Insect hotels are structures made of different materials and shapes, designed to provide shelter and nesting sites for insects.
Initially used by farmers to transport bees and provide them with nesting sites, in recent decades they have become popular in gardening to attract and encourage certain types of beneficial insects. Although their main purpose is to provide a space for solitary bees, there are also hotels designed for other species such as butterflies, ladybirds, lacewings or earwigs.
Several articles have reviewed the usefulness and design of these hotels and the reality is that, while they provide shelter for many insect species, the results are not always as expected. The most effective habitats tend to have reeds or cylindrical holesas some species of solitary bees They nest in "tunnels" where they build cells with mud or leaves, and deposit an egg together with a supply of pollen that will feed the larva when it hatches.
However, other types of receptacles such as those with a slot, aimed at butterflies, and those containing paper, straw, twigs and pine cones, usually aimed at earwigs, ladybirds and lacewings, tend not to attract their target insects, and instead attract wasps, spiders and ants. This is not necessarily a problem, but it is something to keep in mind when selecting them.
Focusing on bee hotels, There are also several factors to be taken into account. Although the aim is to favour native bee species, it has been found that they can compete with introduced bee species and wasps, which can occupy a large part of the cavities. Additionally, the high density of nests in hotels may increase the rate of parasitism, although studies comparing parasitism rates between natural and artificial nests are lacking.
Even so, there are several factors that are in our hands to choose a hotel attractive to solitary bees. It is recommended to use natural materials, such as logs and drilled timber, cardboard tubes or grass reeds. Artificial materials, such as plastic, are not only less attractive to bees, but may also alter the humidity inside the hotel, leading to mould and death of the brood.
Preferences for cavity diameters vary according to the species, It is therefore recommended to use a variety of sizes to attract a wider range of species. In general, the diameter of the holes is usually between 3 and 10 mm, and the length between 8 and 20 cm, with 15 and 20 cm being best. Small species such as those of the genus Ceratinaprefer holes less than 5 mm in diameter, and those of the genus Megachilesizes larger than 5 mm.
It is recommended to place the nests at a heightto hinder access to mammals, to the shelter from the rain and in sunny areasThe shade can encourage wasp nesting. Also, when nests are empty, it is best to clean them annually of spider webs and debris.
Currently, a citizen science project in the UK to find out how to optimise bee hotels, so perhaps we will be able to provide more information soon. For now, it is safe to say that insect hotels are useful for the nesting of solitary bees, and as a sample, here is a photo of the one I gave to my parents a few years ago.
I chose one with habitats for other insects. As the bibliography indicates, I only observed occupancy in the holes dedicated to solitary bees, with the exception of a small wasp nest in the upper receptacle. As a curiosity, one day I also found a cuckoo bee at the entrance, which was on the lookout to parasitise the nests. It was not the kind of visitor I was expecting, but if it is part of our fauna, welcome!
References
Alton, K., & Ratnieks, F. L. (2020). Caveat Emptor: do products sold to help bees and pollinating insects actually work?. Bee World, 97(2), 57-60.
Dave Goulson (25 April 2022). What size should the holes be in a bee hotel? PART I [Video archive]. Youtube. https://www.youtube.com/watch?v=zQQmyz-v8Kg
Dave Goulson (3 August 2022). What size should the holes be in a bee hotel? PART II [Video archive]. Youtube. https://www.youtube.com/watch?v=pxk5jkArC60
González-Zamora, J. E., Hidalgo-Matas, J. A., et al. (2021). Wild solitary bees and their use of bee hotels in southwest Spain. Journal of Apicultural Research, 60(5), 862-870.
Harris, B. A., Poole, E. M., et al. (2021). Consumer-ready insect hotels: an assessment of arthropod visitation and nesting success. Journal of Entomological Science, 56(2), 141-155.
MacIvor, J. S., & Packer, L. (2015). 'Bee hotels' as tools for native pollinator conservation: a premature verdict?. PloS one, 10(3), e0122126.
Rahimi, E., Barghjelveh, S., et al. (2021). How effective are artificial nests in attracting bees? A review. Journal of Ecology and Environment, 45, 1-11.
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