Research to Create Better Gardens for Bees

Summer 2024 

Many gardeners select plants primarily for their color and appearance, often without realizing the valuable wildlife habitat they could also provide. Ornamental plants introduced to residential and public landscapes have been shown to offer great value to bee conservation (Hickman 2023). Bees rely exclusively on a diet of pollen and nectar, making the availability of varied, healthy plants essential for their survival. But what exactly does that mean? What makes a plant and its garden healthy for bees?

This is a question that has been of great interest to bee researchers, horticulturists, and landscape professionals in recent years. For example, we know that certain garden features—such as mulch, the area covered by flowering plants, and a water source—influence bee survival in gardens (Quitsberg et al. 2016; Casey 2018a and 2018b). Several studies have also documented a wide range of bee plant preferences, even among different cultivars of the same plant species (Garbuzov and Ratnieks 2014; Frankie et al. 2019).

Our recent work at the University of California, Davis and the Division of Agriculture and Natural Resources of the University of California has expanded on this work. We conducted replicated field plot studies in Northern and Southern California to compare the bee attractiveness of California native and non-native plants (Figures 1a and 1b). Our observations confirmed large differences between plant species in the number of pollinating visitors they receive per minute. A greater diversity of pollinators—including a richer suite of native bees—visited California native plants when compared to non-native plants (Nabors et al. 2022). However, honey bees (Apis mellifera) showed no significant preference for either California native or non-native plants (Tables 1a and 1b).

This research has informed plant selection and education programs for the public. However, we recognize that for bee-supporting landscapes to take hold in California, nursery producers and landscape designers need ways to quickly know if new plant varieties will be attractive to bees and other pollinators.

To this end, our most recent study evaluated two sampling methods for pollinator attractiveness. Insects are typically measured by timed counts of 3–10 minutes, during which every insect entering an area of observation or landing on the reproductive parts of a flower is counted. This method has been shown to be accurate for bees (Erickson et al. 2022), but we feel it is too time consuming for gardeners and growers.

We have been comparing these more typical three-minute timed counts with an alternative method that consists of three 20-second snapshot counts (Garbuzov and Ratnieks 2014; Mach and Potter 2018). We conducted these counts at replicated research plots especially established for pollinator attractiveness observations (Fig. 2a, 2b) and nursery sites (Fig. 2c, and 2d). We have found that shorter snapshot counts can provide a reliable method to determine the diversity and abundance of bees (Casey, Corkidi, Smith, and Middleton, unpublished).

Pesticides are another key concern in bee survival. We have been investigating the pesticide transport within plants to pollen and nectar so we could help growers understand if pest management practices at the nursery could impact bees when plants reach consumers’ gardens (Fig. 3a, 3b and 3c). This research includes both drench applications of pesticides, which are applied to the soil, and foliar applications, which are sprayed onto leaves.

For all products and at all rates used, the concentration of pesticide residue decreased over time in the nectar of plants treated with drench or foliar applications. However, plants treated with drench applications to the soilless mix had higher pesticide residue concentrations than those sprayed with foliar applications. The concentration of the active ingredients in the nectar of foliar-treated plants was generally below detection limits by six weeks after treatment (Havers et al. 2024).

Although some of the most important steps to protect pollinators must be made in agriculture and on public lands, we also need to protect pollinators in our managed landscapes and gardens. We hope this research will build on past work to provide growers, land managers, and gardeners with the information they need to build better bee gardens.