Wild bees welcome! Myths and facts about wild bee "hotels"

The ideal time to set up a nesting aid for wild bees is in late winter or early spring. During this time, many wild bee species begin to actively search for suitable nesting sites to create their brood cells. Marina Moser, Sonia Bigalk and Sebastian Görn, entomologists at the Stuttgart Natural History Museum, have put together some FAQs for you about wild bee hotels and ideal nesting aids.

• Why is the term "wild bee hotel" actually incorrect?
• What is the difference between wild bees and honey bees?
• And what about the bumblebees?
• What does a perfect nesting aid look like?
• How do I build a nesting aid?
• Help, my nesting aid is not being colonised – what should I do?
• Help, my nesting aid is being colonised by others – what should I do?
• Do I need to clean my nesting aid?
• What else needs to be considered?
• Are nesting aids the solution to bee decline?
• What else can I do to help wild bees?
• Can wild bees sting?
• I would like to find out more about how I can support wild bees. Where can I find good information?

Why is the term “wild bee hotel” actually incorrect?

We humans usually associate the term “hotel” with accommodation where guests can stay overnight in comfort. And this is precisely where the misunderstanding lies: the nesting aids with which we want to do something good for wild bees are much more than just a place to spend the night. Here, female wild bees create several brood cells, in each of which they lay an egg and store a sufficient amount of pollen. The larva hatches from this egg, eats the pollen supply and then pupates. It is therefore better to use the term “nesting aid”, as this is where the wild bees are supposed to settle.

In the horned mason bee (Osmia cornuta), the offspring is already fully developed by the end of the summer and overwinters in its brood cell as an “adult” insect. As one of the early wild bees, the males of the horned mason bee hatch at the beginning of March, while the females hatch one to two weeks later.

Fun fact: It is very easy to distinguish the males from the females of the horned mason bee: The males are slightly smaller and look like they have a moustache due to the white hairs on their face. Females have black hairs on their face and two small horns, which are the reason for the name “horned mason bee”.

What is the difference between wild bees and honey bees?

The biggest difference lies in their way of life. Honey bees are known for their eusocial way of life, i.e. they form a colony with an egg-laying queen, several thousand female workers and (at least temporarily) males, the so-called drones. Their division of labor and their communication, in which the workers exchange information about the best food collection sites through waggle dances, are widely known. In contrast, the majority of our 604 wild bee species are also known as solitary bees. They do not form colonies; each bee is responsible for itself and the care of its offspring. As wild bees generally only have relatively short flight times of a maximum of eight weeks, most wild bees never meet their offspring.

And what about the bumblebees?

Bumblebees are wild bees, although their way of life is similar to that of honey bees. Bumblebees form colonies, but with significantly fewer individuals than honey bees. In a bumblebee colony, there is also a division of labor between the queen, who lays eggs and is thus responsible for reproduction, and the workers, who care for the queen and her offspring. In contrast to the queens of honey bees, however, bumblebee queens have no special physical adaptations to this way of life; they look like a large worker bee. This way of life is referred to as “primitive eusocial” and can also be observed in some species of sweat bees.

What does the perfect nesting aid look like?

Around 20 of the wild bee species found in Germany nest in existing cavities. They are therefore potential nesting aid inhabitants. Most of these wild bees prefer holes with a diameter of 3 to 9 millimeters. Unlike in most wild bee “hotels” found on the market, the cavities should be up to 20 cm deep, but at least 10 cm, so that the wild bees can create several brood cells in a row. The nesting aid should be well protected against rain and fully exposed to the sun.

How do I build a nesting aid?

If you want to do it yourself, bamboo tubes with an internal diameter of between 3 and 9 mm are a good option. These are cut off behind the thickened nodes so that the natural closure of the bamboo tube forms the back wall. If there are several such knots over a length of 10-20 cm, the seals can be broken with a drill or a thin rod. It is important to make the tubes’ inner walls as smooth as possible so that there are no splinters along the entire length. The tubes can then be tied together in bundles or arranged horizontally in an existing container (e.g. a wooden box or tin cans).

Alternatively, wood with boreholes that imitate the natural feeding tunnels of beetles is also suitable. Hardwood, e.g. oak, beech, ash and hornbeam, is particularly recommended for this purpose. The drill holes should also have a diameter of 3 to 9 millimeters, while a depth of approx. 10 cm is sufficient. When drilling holes, it is important that the wood does not develop cracks through which moisture and parasites can enter. To prevent cracks, a distance of approx. 1 cm between the drill holes is recommended. With logs, it is important to always drill across the grain, i.e. not into the end grain, but from the “bark side”. Here, too, it is essential that the surface is free of splinters, which is easily achieved with sandpaper and a little patience.

Help, my nesting aid is not being colonised – what should I do?

If a nesting aid is not colonised, there may be various reasons for this. Cities, with their warm microclimate and diverse range of flowers, are often attractive habitats where the horned mason bee and its relative, the rusty mason bee (Osmia bicornis), can be found almost everywhere. In the vast majority of cases, it can therefore be ruled out that there are simply no bees that could colonize the nesting aid.

