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Spraying herbicides to control invasive plants has little impact on rodents

By Stephanie Fox
Medill Reports

This article first appeared in Empower magazine online.

Photos and videos in slideshow: (Stephanie Fox unless noted)

Under the overcast sky, wrapped in clothes not nearly warm enough for the misting, cold weather, graduate biology student Nick Steijn and his group of volunteers spent the bitter Friday morning trudging through the Illinois Nachusa Grasslands’ tall grass and prickly shrubs to gather data for his thesis. The data involve small, captured rodents.

After analyzing the rodents collected over the course of his two years in graduate school at Northern Illinois University, DeKalb, Steijn, 25, concludes that the specialized herbicides don’t harm the rodents but adds that more research needs to be done.

He hypothesized that using herbicides, or a combination of chemicals sprayed to destroy unwanted vegetation, at Nachusa Grasslands in Franklin Grove, Illinois, could impact small rodent populations. Instead, his research revealed evidence that herbicide use has little-to-no effect on small rodents such as mice and voles in Illinois grasslands.

“I’m not sure what I can say with confidence other than, whatever effect there is, it’s apparently not very strong,” said Steijn. “In other words, spraying an area with herbicide does not have a strong effect on the small mammals in the area.”

Steijn’s research focuses on specialized herbicides such as Transline and Crossbow, which only kill certain types of plants. One invasive plant these herbicides kill is red clover (Trifolium pratense).
“I’ve been attempting to knock back red clover,” said Steijn, who spends 12 days in a row, four times a year conducting fieldwork concentrated on the impact of the herbicides on small rodent populations at Nachusa Grasslands.

His most recent trips to the grasslands, between September 22 and October 4, marked his ninth and final fieldwork session. The rest of his time as a master’s student will consist of writing his thesis and interning with the Illinois Department of Natural Resources.

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Nick Steijn prepared to insert a Passive Integrated Transponder (PIT) tag into the loose skin behind a vole’s skull. (Stephanie Fox)

During his four 12-day cycles of field work in April, June, August and late-September/early-October, Steijn compared small mammal communities at designated 60-by-60-meter sites where red clover had either been sprayed or not sprayed with herbicides. He executed this by setting traps overnight filled with peanut butter and oats. The next morning, he and his volunteers would spend about three hours driving from site to site in a Kawasaki UTV to check the traps, identify the rodents and release them.

The location and information about the trapped rodents, such as their species, gender, length and weight were recorded. Steijn then collected hair samples to be analyzed by his thesis instructor at Northern Illinois University Holly Jones to determine what the small rodents on the prairie eat. Before releasing the rodents, he tagged them with Passive Integrated Transponder (PIT) tags—the same, grain-of-rice-sized tags implanted into pet cats and dogs. Steijn inserted tags into the loose skin behind the head of smaller rodents with an extremely sharp and frighteningly thick needle, and clipped tags onto larger rodent’s ears such as the 13-lined ground squirrel so the PIT tags could be scanned later to identify rodents that had previously been trapped, tagged and released—though, Steijn said “recaps,” or rodents caught multiple times are rare due to the number of rodents in the prairie and the size of Nachusa Grasslands.

“There’s a couple of species that we catch a lot of and a couple of species that we hardly ever catch. The super common ones are deer mice, white footed mice and prairie voles,” Steijn explained. “Some of the more uncommon species are meadow jumping mice and thirteen lined ground squirrels.”

During his final trapping, Steijn caught deer mice, voles, shrews and 13-lined lined ground squirrels.

Steijn chose to spray herbicides on red clover because of its abundance in Nachusa Grasslands.

“[Red clover is] not the most aggressive invasive plant, so here at Nachusa they don’t devote a lot of time and effort to controlling it. There are other invasive plants where if you ignore them, they will start to take over completely…Because they’re actually so good at getting control of those invasive plants here at Nachusa red clover is one of the few non-native plants that’s actually kind of abundant.”

Steijn also believed that utilizing red clover for his experiment could have national implications because of its prevalence across the country.

“It’s all over the United States, but it’s native to Europe, Western Asia and Northern Africa.” The invasive species was originally brought to the U.S. for agricultural restoration. Farmers planted red clover in their fields more than 100 years ago because it returns nitrogen to the soil. It also proved beneficial to cattle farmers who could feed the fast-growing plant to their cows.

But today, the invasive species hinders restoration efforts.

“Now when we’re trying to convert corn fields to prairies, it pops up everywhere,” Steijn said. For years, The Nature Conservancy, which maintains Nachusa Grasslands, has purchased surrounding land devoted to agriculture to reconnect and restore remnant prairie, woodlands and wetlands to their original, natural state. Nachusa Grasslands is currently one of the largest and most biologically diverse grasslands in Illinois, at about 3,600 acres, but managers hope to continue expanding the conservation efforts.

One of these efforts, is spraying herbicides. Steijn’s results can be used by The Nature Conservatory to help it determine the impact of this strategy.

“From a manager’s perspective this [inconclusive results] is a good thing because they do use a lot of herbicide out here,” said Steijn. “In this instance, with herbicides and small mammals, it’s important to know that there either is no impact or only a small one because that’s information that the managers can use in how they decide to use herbicides.”

Jones, who has conducted her own research at Nachusa Grasslands since 2012 while overseeing Steijn and other master’s student’s small rodent experiments, agreed that inconclusive results are significant.

Nick Steijn holds a deer mouse–one of the most commonly caught small rodents–during his experiment at Nachusa Grasslands. (Stephanie Fox)

“Knowing that it [Steijn’s research] is potentially inconclusive or that we potentially need more time is a critically important piece of information,” Jones said. “Just because [Steijn’s] hypothesis wasn’t supported doesn’t mean that this wasn’t a hugely successful piece. It completely was…His project was really designed to help inform managers [about] what actions they’re doing out on the prairie and how that impacts animal communities.”

The fact that Steijn found herbicides don’t have a large impact on small mammal communities is a huge benefit to managers because they use a lot of herbicides to try and control unwanted plants in restoration throughout the world.

“I’m not saying that more research doesn’t need to be done at all those places. But, at least at Nachusa, his research is showing that it’s okay” for managers to keep spraying herbicides, said Jones.

Photo at top: The invasive species red clover. (Petra Österreich)