VIRGINIA TURFGRASS JOURNAL: Margaret J. Couvillon, Ph.D., Assistant Professor of Pollinator Biology and Ecology, Department of Entomology, Virginia Tech

Ornamental Plant Choice Impacts the Abundance and Diversity of Insect Pollinators in Gardens
This article is based upon the following peer-reviewed study: MC Palmersheim, R Schürch, ME O’Rourke, J Slezak, and MJ Couvillon. “If You Grow It, They Will Come: Ornamental Plants Impact the Abundance and Diversity of Pollinators and Other Flower-Visiting Insects in Gardens.” Horticulturae 8, no. 11 (2022): 1068.

Plants are mostly stationary. Although they can move a little bit, growing upwards and outwards, these movements are small, slow, and generally insufficient for finding a mate, especially if you want to reproduce sexually every year. Sexual reproduction helps maintain genetic diversity in a population and is a winning strategy. Therefore, over millions of years of co-evolution, plants have solved this sessile challenge by developing close relationships with some animals to serve as the vectors to transport pollen, the male sex cells. That close relationship is pollination, and about 88% of all flowering plants need that animal-assist to make the magic happen (Ollerton, Winfree & Tarrant 2011). Of the animals that participate in pollination, insects are by far the most consequential. If we line up the plants, both wild and agricultural, that need pollination services, our heavy lifters are insects, such as bees, butterflies, and flies. Therefore, while birds and bats are also valuable in this process, there is a critical need for the contribution of insect pollinators (Klein et al. 2007; Ollerton 2021).

The large role of insects in plant reproduction makes their recently recorded declines even more alarming: insects in general, and pollinators in particular, are decreasing in numbers (abundance) and species richness (diversity) (Hallmann et al. 2017; Sánchez-Bayo & Wyckhuys 2019). These declines have been precipitous, upwards to 75% by some measures, and likely a reflection of the larger threats to wildlife in general that humans have bequeathed to our world. For insect pollinators, of course, their declines have direct consequences on our global food security, as 75% of food crops need insect pollination services (Klein et al. 2007).

Both wild and managed insect pollinators, like honey bees, face challenges from pesticides, pests, pathogens, and poor nutrition, where a lack of abundant, diverse flowers in a landscape can have direct and indirect negative consequences on insects (Goulson et al. 2015). Media and popular press coverage has been wide: everyone wants to feed hungry pollinators. One activity that is immensely popular among concerned citizens is to establish a pollinator garden, where ornamentals are planted with the express purpose of feeding pollinators. Groups ranging from schools to libraries to museums to private individuals are frequently approaching perceived experts (i.e., an Assistant Professor of Pollinator Biology) for advice on what plants to use. When I started receiving such phone calls in 2017, I realized that all I could do was direct people to existing plant recommendation lists. I quickly realized, however, that a non-specialist gardener, such as myself, might experience barriers against making informed choices about plants for pollinators. For example, recommendations from organizations like the Xerces Society publish lists that appear based on expert opinion but lack empirical backing and/or do not cite sources. There are also inconsistencies among plant recommendations, where there is very little overlap, even within regions (Garbuzov & Ratnieks 2014a). Sometimes recommendations are too general, even if we know that different cultivars might vary in as much as tenfold in their attractiveness to insects (Garbuzov & Ratnieks 2014b). Additionally, some recommended plants required greenhouses for growth from seed, which would definitely be a challenge to someone like me. Lastly, some gardeners might want to target butterflies, while another, next to an apiary, might wish to cater to honey bees. I was discouraged by the lack of available data in existing recommendations. So I decided to make my own, targeted to our Mid-Atlantic region, but hopefully also useful to gardeners everywhere.

Fortuitously at this moment, my lab was approached by Kaeser Compressors, whose USA office is based also in Virginia, because they wished to commemorate the 100th anniversary of their parent company Kaeser Kompressoren of Coburg, Germany with an environmentally-focused project benefitting the communities they serve. With Kaeser’s involvement in the food processing industry and recognizing the pollinator crisis’ broader impact on the world’s food supply, it seemed a great fit. Plus, it didn’t hurt that their corporate colors are yellow and black. With their backing and with the help of two colleagues, Drs Scoggins and O’Rourke, from another department on campus, we recruited a new graduate student, Micki Palmersheim, and started planning. We broke ground on our experimental gardens in September 2019, wrapped up the field portion of the project in August 2021 and graduated Micki a few months later, and published our research in a peer-reviewed journal in 2022 (Palmersheim et al. 2022). It has been a wonderful journey that has resulted in a tangible product that we hope will be useful to others.

In our study, we designed, planted, and monitored two experimental gardens in Blacksburg, VA, for two summers (2020–2021). One garden was located at the Urban Horticulture Center, and the other was located at the Turfgrass Research Center (Figure 1), both affiliated with Virginia Tech. With these gardens, we investigated the effects of 25 ornamental landscape plants, readily available in nurseries, on flower-visiting insects. Plants were either native or non-native to the mid-Atlantic of the USA, and they were either perennial or annual. We surveyed the number and taxonomy of insect pollinators through non-destructive sampling and analyzed the data to determine the plants that brought an abundance and diversity of insects to gardens. Our goal was to provide everyday gardeners with data-driven recommendations to help them create customizable, pollinator-friendly gardens.

Over the two summers, we observed 26,804 insects, a majority of whom were from the insect order Hymenoptera, mostly bees (Figures 2 & 3), but also some wasps. We also saw butterflies and moths (Lepidoptera, Figure 4), flies (Diptera), beetles (Coleoptera), and some grasshoppers and crickets (Orthoptera). We could not identify some insect visitors with our methodology, but these were a small proportion of our data (Palmersheim et al. 2022).