• Location: It is much more likely that the location is not ideal for wild bees. Wild bees prefer sunny places that are also protected from rain and excessive wind. The nesting aid should therefore face south if possible and not be shaded by trees, buildings or the roof of the nesting aid.
• Choice of material: In addition, unsuitable materials could deter wild bees from nesting. Wood that is too soft or cracked or tubes with splinters are not well accepted, as softwood retains moisture and the bees can tear their sensitive wings on the splinters when they hatch.
• Unsuitable hole sizes: Different wild bee species have different preferences when it comes to the size of cavities. Small cavities (2-4 mm) are colonized by smaller species, but these are much rarer than the two common mason bee species, which prefer diameters of 6-9 mm. Perforated bricks with diameters of 2 cm or more are generally not colonized - unless they are filled with bamboo tubes of a suitable size.
• Unfavorable time of year: The individual wild bee species are only active for a few weeks and most cavity-dwelling species nest in spring or summer. If the nesting aids are only set up or hung up in summer or autumn, they may not be colonised by the early mason bee species until the following spring.
• Too much disturbance: Nest boxes should be moved as little as possible after they have been erected. Disturbance caused by constant observation or the associated shading can also cause the bees to look for a more suitable nesting site.

Help, my nesting aid is being colonised by others – what should I do?

Other insects such as wasps, flies and spiders may also show an interest in nesting aids. The spectrum ranges from insects that are simply looking for a sheltered place to rest or nest to parasites that attack wild bees or predators that want to eat them. In individual cases, an encounter may end fatally for the individual wild bee. Ultimately, however, eating and being eaten is also part of the natural network. And lastly, wasps, flies and spiders also have a right to exist and fulfill important functions in the ecosystem.

Fun fact: Female mason bees often leave the outermost brood cell empty. This serves as protection against parasites or hungry birds, which in the best case (for the bee) look for their host or food elsewhere after finding this brood cell empty.

Do I need to clean my nesting aid?

The experts are not yet in complete agreement on this. However, one thing is certain: closed brood cells should never be opened. Passages that have not been colonised in a year and do not have a nest plug can be carefully freed of cocoon and clay residues in winter (until the end of February). However, caution is advised here: The outermost nest closure and therefore the outermost brood cell could fall victim to a hungry bird in winter, but the rear brood cells of the tube may still be intact and must not be destroyed during cleaning. Careful observation of nesting activity in spring and summer is essential to determine which nesting tunnels are no longer occupied. If a formerly much sought-after nesting aid is no longer colonised after a few years, wild bee expert Paul Westrich recommends placing the nesting aid in a dark cardboard box with a 1-2 cm wide exit hole at the end of February. This allows the hatching wild bees to find their way out, but in most cases, they no longer find the nesting aid attractive. As soon as all the bees have hatched, the nesting aid can be cleaned and is ready for colonisation again the following year.

What else needs to be considered?

The nesting aid must not be taken indoors in winter. Wild bees determine the right time to hatch based on the outside temperature - unlike plants, which are mainly guided by the length of the day. If nesting aids are taken indoors for the winter, the unusually high temperatures confuse the wild bees, which then hatch far too early in the home or cellar but are unable to find food outside in nature.

Are nesting aids the solution to “bee decline”?

Nesting aids are not the solution to stopping bee decline on a large scale. Nesting aids only support around 20 (i.e. 3.3 %) of the 604 bee species recorded in Germany. In addition, they are useless for most of the truly endangered species, as three out of four wild bee species build their nests in the ground. However, they are certainly a support for wild bees and they provide a unique opportunity for experiencing nature as observing the busy bees can be fascinating, especially for children. Everyone can, may and should enjoy their nesting aid.

What else can I do to help wild bees?

• Provide food plants: Make sure you have a variety of bee-friendly plants growing in your garden, on your balcony or in your town. Choose native flowers, shrubs or trees that provide bees with plenty of nectar and pollen throughout the year. Flower mixtures are also a popular method, but it is essential to use native seeds!
• Mow less: Instead of constantly mowing at short intervals, only mow twice a year. This gives flowering plants enough time to bloom and then form seeds. The mowed material should be removed.
• Avoid fertilizers and pesticides: Wildflowers prefer nutrient-poor soils. This is why fertilizers tend to harm the flowers. Insecticides are an absolute taboo in a bee-friendly garden!
• Other nesting options: Three quarters of native wild bee species nest in the soil. Therefore, even a piece of open ground is real wild bee protection! The site should be as sunny as possible and the soil must be neither too firm nor too crumbly. Unwashed sand, for example, is a good material for ground nesters.
• Get active! Talk to friends, family and neighbors about the importance of wild bees and help them to contribute to the protection and promotion of wild bees themselves. Get involved in local nature conservation projects or support relevant organizations. Share this article - sharing is caring ;-)

Can wild bees sting?

It depends. The females have a defensive stinger, which they can certainly use for defense. However, most wild bees are extremely peaceful and, compared to honey bees and some bumblebees, do not show any defensive behavior if you get too close to their nest.

Fun fact: Unlike honey bees, the sting of wild bees has no barbs, so in an emergency wild bees can sting several times to defend themselves against predators, nest competitors or other threats.

I would like to find out more about how I can support wild bees. Where can I find good information?

In the information material of the Bunte Wiese Stuttgart: https://www.buntewiese-stuttgart.de/infomaterial.html

Or in the books by Paul Westrich:

• Paul Westrich: Die Wildbienen Deutschlands, 2. Auflage. Ulmer Verlag, Stuttgart 2019, ISBN 978-3-8186-0880-4. 
• Paul Westrich: Die anderen Bienen. Verlag Dr. Friedrich Pfeil, München 2014, ISBN 978-3-89937-136-9.

Practical gardening tips:

• Dave Goulson: The Garden Jungle: or Gardening to Save the Planet. Vintage (Penguin Books), London. ISBN: 9781784709914. 
• Ulrike Aufderheide: Tiere pflanzen. Pala, Darmstadt, ISBN 978-3-89566-388-8. Praktische Tipps für Lebensräume im Naturgarten.