitum collects hair-like plant fibres and packs them into bundles, such that the material resembling cotton wool. And I recently made the discovery that a megachilid found only in WA, Rozenapis ignita, the sole species in its genus, packs its nest with banksia cone fuzz – another unique oddity of this species.
The Hylaeinae include species in the genera Hylaeus, Meroglossa, and Amphylaeus. Hylaeine bees swallow pollen and therefore in their evolutionary history have lost scopae, and therefore appear relatively hairless, like wasps. They are typically black with yellow or white facial markings, and may have a yellow “badge” on their thorax, as well as a yellow collar or yellow legs. Like all colletids, they line their nests with cellophane-like secretions produced from their mandibular glands, and also seal their nest with this secretion. These secretions are formed by polyester, and are thick, strong, waterproof, and insoluble in different solvents, as well as resistant to fire, high temperatures and various chemicals; bioengineers are looking to synthesise this material as a biodegradable alternative to plastics!
Which particular species use bee hotels depends primarily upon the region – whilst some species occur across most of Australia, others have a geographic distribution restricted to only a small area. Bee hotels also are highly attractive to some species, whereas for other species in a region, despite nesting in nature in pre-made cavities, they seem to eschew these artificial structures we put on offer.
Even for a well-designed bee hotel, where you install it plays a big role in whether you will get bees checking-in. Bee hotels should be installed at heights of between one to two metres from the ground. Hanging them under eaves or on tree branches are good locations. Bees prefer sunny locations, so make sure the bee hotels is not placed in the shade, and receives sunlight, especially in the morning; however be sure to place the bee hotels where they will not receive the direct midday sunlight to prevent larvae from “cooking” inside! Avoid placing a bee hotel where there will be frequent human traffic passing it as this may deter the bees from using it.
Checking in at bee hotels
Despite the common name of these structures, native bees don’t really use them as “hotels”; rather, bee nurseries might be a better descriptor (but it’s not as catchy). The native bees do not actually live in these structures, but rather, the females use them as nesting substrates. The majority of native bees are solitary, and instead of living in colonies, they live alone. They also do not care for their offspring, and instead their parental duties are limited to gathering food in the form of pollen and nectar, depositing it in a nesting cavity, laying an egg on the food store, and then sealing up the nest. The egg then hatches, and as a larva, eats the store of food it’s mum has stock-piled in the cell, undergoing a number of larval instar stages (typically four), pupating, and then metamorphosing to emerge as an adult bee. Within a nest cavity, a female will create a number of cells, usually separated by a partition made of mud, resin, sand or other materials, depending on the species. Each cell is provisioned sequentially, starting with the innermost cell. Once a female has completed the final cell in the nest she will seal the nest entrance up with material (which varies by species, as noted above), as protection from the elements, predators and parasitoids. The number of cells per nest depend on the length of the nest cavity, the particular bee species, and even within a species, as how many cells a female will create depends on how much resources she has. In general, larger cells containing fertilised, and therefore female eggs, are created first, and the outermost cells that are created later are smaller and contain unfertilised and therefore male eggs; bees, like all Hymenoptera (the insect order including bees, wasps, ants and sawflies) have haplodiploid sex determination, where females are produced from fertilised (diploid = two sets of chromosomes, one from the mother and one from the father) eggs, and males from unfertilised (haploid = one set of chromosomes, only from the mother) eggs. Offspring from the eggs laid last must emerge first otherwise their siblings become trapped behind them in the nest. With males emerging first, this allows the males to build up energy reserves and stake out good flowers in preparation for when the females emerge. The duration between when an egg is laid and the resultant bee emerges depends on the species, the temperature, and when the egg is laid. Within a season, Hylaeus tend to develop faster than Megachile, with the former taking about 1 month to emerge, and the latter 2-3 months. Warmer temperatures tend to speed up development. However, in both groups, if eggs are laid towards the end of the activity season, the offspring will undergo diapause – suspend their metabolism – until the next season when favourable temperatures recommence. As a bet-hedging strategy, some species may even diapause for more than one season. The triggers for diapause, breaking diapause, and emergence differ between species, and scientists are only just starting to understand the cues involved in these complex lifecycles for a select few species.
Who else checks in?
Even for the best-made bee hotels, other creatures like to inhabit these structures. There are a large variety of cavity-nesting wasps that also nest in bee hotels. Some of these wasps, known as mason or potter wasps in the subfamily Eumeninae, seal up their nests with mud. Other wasps, genus Isodontia, leave grass or sticks protruding from the entrance. The key difference in all cases is that the wasp mother stocks the nest with animal matter (cockroaches, spiders, caterpillars etc.) rather than nectar and pollen. Spiders and cockroaches and various other insects also like to make use of bee hotel cavities. Then there are some sneaky guests – the parasitoids. These even include bees! Megachilids in the genus Coelyoxis, known as “cuckoo bees”, have evolved a sneaky life-history strategy known as kleptoparasitism or brood parasitism, where they eschew all the hard work of a typical bee mother. Instead of foraging for food to provision nests to feed their progeny, they lay their eggs in the nest of a host species. When the cuckoo bee egg hatches, the larva feeds on the food intended for the host larva, as well as often consuming the host egg or larva too! There are also numerous wasp species that parasitise native bee nests, and some beetles in the family Meloidae. With a supply of pollen, nectar and larvae, bee hotels can also be targets for ant colonies.
Avoiding having these unwanted guests occupy a bee hotel is nigh impossible. Some of these, like “legitimate” nesting wasps, are welcome as biocontrol agents, however the others are more problematic. Whilst in nature there is a balance of parasitoids and hosts, and the presence of parasitoids indicates a healthy host population, we may artificially increase the relative abundance of parasitoids relative to hosts and upset this balance. For this reason it is adviseable that bee hotels are dispersed around, rather than having one huge bee hotel and many bee hotels all aggregated together.
Putting bee hotels out just before the start of the native bee activity season can help improve the chances it will be occupied by the intended guests.
Ants can be a real pain, and often the only way to deal with them once they have found a bee hotel is to tip the entire ant colony out, wash out the bee hotel, and relocate it. Some strategies to avoid ants getting to it in the first place is to hang it on a thin piece of rope out on a tree limb rather than tied against the tree trunk, and coating the rope with Vaseline.
In summary, bee hotels are just one way that we can help conserve native bees. However, not all bees will use bee hotels, and the design of a bee hotel is critical for it being used by native bees. Perhaps the greatest value in bee hotels lies more in allowing us to see these amazing creatures in our own backyards, and forge a connection with them.
Learn more! To learn more about the best design for bee hotels, what species use them, and what flowers are most preferred by the native bees that check-in to the bee hotels, all this and more is covered in the book “Bee Hotels for Australian Bees”. This 64pg book, filled with high-quality photos and scientifically-verified information is available as an e-book or printed version. Email firstname.lastname@example.org to order a copy.
You have the opportunity to participate in a global citizen science project on bee hotels. Kit Prendergast, PhD researcher, has set up a citizen science project for people across the globe to document their use of bee hotels. Such data will be invaluable to identifying the optimal range of bee hotel designs, and the range of species that use them. To do so, go to the Facebook group “Bees in the burbs in a biodiversity hotspot”: https://www.facebook.com/groups/Beesintheburbs/
LinkedIn: Kit Prendergast