In 24 gardens scattered across Zurich, volunteers armed with specimen jars and clipboards spent hours watching bumblebees, solitary bees, and hoverflies move from flower to flower—a patient, meticulous effort that has revealed something surprising about cities and the insects that sustain them. Researchers from the Swiss Federal Research Institute for Forest, Snow and Landscape spent 9 hours per volunteer in each garden, capturing and cataloguing every insect visitor, and what they discovered challenges a simple assumption: that urban gardens alone can solve the pollinator crisis.
The study matters because cities are now major refuges for wild pollinator species, yet we've known little about which insects actually thrive where, or why. As urbanization accelerates and agricultural landscapes shrink, cities have become unexpectedly vital for conservation. But it turns out that planting flowers—even diverse flowers—is only half the story.
The researchers measured even the tongue lengths of individual insects to understand their feeding habits. This detail reveals how nature works at granular scales: large wild bees like bumblebees have long tongues that can reach nectar hidden deep in specialized flowers, while hoverflies with shorter tongues depend on easily accessible blooms. What they found was encouraging and sobering in equal measure. Large wild bees remain surprisingly active in flower-rich gardens even in the densest urban centers, likely because their size allows them to fly across stretches of pavement to reach individual pockets of flowers. Smaller solitary wild bees thrive too when gardens offer diversity, finding both nutrition and nesting spaces within single plots. In this way, specialist flowers can be pollinated effectively almost anywhere in the city.
But the picture fractured when researchers looked at hoverflies and beetles. These insects became rare wherever cities grew denser, regardless of how lovely the flower selection was in any single garden. The problem wasn't botanical; it was spatial. Hoverflies, whose larvae eat aphids, require many connected green spaces to sustain populations. Beetles that breed in dead wood cannot find suitable breeding grounds when concrete dominates entire neighborhoods. The result is a collapse in pollination services in heavily built-up areas—fewer insects mean fewer plants setting fruit and seeds.
This discovery points to a limitation that no individual gardener can overcome alone. David Frey, the WSL visiting researcher who conducted the experiment, acknowledged the value of private action: "It's always worth it to do something for biodiversity, even in small spaces." Soil quality improves, recreational value increases, and every flower matters. Yet his colleagues emphasized the hard truth. As Merin Reji Chacko put it, beetles and hoverflies "depend on habitats in entire neighborhoods and not just single gardens."
The study's findings, published in the Journal of Applied Ecology, demand action at multiple levels. City planners must weave green corridors and habitat patches into dense neighborhoods. Municipalities must protect existing green spaces rather than let them erode. Private gardeners should plant diverse flowers and leave dead wood where they can. None of these actions alone is sufficient, but together they can reshape how cities function—not as concrete monocultures but as layered ecosystems where pollinator populations can actually sustain themselves and the flowering plants they depend on.