Bees are among the most important pollinators in the natural world, quietly sustaining ecosystems and food production. While honeybees often steal the spotlight, a vast number of solitary and primitively eusocial bees play equally vital roles. But across both urban and natural landscapes, many of these species are facing worrying declines.
As cities expand, they’re increasingly being seen not just as threats to biodiversity, but as potential refuges for pollinators. Yet urban environments are very different from natural ones. The heat generated by buildings and concrete – known as the urban heat island effect – and the way green spaces are managed (often with little consideration for flowering plants) could be affecting bees in ways we’re only beginning to understand.
As part of a recent study led by my former PhD student Muzafar Sirohi, we explored how urban conditions might be influencing the timing of bee emergence and the sex ratios of different species. This work formed part of Muzafar’s PhD research, and I was pleased to be part of the team that supported and collaborated on the project.
We found that several solitary bee species were producing females before males – a reversal of the more typical pattern known as ‘protandry’, where males emerge first. Most bees in the families Apidae and Megachilidae did follow the usual male-first pattern, but there were some interesting exceptions, including Nomada marshamella and Nomada fabriciana. Soil-nesting species also showed a lot of variation in emergence timing, likely influenced by microclimatic differences in urban soils.
When we looked at overall sex ratios, patterns varied across bee families. In Halictidae, females were more common, whereas Apidae and Megachilidae were skewed towards males. Interestingly, the Colletidae family showed no strong bias either way. However, in five species from the Andrenidae and Halictidae families, we saw a clear difference between urban and natural environments: urban populations had a higher proportion of males.
This could suggest that urban habitats – especially those with limited floral resources due to mowing, paving, or the removal of wild plants – may not be supporting as many female bees. Since females are the ones responsible for nest-building and and potentially pollination, as they visit more flowers, this imbalance could have long-term effects on bee populations and the pollination services they provide.
Our study adds to the growing body of evidence that urban environments can support pollinators – but only if managed thoughtfully. Cities need more than just green space: they need flowering plants, nesting habitats, and careful planning that recognises the delicate balance of bee ecology. With the right actions, we can make urban areas part of the solution to pollinator decline.
Here’s the reference with a link to the study:
Here’s the abstract:
Solitary and primitively eusocial bees are essential pollinators of plants. However, recent observations indicate a decline in their populations in both urban and natural environments. Urban areas are increasingly recognized as potential habitats for bee conservation. Nonetheless, these urban habitats can influence the taxonomic and functional diversity of bee populations. Therefore, we hypothesize that the distinctive warmer climate of urban areas – resulting from the urban heat island effect – along with the potential scarcity of floral resources, contributes to shifts in emergence patterns and the sex ratio of solitary and primitively eusocial bees. We found that many solitary bee species produced females before males. Additionally, most species within the Apidae family were recorded as protandrous, with the exceptions of Nomada marshamella and Nomada fabriciana. All species of Megachilidae were found to be protandrous. We also observed significant variation in the emergence patterns of soil-nesting species. Notably, we did not find any relationship between sociality and nesting preferences in relation to sex-biased emergence. The overall sex ratio varied among different bee species and families. In Halictidae family, sex ratios were biased towards females, while the Apidae and Megachilidae families exhibited a skewed ratio towards males. The sex ratio in the Colletidae family did not show any significant difference. However, among the Andrenidae and Halictidae families, we identified five species with significantly different sex ratios between urban and nature areas, with a higher proportion of males observed in urban sites. This suggests that these species may have been affected by limited food resources, potentially due to urban management practices such as the removal of floral resources. This could lead to increased competition for resources among the species.
