The Little Known World of Urban Bees 🐝

Sweat bee. Photo: Glenn Seplak

Bees and flowers go together like bread and butter. Bees are almost entirely dependent on flowers to meet their nutritional needs. Adult bees consume nectar to provide them with the energy they need to mate (if they are male) and to build and provision a nest (if they are female). Mother bees feed pollen to their young. Pollen is a high-protein food that supplies baby bees with the nutrition they need to grow and transform from a tiny larva into an adult bee.

On the other side of this relationship are the flowers, most of which require bees for pollination. In fact, nearly 90% of flowering plant species worldwide are dependent on animals for pollination, and of those animals, bees are by far the top performers.

But the relationship between bees and flowers isn’t as simple as it seems at first glance. A mother bee won’t collect pollen from just any flower. Most bee species are specialists to some degree. In other words, they are picky eaters. You might find these picky eaters in a wide variety of flowers consuming nectar, but on closer look, you would discover that they only collect pollen from a limited subset of these flowers. And these specialists are not all equally picky. For example, in the Sonoran Desert, you’ll find some species of bees that will collect pollen from just about any cactus flower, while others focus on the flowers from only a few genera within the cactus family.

Honey bee. Photo: Glenn Seplak

Our most familiar bee, the honey bee, is the exception. Honey bees are the ultimate generalists, collecting nectar and pollen from an extremely wide range of plant species. They are exceptional in many other ways as well. First of all, they are social and live in a hive with hundreds to thousands of sister bees. In contrast, nearly all other bee species are solitary, meaning that they live completely alone. Another big difference between honey bees and most other bees is that you’ll find honey bees out foraging on flowers at any time of year, as long as it’s not too cold, whereas the foraging activity of most solitary bee species is highly seasonal.

Like bees, plants can be specialists or generalists and everything in between. Some specialist flowers are dependent on a small number of bee species for pollination, while generalist flowers may be pollinated by a wide variety of bees as well as other types of insects. These relationships between bees and flowers have been shaped over thousands to millions of years of evolution, and these relationships are undoubtedly being affected by climate change.

Understanding the effects of climate change on these relationships is one of the main goals of a new long-term research program that began nearly two years ago. In the summer of 2016, volunteers and I began sampling the bee fauna of Las Milpitas, an urban farm that belongs to the Community Food Bank of Southern Arizona and lies just south of ‘A’ Mountain along the banks of the Santa Cruz River. This research project grew out of a larger effort to increase the quantity and quality of pollinator habitat in the City of Tucson. From research that has taken place in other, much larger cities, we know that urban environments can support a great diversity of bees. Moreover, by supporting the bees, we are supporting local food production, an industry that has seen tremendous growth in Tucson in recent years.

Photo: Lois Settlemeyer


Much of the research on plant-pollinator interactions and climate change has focused on the plant perspective. We are interested in the bees. The Sonoran Desert is a global bee hotspot, with more species of bees than almost anywhere else in the world, but surprisingly we know relatively little about our exceptional bee fauna. Not only is the natural history of most species in the Sonoran Desert poorly known, but furthermore, there have been very few studies that have tried simply to enumerate the many different species found here in our desert.

We are sampling the bee fauna in Las Milpitas every other week in order to capture the diversity of bees that occurs over the course of a full year. Just as plants don’t bloom all year long, most bees are not active all year long. Because the foraging activity of most bees is highly seasonal, the diversity of bee species that are out foraging changes from one month to the next.

By sampling bees over the course of a year and over multiple years we are gathering data that will help us better understand how changes in temperature and precipitation might be affecting our bee fauna. Typically, solitary bees spend most of the lives in their natal nest in a state of developmental diapause. They will emerge from their nest when conditions are “right,” and hopefully, when the plants they require for food are blooming. A mismatch in the timing of bee and flower activity could spell disaster for a mother bee as she may be unable to find food for her offspring.

Pan traps used to sample bees. Photo: Kim Franklin


Las Milpitas is a beautiful farm right in the heart of our city, but it’s just one place. To better understand the bee fauna of the Tucson Basin, we need to sample not only across time, but also across space. This spring we started to grow the spatial extent of our research by involving Sunnyside High School students who are sampling the bee fauna on their campus using the same methods that we employ in Las Milpitas. We hope to add more schools to this research project in coming years.

The Sunnyside students are also DNA barcoding the bees they sample in order to identify them to species. In other words, they are reading the DNA of their bees and matching it with DNA sequences that exist in public DNA reference libraries. Establishing a streamlined protocol to quickly identify the collected bees via DNA barcoding adds an exciting dimension to this research, and it gives students an opportunity to participate in authentic research and to make discoveries about the biodiversity that exists in their own backyards.

Ultimately we hope to see this project to contribute to the growth of a BiodiverCITY, a city in which wildlife thrives alongside its human habitants. Tucson already holds a wealth of biodiversity, but as the city grows, we will have to work to keep it that way. Pollinator-friendly gardening practices, landscaping that utilizes native plants, and public and private spaces that provide nesting habitat, all support our exceptionally diverse bee fauna, and also a tremendous diversity of other animals as well. Moreover, all of this plant and animal diversity contributes to the vitality of our local economy and our quality of life!

Check out this fun video on Mariposas of the Milpa, edited by Jay Pierstorff.

Cactus bee. Photo: Glenn Seplak







Written by Kim Franklin, Desert Museum Conservation Research Scientist




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