Strips of native prairie planted within agricultural monocrops are not an “ecological trap” for native pollinators, but also do not reduce the runoff of insecticides that may pose a threat.
by Mary C Linabury, graduate writing fellow and PhD candidate at Colorado State University
Prairie strips: a refuge in an agricultural desert or an ecological trap for native pollinators?
Agriculture is essential, but the lack of diversity in farmland can create ecological barrens. “Prairie strips” are long, thin strips of tallgrass prairie vegetation planted adjacent to agriculture that have potential to bring ecosystem services into agricultural spaces. By planting native prairie zones in line with monocrops, farmers can limit the negative impacts of agricultural practices and gain the services provided by native prairies, such as increased pollination, enhanced microbial diversity, and containment of nutrient runoff.
Despite their host of benefits, there is concern that these prairie corridors may, in fact, be exposing native pollinators to harmful insecticides. Prairie strips are planted in systems that commonly utilize neonicotinoid insecticides, or “neonics.” Only a small percentage of neonics end up in the target plant tissue and instead infiltrate into the surrounding ecosystem. “There are many different ‘compartments’ of the landscape that [neonics] migrate into once they’re applied,” Prairie strip researcher, Dr. Corinn Rutkoski (Image 1), highlights. Possible compartments for these neonics include deep soil layers within the prairie strips or within the prairie plants themselves. Run-off insecticides may be sequestered in plant tissues where pollinators could be exposed, but it is unknown if prairie strips accumulate neonics in concentrations lethal to insects.
Credit: Gabriel De La Rosa, CC BY-SA 4.0.
In a recent paper published in Agriculture, Ecosystems & Environment, Dr. Corinn Rutkoski and colleagues sought to determine if prairie strips are a haven or a trap for native insects while investigating how prairie strips modify the movement of neonic insecticides through a maize crop system. They found that insecticides that accumulate within prairie strips are well below the threshold that harms native pollinators, but also do not stop the runoff of insecticides. Corinn summarizes: “Prairie strips are not a silver bullet for farm ecosystem services. There are multiple benefits and drawbacks to consider.”
Tracing the movement of neonic insecticides through collaboration
Credit: Corinne Rutkoski, used with permission.
Prairie strips have been widely studied at the flat Kellogg Biological Station LTER, but Corinn wanted to understand how topography influenced insecticide runoff, so she brought her knowledge to the sloping farm fields of Iowa to collaborate with the Iowa State University prairie strips research site (Image 2).
Prairie strips may absorb insecticides through multiple mechanisms. For one, the deep root system of prairie strips may increase leaching of the neonics into deep soil layers and into groundwater. To test this, Corinn and colleagues collected soil samples and groundwater to understand how the neonic insecticides moved and accumulated through compartments within the system (Image 3).
Prairie strips may also absorb insecticides by accumulating neonics within prairie plant tissue, so Corinn collected and analyzed tissue from two representative plant species: the black-eyed susan (Rudbeckia hirta) and the common milkweed (Asclepias syriaca)(Image 4). Both species are common within prairies, but the common milkweed also serves as an essential food source for monarch butterfly larvae, making the role of milkweed of special conservation interest.
Prairie strip plants do not pose a lethal threat for insects, but don’t inhibit insecticide runoff
One major expectation of the study was that prairie strips would act as catchment areas to reduce the runoff of insecticides. Surprisingly, this was not the case. Prairie strips did not reduce insecticide runoff to downslope areas, as demonstrated by high neonic concentrations in soil at the base of farm fields. Corinn concludes, “…the down slope movement of the neonics is happening regardless of the presence of prairie strips.” Previous studies have shown that prairie strips have the potential to reduce neonic runoff, though Corinn suggests that their findings may be confounded with another feature on the agricultural landscape: “grass waterways, meant to prevent erosion, may have acted as a waterslide for insecticides.”
Credit: KBS LTER, CC BY-SA 4.0.
Second, Corinn and her colleagues expected that prairie plants would take up neonic insecticides. Luckily, neonics were only found in two of the plant tissue samples, and at levels well below the lethal dose for insects. Corinn explains, “I expected that prairie plants were going to be sponges for the insecticides applied to surrounding crops, and that we would see insecticides spike in plants. I was relieved that we didn’t…it means that the prairie strips were a relatively safe habitat for visiting pollinators.”
These results are promising, but don’t necessarily mean prairie strips should be considered “ecological safe havens,” as they do accumulate some level of neonics. Even at low doses, neonics can harm pollinators through repeated exposure.
Prairie strips seem safe for visiting insects, but long-term studies are needed
This research is an important “snapshot” of the prairie strip-agricultural system, but Corinn underscores the need for longer-term assessment of neonic insecticides within prairie strips and a broader assessment of focal prairie species. Corinn stated that there may still be, “…negative consequences from insects being exposed to the neonics at sublethal doses repeatedly. So even if the insects aren’t coming into contact with a high concentration of the insecticide, if they’re continually returning to flowers with a low dose of the neonics, there can still be negative consequences for their reproduction or their ability to forage.”
Credit: KBS LTER, CC BY-SA 4.0.
She also highlights that a longer-term study would allow us to understand the role of prairie strips across years with variable precipitation. The study occurred during a low rainfall year which may have altered the way in which the neonic insecticides moved through the system. A longer-term study that encompassed years with variable rainfall would allow for a stronger conclusion on the role of prairie strips in agricultural systems.
Prairie strip research remains a focus of the Kellogg Biological Station LTER site, so these questions may soon be answered. For those interested in prairie strip research, Corinn provides an enticing description of her experience, “I think prairie strips are a really beautiful compromise between working lands that provide food, fuel, fiber that we need and the habitat that we need to restore. It’s such a wonderful system. I feel lucky to have worked in it.”
The two focal species used to assess insecticide content in prairie strip plant tissue: black-eyed susan (Rudbeckia hirta, left) and common milkweed (Asclepias syriaca, right). Credit: KBS LTER
Research will continue, but native pollinators need habitat now
Prairie strips will likely serve an important role in the future of sustainable farming, but they are not a panacea for rogue insecticides and the decline of native pollinator habitat. Corinn promotes the hard work of the Xerces Society of Invertebrate Conservation. Their website’s “Pollinator Conservation Resource Center” provides resources to aid in the creation and maintenance of pollinator habitat with region-specific advice.Find those resources and more through this link: Pollinator Conservation Resource Center | Xerces Society
