If you want to do something good for beneficial insects in your home garden, you can offer them a rich flower buffet. Even in densely built-up areas, bumblebees and some wild bees find their way to these flower islands. However, this is not enough for other pollinators, including hoverflies and beetles. They rely on better connected habitats that offer them sufficient opportunities for the development of their larvae, a study shows. Insect-friendly urban planning therefore requires interaction between private garden owners and urban actors.
In densely built-up cities, gardens and green spaces can provide refuge for beneficial insects. But which species can benefit from the small green and colorful islands in the middle of gray concrete deserts full of sealed areas? Can amateur gardeners who want to grow their own fruit or vegetables in the middle of the city hope that pollinators will find their way to them? And how can cities be planned and developed so that they also offer a living space for our six-legged helpers?
Counting insects in gardens
A team led by Merin Reji Chacko from the Swiss Federal Institute for Forest, Snow and Landscape Research in Switzerland looked into these questions. In 24 gardens in Zurich, volunteers documented several times for nine hours at a time how many insects of which species pollinated flowers there and how successful the pollination was, i.e. how well the plants then formed seeds. The researchers related these factors to the structural density around the respective garden and to the local variety of flowers within the garden.
In order to make the results comparable, Reji Chacko and her team set up several flower pots in each garden before the counting began with four selected plants that have varying degrees of attraction to certain pollinators due to their flower shape – from the wild carrot, whose flat umbel flowers provide easily accessible nectar and are also popular with insects with short mouthparts such as hoverflies and beetles, to the comfrey, which hides its nectar so deeply in its calyxes that only insects can long trunk like bumblebees and certain bees can reach it.
Many flowers alone do not help everyone
The result: “Both pollinator diversity and flower visitation rates decreased with increasing building density and the loss of flower diversity,” the researchers report. However, important differences emerged between the various types of insects: “While small solitary wild bees and large social bees such as bumblebees were significantly more common in flower-filled gardens, no such connection was apparent for hoverflies, beetles and small social wild bees,” said the team.
No matter how colorful and diverse a garden was on a local level: if there were mainly sealed areas around it, beetles and hoverflies stayed away. As the researchers explain, many useful beetles require dead wood as a breeding ground, which is in short supply in cities and beautifully landscaped gardens. The hoverflies, in turn, feed on aphids in the larval stage. Where these are missing, adult animals cannot develop. If a garden is not connected to habitats in which beetles and hoverflies can reproduce, these pollinators will be absent. As a result, plants such as wild carrots, which rely primarily on these species, are hardly fertilized in city gardens.
Private and municipal use
For plants like comfrey, which are pollinated primarily by bumblebees and bees with long trunks, it made little difference in the experiment where in the city they grew. Because their pollinators can also overcome larger asphalted areas and are often attracted to flower-filled gardens. “It’s always worth doing something for biodiversity in a small area, even if you have a very isolated garden in the city center,” says Reji Chacko’s colleague David Frey.
However, this is not enough as a sole strategy to protect insects. According to the researchers, not only private individuals are in demand, but also municipal actors. As the study has shown, not all important species of pollinators benefit from isolated flowering islands. “The loss of habitat in settlements should therefore not only be countered by selectively increasing the availability of flowers, but also through landscape-wide measures, such as creating and connecting specific habitats for different pollinator groups,” recommend Reji Chacko and her team.
Source: Merin Reji Chacko (Federal Research Institute for Forest, Snow and Landscape, Birmensdorf, Switzerland) et al., Journal of Applied Ecology, doi: 10.1111/1365-2664.70384