What did we find? First and foremost – plant choice matters on both the abundance (Figure 5) and diversity (Figure 6) of garden insect pollinators. Our most visited plant in terms of numbers – the black-eyed Susan – had a 42-fold higher abundance than our least visited, the petunias. Actually, our top six plants in terms of insect abundance (black-eyed Susan, brown-eyed Susan, and Joe-Pye weed, purple coneflower (Figure 2), Helen’s flower (Figure 3), and sedum) all received on average more than 90 visitors per plant plot (Figure 5), with the top 3 as true prize winners, with a whopping 200+ visitors per plot across a year (Palmersheim et al. 2022).

And what about diversity – did the same plants that brought high abundance also bring high diversity? There was some overlap (Figure 6), as the top 6 plants were purple coneflower, zinnia, Helen’s flower, yarrow, catmint, and dwarf goldenrod. We looked at this more formally, and we found that the plants with a middle score of visitor abundance tended to have the highest score of visitor diversity (Palmersheim et al. 2022). In other words, the plants with very low or very high abundance had the lowest diversity. We do not know the exact reason why this might be, but one possibility could be that if a plant was particularly good, especially in terms of collection efficiency, for one type of insect, that insect might overwhelm (abundance) and deplete the resources, discouraging different (diversity) visitors to then come.

Did plant lifespan (whether is was annual or perennial) or plant nativeness (whether it was native or non-native) impact the abundance and diversity of visitors? In terms of abundance, there was a clear answer: native perennials outperformed non-native perennials and non-native annuals. For diversity, perennials, both native and non-native, did better than the non-native annuals (Palmersheim et al. 2022). These results are not too surprising, as previous studies have generally characterized perennials as better than annuals because they have many years to accumulate resources for feeding pollinators (Parrish & Bazzaz 1979; Fussell & Corbet 1992).

One thing important to remember is we are not saying that annuals are without garden value. Many (although not all) of them possess little to no foraging resources for insect pollinators (Strzałkowska-Abramek 2019); however, they are some of the easiest flowers to grow (Weisenhorn 2021) and provide long-term color aesthetics, providing beauty to a garden. For the wildlife-conscious gardener, their addition can be balanced against the inclusion of other, more rewarding plants that do provide foraging resources for the hungry insect pollinators.

Our study was necessarily limited to our selection of ornamental plants, which of course is a mere slice of what is potentially available to gardeners, and we would expect different results with different plants, cultivars, or maybe some variation across locations and seasons. However, our results do clearly show that choice matters, and the inclusion of some of these easy-to-grow, easy-to-find plants might bring a big boost to the buzz factor of your garden. In fact, as part of our original research article, we included tables with our insect counts and plant cultivar information. This will allow for plant selection that targets specific insect taxa, allowing for customization in a way that was missing from other plant recommendation lists. The article is open access and therefore freely available to anyone that wants to take a look.

So if your thumb isn’t as green as you would like, but you still wish to create a beautiful garden for both you and insect pollinators, let these plants be your guide. You can optimize for flower-visiting insects in a data-driven manner to create a splash of color that also helps to feed the hungry bees.

References

Fussell, M. & Corbet, S.A. (1992) Flower usage by bumble-bees: a basis for forage plant management. Journal of Applied Ecology, 451-465.

Garbuzov, M. & Ratnieks, F.L. (2014a) Listmania: the strengths and weaknesses of lists of garden plants to help pollinators. BioScience, 64, 1019-1026.

Garbuzov, M. & Ratnieks, F.L. (2014b) Quantifying variation among garden plants in attractiveness to bees and other flower‐visiting insects. Functional Ecology, 28, 364-374.

Goulson, D., Nicholls, E., Botías, C. & Rotheray, E.L. (2015) Bee declines driven by combined stress from parasites, pesticides, and lack of flowers. Science, 347, 1255957.

Hallmann, C.A., Sorg, M., Jongejans, E., Siepel, H., Hofland, N., Schwan, H., Stenmans, W., Müller, A., Sumser, H., Hörren, T., Goulson, D. & de Kroon, H. (2017) More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS ONE, 12, e0185809.

Klein, A.-M., Vaissière, B.E., Cane, J.H., Steffan-Dewenter, I., Cunningham, S.A., Kremen, C. & Tscharntke, T. (2007) Importance of pollinators in changing landscapes for world crops. Proc Biol Sci, 274, 303-313.

Ollerton, J. (2021) Pollinators and Pollination: Nature and Society. Pelagic Publishing Ltd, Exeter, United Kingdom.

Ollerton, J., Winfree, R. & Tarrant, S. (2011) How many flowering plants are pollinated by animals? Oikos, 120, 321-326.

Palmersheim, M.C., Schürch, R., O’Rourke, M.E., Slezak, J. & Couvillon, M.J. (2022) If You Grow It, They Will Come: Ornamental Plants Impact the Abundance and Diversity of Pollinators and Other Flower-Visiting Insects in Gardens. Horticulturae, 8, 1068.

Parrish, J. & Bazzaz, F. (1979) Difference in pollination niche relationships in early and late successional plant communities. Ecology, 60, 597-610.

Sánchez-Bayo, F. & Wyckhuys, K.A.G. (2019) Worldwide decline of the entomofauna: A review of its drivers. Biological Conservation, 232, 8-27.

Strzałkowska-Abramek, M. (2019) Nectar and pollen production in ornamental cultivars of Prunus serrulata (Rosaceae). Folia Horticulturae, 31, 205-212.

Weisenhorn, J. (2021) Annual flowers that attract pollinators. University of Minnesota Extension.

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