Most sun-starved Minnesotans are happy to spend spring and summer outdoors. With every lake excursion and walk in the woods come necessary evils. Bug spray, sunscreen, protective clothing, and a daily tick check are the bare minimum for staying healthy. But with the rise of tick-borne disease in the United States showing no sign of slowing down, what if there were other land management tools designed to reduce the risks ticks pose to human health?
Chris Wojan, a graduate student in the Ecology, Evolution and Behavior program, thinks fire could help. Preliminary results from his graduate research suggest that conducting prescribed burns at least every four years could significantly reduce the number of ticks in a landscape.
“Prescribed burns can incinerate local tick populations, and they can also alter the habitat in a way that might affect tick populations more long term,” says Wojan, a tick ecologist who’s spent the last few years investigating tick populations in a long-term fire experiment at Cedar Creek Ecosystem Science Reserve (CCESR).
Prescribed, or controlled, burns are typically implemented to restore native landscapes and prevent decaying material from fueling future wildfires. What scientists still aim to better understand are the broader impacts of these burns, including reducing tick populations. Researchers at Cedar Creek have conducted controlled burns as part of a broader fire ecology experiment for more than 60 years in an effort to describe how fire, and frequency of fire, affects a landscape over time. Understanding how fire impacts landscapes is crucial to understanding the natural history of Minnesota given fire was once more common across the United States. Field sites in the experiment are burned with seven different burning frequencies – some are burned annually while others are left untouched.
Wojan studied tick populations in each of these treatments by dragging a white cotton cloth along the ground for about 100 meters and then inspecting it for specimens. “You could say I developed a trained eye for it,” he says. “It’s a weird skill to have, I suppose, but I can pick out ticks pretty easily from other little specks.”
Particularly difficult to spot is the deer tick, also known as the black-legged tick. The nymphs are the size of a pinhead and the adults are no bigger than a sesame seed. They are also Wojan’s primary subject and the transmitter of Borrelia burgdorferi, the bacteria that causes Lyme disease. Deer ticks – and Lyme disease – have probably always been prevalent in the east coast and the midwest. But it wasn’t until 1976 that the first case of Lyme was officially reported, and since, the number of ticks carrying B. burgdorferi have skyrocketed. Since then, the disease has increased more than fivefold in Minnesota. But how?
Scientists have some theories. Climate change and deforestation may play a role, for one – deer ticks like to hang out in tall, grassy areas along the edge of wooded areas (i.e. “dappled shade,” says Wojan) and emerge to feast on hosts when it’s hot and humid. Another common explanation is that a crash in the deer population from overhunting caused the deer tick population to diminish. White-tailed deer are a major source of “blood meals” for deer ticks at many stages of their life cycle. “Then, as deer populations rebounded, the ticks may have found perfect habitat in their old range of centuries past, with Lyme disease being more noticeable given the relative lack of other infectious diseases in the northern US of the 1970s and 80s,” says Wojan. Hunting regulations allowed deer populations to come back in the 20th century. “But the other thing – and maybe this is speculative – is that there was lots of fire suppression in the 1900s that may have built up a lot of really good tick habitat like leaf litter, canopy, etc.”
Fire used to be a regular part of the natural history of the United States. Without it, savanna and prairie may have grown dense and shrubby. As the tick population increased due to increased access to ideal habitat, young ticks may have emerged and attached to rodent hosts carrying Borrelia burgdorferi. As they grew into adults, they could have infected larger hosts, like deer. On and on.
Today, fire could interfere with several stages of the deer tick’s life cycle. Wojan has yet to parse the details out, but he feels fairly confident that at the very least, fire could make Minnesota habitats less appealing to the parasite. “Deer ticks are fairly susceptible to drying out,” says Wojan, “so they prefer moist habitats like leaf litter and lots of canopy cover.” Frequent fire could certainly impact those environmental factors. Fire might also play a role in what kinds of hosts visit the landscape. The dynamics of these landscapes are extremely interwoven.
Prescribed burns could have a widespread impact on the future of tick borne diseases, and not just involving deer ticks. Wood ticks and lone star ticks are other common vectors of disease that may be impacted by land management. But for now, Wojan feels preventative measures like protective clothing and daily tick checks can do wonders in keeping tick-borne disease at bay. He maintains a fairly non-biased view of ticks.
“I prefer to keep a neutral stance about nature, including ticks,” he says. “They’re just doing what they evolved to do. And humans treat their landscape pretty parasitically, too, so who am I to judge?” –Adara Taylor