Anyone who truly wants to do something for insects…

The bee season is in full swing and they are searching everywhere. Bees They are searching for nectar-filled flowers. Where can they and other hungry pollinating insects find them in densely populated cities?

WSL researchers addressed this question in a complex experiment. Around 30 tireless volunteers repeatedly sat in front of flowerpots in 24 different gardens in the city of Zurich over the course of a month, meticulously recording and catching every insect – for nine hours at a time.

This study reveals for the first time which insect species visit which gardens and flowers throughout the day and how well they pollinate the flowers.

“We looked at urban gardens located in different densities of urban areas and offering varying ranges of flowers. We found significant differences in the prevalence of various insect species,” summarizes ecologist and WSL visiting scientist Merin Reji Chacko.

Analyzing such differences helps researchers figure out what a city looks like where pollinating insects are a common sight. Finding the right home.

This is important because many of these insect species, such as bumblebees, play a vital role in the pollination of wild and cultivated plants. Cities can host a great diversity of wild bees and are therefore important for their protection.

Measuring insect tongues

To find out how the insects adapt to their environment, the researchers even measured the tongue length of individual animals. This reveals which flowers an insect can use:

Only pollinators with long tongues, such as bumblebees, can reach the deeply hidden nectar of specialized flowers. Pollinators with shorter mouthparts, such as hoverflies, on the other hand, depend on easily accessible flowers.

The researchers found that particularly large wild bees such as bumblebees are active in flower-rich gardens in the densely built-up city center, presumably because their size allows them to fly over larger asphalted areas to individual "flower islands".

They thus benefit from a diverse range of flowers, even in densely built-up city centers. But also Small wild bees with relatively long tongues benefit from the rich A buffet in the garden, presumably because they are small and can thus find enough food and nesting places.

Plants whose nectar can only be reached by insects with long tongues can therefore be pollinated almost anywhere in the city.

Flowers lack pollinators

However, not all insects pollinate flowers everywhere: hoverflies and beetles are seen less and less frequently as urban development becomes increasingly dense – regardless of how attractive the range of flowers in a particular garden is or how long their tongues are.

The reason: These groups cannot find a suitable habitat in densely populated areas. The feather-light hoverflies, which feed on aphids as larvae, need plenty of green spaces to thrive in the city.

Many beetles that breed in deadwood, Furthermore, no suitable ones can be found in concrete-covered areas. Breeding grounds. The consequence is that flowers pollinated by such insects are less frequently pollinated in densely built-up urban areas and therefore produce fewer seeds and fruits.

Involvement of private individuals and cities is needed.

The results of the study clearly show that it is necessary to use the technology at all decision-making levels. – from private to Urban planning level – needed to promote biodiversity and ecosystem services such as pollination in the city.

Private garden owners can achieve a lot, says ecologist and WSL visiting scientist David Frey, who led the experiment as part of his doctoral thesis at WSL:

"It's always worthwhile to do something for biodiversity, even in a small space. Even if you have a very isolated garden in the city center. Planting many different plants also has benefits." positive effects on soil quality and even the The garden's recreational value.

But the efforts of individuals are not enough. Beetles and hoverflies, for example, depend on habitats throughout the entire city district, not just in individual gardens.

Co-lead author Reji Chacko says: “We have an astonishingly high level of biodiversity in cities. But it is important that we ensure that the green spaces we have are protected. This is especially true when we densify cities inwards and upwards, as is planned, for example, in the Zurich city master plan 2040.”

original publication

Reji Chacko M., Frey DJ, Albrecht M., Ghazoul J., Moretti M. (2026) No one-size-fits-all: trait-dependent effects of local plant diversity on pollinators and pollination service in a densifying city. J. Appl. Ecol. 63(5). doi.org/10.1111/1365-2664.70384

Source / Photos: wsl.ch