Silent Spring 50th Anniversary Essay Series
Investigating the use of Herbicides in an Endangered Species' Habitat
U.S. Fish and Wildlife Service
Environmental Contaminants Specialist
Rachel Carson’s research in the 1950s on the effects of pesticides to the American robin sparked awareness of and a concern for the risks of chemicals to human and wildlife health. Carson’s research led to the banning of the pesticide DDT and to the Environmental Protection Agency‘s review and regulation of all pesticides. Although regulated, chemicals are widely used in the environment and there is evidence that some chemicals used today can cause a health risk to wildlife, something Carson warned us about decades ago. The U.S. Fish and Wildlife Service maintains Carson’s legacy of due diligence and continues investigations on the effects of chemicals on wildlife today.
The Hine’s emerald dragonfly (Somatochlora hineana) is the only federally-listed dragonfly protected by the Endangered Species Act. Most of its range is within the Great Lakes Basin, with the largest population found in Door County, Wisconsin. This species requires high quality wetlands fed by calcareous groundwater seeps underlain by dolomite bedrock, a habitat that is critical to the species’ survival. The dragonfly larvae spend 4 to 5 years as aquatic invertebrates, undergoing multiple molts until they crawl onto nearby vegetation and molt one last time into the adult dragonfly. During times of low surface water in mid-summer and fall, the aquatic larvae use crayfish burrows as safe havens, retreating into them to survive. The dragonfly’s extended development time and relationship with crayfish makes the Hine’s unique among other dragonfly species. The Hine’s may be susceptible to the effects from chemicals because their exposure to them in wetlands occurs over a long time.
In most areas where the Hine’s is found, invasive plants such as reed canary grass, buckthorn and common reed are out-competing native vegetation and changing the natural hydrology of those wetlands. Land managers have a pressing need to control invasive plants to preserve native wetland communities in Hine’s habitat and are asking the Service for approval to use herbicides. While controlling invasive plants is recommended in the Services’ Hine’s Emerald Dragonfly Recovery Plan, herbicides could harm the very species the Service is trying to conserve.
Herbicides are composed of an active ingredient and a number of other chemicals which are identified merely as “others” or "inerts” on the product label, although those chemicals usually constitute more than 50 percent of the product. Also, users often add surfactants, chemicals that help the herbicide absorb into the plants, to improve their effectiveness. There is limited information on the toxicity of the “inert” chemicals and surfactants to federally-protected wildlife like migratory birds and endangered species. Most EPA mandatory studies are conducted in the lab, not in the field, using test species that may not represent federally-protected wildlife. Additionally, the lab studies typically look at the toxicity of the active ingredient alone and do not test for toxicity of the product as it is actually used, combined with the “inert” chemicals or with surfactants. Depending on the composition and interaction of the chemicals, their modes-of-action, differences in species' sensitivities to the chemicals, and the concentrations of the chemicals used, wildlife can be harmed by herbicides even if the herbicides do not directly kill them. For example, some herbicides were found to reduce the population of Behr's metalmark butterfly by reducing the number of caterpillars that emerged into adults (Stark et al. 2012).
The Service is working with the Environmental Protection Agency and the University of South Dakota to create a decision-making model for the Hine’s that weighs the risks of chemical exposure to various herbicides and application practices. The Service is also working with research scientists from the University of South Dakota and The Nature Conservancy to conduct lab toxicity tests; field research; controlled studies in a natural environment (i.e., mesocosm studies); and research to assess the effects of herbicides on captive-reared Hine’s larvae, crayfish and their food sources (other invertebrates). These studies will help ensure the recovery of this dragonfly’s populations by identifying herbicides that will control invasive plant species but avoid or minimize harm to the ecosystem.
This research will help land managers determine which herbicides and surfactants are the safest to use; it will also help us understand how our management actions impact Hine’s populations and their ecosystems. The recovery of the endangered Hine’s emerald dragonfly is dependent on using the best management practices available that avoid harm to the species, yet maintain its habitat. This collaborative research project allows the Service to remain vigilant to the effects that chemicals can have on wildlife, a passion of Rachel Carson’s and a mission of the Service.
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1Stark, J.D., X.D. Chen, and C.S. Johnson. 2012. Effects of herbicides on Behr’s metalmark butterfly, a surrogate species for the endangered butterfly, Lange’s metalmark. Environmental Pollution 164 (2012) 24-27