Pastures as Potential Pollinator Habitat

Story by Skye Bruce

When you think of pastures, what do you see in your mind’s eye? Perhaps a field of grasses, speckled with cows, and maybe some clover? As a graduate student in Entomology at UW-Madison, I’m interested in whether these grazed landscapes contain habitat for beneficial insects, and how grazing management methods affect insect communities. Specifically, what floral resources and host plants are in pastures? Are pollinators, like butterflies and bees, using them?

Skye surveying a cattle pasture for butterflies and flowering plants near Dodgeville, WI. Skye is a PhD candidate in Dr. Claudio Gratton’s lab at the University of Wisconsin–Madison, studying butterfly landscape ecology.

From a conservation perspective, grazed lands are an important, but understudied, opportunity to increase and improve habitat for beneficial grassland insects. In Wisconsin, eighty percent of land cover defined as “grassland” is grazed. Grasslands, by definition, require a disturbance regime to keep woody species at bay and encourage regrowth of grasses and flowering plants. On protected grasslands, this disturbance regime almost always involves prescribed fire or mowing. Both management practices have received attention from researchers interested in understanding how mowing and fire frequency affects the grassland ecosystem. It is generally agreed upon that leaving some proportion of the area as “refuge” (not burned or mowed) is a part of grassland best management practices to protect biodiversity.

Rusty-patched bumble bee, an endangered species, nectaring on thistle in a pasture (foreground). In the background, field technician for the Gratton lab, Violeta Calderon (UW-Madison ‘22), helps me survey the pasture for flowering plants and butterflies. Dairy cows graze in the pasture behind her.

Butterflies have three fundamental needs in order to survive and grow their populations: food (host plants and nectar plants), shelter (a place to become a chrysalis and/or overwinter), and non-toxic habitat (free of insecticides). Many butterfly species feed on a specific host plant as a caterpillar: think of a monarch caterpillar munching on a milkweed plant. But as an adult, a monarch butterfly needs nectar and can drink nectar from many different flowering species. So, like many butterfly species, the monarch requires a specific host plant and also other flowering species, blooming across the season. 

From an agricultural perspective, whether grazing can be done in a wildlife-friendly manner (and how) is a hotly debated topic. Some evidence suggests that low-intensity, rotational grazing is better for butterfly populations than high-intensity, continuous grazing. Although grazing continues to be a significant proportion of Wisconsin’s agriculture, we lack regional research about pollinator-friendly grazing. Graziers’ experiences differ regionally, based on their environment and even herd forage preferences. For example, I’m interested in understanding how the host plant of the monarch, milkweed, is affected by grazing. Milkweed is a native grassland forb, a common occurrence in Wisconsin pastures. Some cattle eat milkweed, while others preferentially forage on other pasture vegetation and avoid milkweed. 

Common milkweed (foreground), as its name suggests, is commonly found in grasslands throughout the state, including pastures. It is the host plant for the monarch butterfly. Monarchs require milkweed for their caterpillar offspring to eat, and they also require a diversity of flowering plants as a source of nectar in their adult stage.

To better understand how grazing systems affect pollinators, Gratton lab field technician, Violeta Calderon (UW-Madison ‘22) and I surveyed 30 cattle pastures in southern Wisconsin over summers 2021 and 2022. Working with graziers, I learned about their grazing methodology: whether they were rotating cattle, and if so, how frequently. We surveyed the vegetation of each pasture to assess how many different blooming species were present, and measured the vegetation height and the amount of flowers in the pasture. And of course, we cataloged butterfly diversity and abundance on each pasture.

We observed a grand total of 2541 individual butterflies over the two summers, and we had a blast with the field research. Farmers who allowed us to survey their pastures were very welcoming to us, and one of the best parts about this research experience was learning from them about their farms. As much as our research uncovers knowledge that is new to science, we as researchers have much to learn directly from farmers, who are deeply connected to their land and herd. I learned as much or more from our conversations as I did from surveying the pastures, and for this reason I have the utmost gratitude for farmers who have supported this research.

A black swallowtail butterfly (foreground) nectars on red clover in a pasture outside Monticello, WI during one of our surveys.

While the final analyses are not yet complete, our preliminary results suggest that more butterflies overall are found on rotationally grazed land compared to continuously grazed land. However, continuously grazed pastures contain about the same butterfly diversity as rotationally grazed lands. These results will be important to land managers and graziers in our region who value pollinators and want to help their populations grow. My future analyses will focus on more overall butterfly community-level questions, as well as dive into the effects of grazing management methods on monarch butterflies, specifically.

When we began this research, we were not even sure if we would find any butterflies (or flowers) regularly on pastures. Would there be enough habitat for butterflies to be present? The data are clear: pastures are habitats for butterflies in Wisconsin, and if we can manage them in pollinator-friendly ways, pastures can be excellent sources of food and shelter to help boost butterfly population numbers.