Penn State graduate student Emily Erickson observing pollinators. She and her co-authors recently published a study that advances our understanding of how to design urban and suburban landscapes that are practical for humans and pollinators. Photo credit: Emily Erickson.

By Melissa Mayer

Melissa Mayer

When it comes to flowers, the traits humans prefer—things like low pollen production, brighter colors, and changes to the height and shape of plants—are a mixed bag for pollinators. Plants bred for larger flowers or extended bloom times may be a boon for some hungry pollinators, but structural changes in the plants can make it harder for pollinators to handle the flowers, access nectar, or even find the flowers in the first place.

In a recent study published in Environmental Entomology, one group of researchers tracked pollinators visiting annual ornamental plants at exurban sites. The study is part of Protecting Bees, a project headed by Cristi Palmer, Ph.D., of Rutgers University and funded by the United States Department of Agriculture’s Specialty Crop Research Initiative (USDA-SCRI). The project, which includes work from universities all over the United States, hopes to tease out the subtle relationships among pollinators, plants, and pests.

Emily Erickson

The Environmental Entomology paper focuses on one prong of this research: understanding what makes ornamental plants attractive to pollinators. “It has been exciting to work in a system that is relatively new in the world of pollination ecology,” says Emily Erickson, a graduate student in the department of entomology at Pennsylvania State University and one of the study’s authors. “I think this research is an important step to understanding how to design urban and suburban landscapes that are practical for humans and pollinators,” she adds.

A Bee-utiful Experiment

Bee foraging on a flower. Photo credit: Emily Erickson.

The research team selected two semi-natural sites with established mutualistic relationships between flowering plants and local pollinators, including a diverse community of bees. For good measure, they added a honeybee colony near each site.

The team grew five cultivars for each of the five most popular ornamentals known to attract pollinators: marigold, zinnia, Egyptian starcluster, sweet alyssum, and lantana. They arranged these in a random design and observed them at least twice a day for the full blooming season for each plant, essentially early July through late September of 2016 and 2017.

The researchers found that these ornamental plants drew a wide range of visitors from two or three pollinator groups­, including bees, flies, and butterflies and moths. The floral features that influenced visitation patterns varied. It seems that many traits—color, nutritional quality and quantity, odor, and shape—all play roles in how attractive a given flower is for a given pollinator.

Putting the Data in Context

Since ornamental plants are usually cloned or inbred, genetics can’t explain most of these variations. Factors like differences in the local pollinator community, background flowers, or the general landscape as well as unpredictable fluctuations in the environment may. It all comes down to environmental context.

“These were exciting observations because it reinforced our understanding that plant-pollinator interactions are very complex and nuanced—which is not how we have historically approached plant selection for creating pollinator habitat in urban environments but will be an important consideration going forward,” explains Erickson.

Pollinator communities include both specialists—who forage for pollen from only one or a few plant species—and less picky generalists. It turns out annual ornamentals only attract generalists. This means these plants can’t support a complex, stable pollinator community on their own. Still, their extended bloom times or showier flowers may make a difference for generalists.

Syrphid fly visiting marigold. Photo credit: Emily Erickson.

“Since we know that urban areas are already dominated by generalist pollinator species, our conservation goals differ and so too should our approach,” says Erickson. “In these environments, ornamental plants that attract an abundance and diversity of pollinators may be a good solution for meeting the intersection of human aesthetic preferences and the foraging needs of the local generalist pollinators.”

Bee-Friending Local Pollinators

For people planning gardens with pollinators in mind, Erickson has some advice. First, choose a wide variety of flowers that bloom at different times. “This will ensure that the garden can attract a diversity of insect taxa and that visiting pollinators will have season-long access to resources,” she explains.

If you can, go with native plants or look for cultivars that resemble their wild types in shape and color. For example, flowers that are “doubled” to produce extra petals might be lovely to humans but super difficult for pollinators to handle. And don’t discount perennials, which have a shorter breeding history than annuals.

The best bet is to simply “ask” your local pollinators. According to Erickson, “plant many different things, and the visiting pollinators will quickly determine which they most prefer!” For professional guidance, she recommends this Protecting Bees tool that suggests plants based on your precise location, characteristics of your outdoor space, and even the kinds of pollinators you want to attract.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.