Warmer winters are allowing lone star tick (Amblyomma americanum) populations to expand northward in the U.S., but a new analysis of historical trends argues land management and deer numbers are the primary factors. Contributing to this analysis is a fresh translation of a 1754 Swedish-language report by Finnish naturalist Pehr Kalm. The translation, conducted by Swedish entomologist Anders Lindström, Ph.D., provides clearer evidence of the lone star tick’s range at the time. Kalm’s description of the lone star tick (female shown here) notes that it can cause highly itchy swellings “that even if one is assured that it will be worse if they are touched, it is almost impossible to keep the fingers away because you want to scratch and claw them.” (Photo by Ilia Rochlin, Ph.D.)

By Leslie Mertz, Ph.D.

Leslie Mertz, Ph.D.

After essentially disappearing from the northeastern and Great Lakes states for a century, lone star ticks are not only returning, but overrunning some of their old haunts.

“I live on Long Island and we have tons of lone star ticks. They are everywhere, including our backyard, and we are exposed to them every day in the summer,” says Ilia Rochlin, Ph.D., entomologist at Rutgers University’s Center for Vector Biology and Stony Brook University Department of Microbiology and Immunology, and lead author of a sweeping new analysis, published in January in the Journal of Medical Entomology, that explains how land-management practices—and not just climate change—have driven the rise of these irritating and sometimes disease-causing pests.

To understand today’s return of the ticks, Rochlin and fellow researchers Andrea Egizi, Ph.D., of Rutgers, and Anders Lindström, Ph.D., of the National Veterinary Institute in Sweden, looked back in time. Their starting point was the mid-1700s, when European naturalist Pehr (Peter) Kalm led the first major expedition to document the flora and fauna of the New World. For the new report, Lindström conducted a fresh translation of Kalm’s 1754 Swedish-language report, which included this graphic description the lone star tick (Amblyomma americanum):

“It is found in great numbers in the forests, both in New Jersey and Pennsylvania, even more are found in Maryland and the cities situated further south. … There will commonly, where they have bitten or sucked themselves in, form a hard lump, big as a (garden) pea or bigger. These lumps often last a full 6 weeks before they disappear. After a few days in the forest, one could well have 50 to 100 such lumps that made an indescribable inconvenience. One could then almost never scratch enough. The worst was when some lumps itched and the others burned, so that you did not know where to turn.”

Following Ticks Through Time

Changes in forest management are behind today’s expansion of lone star ticks’ range into parts of New England, and climate change is helping them spread to other northern states from the Atlantic Ocean to the Great Lakes, according to a new study in the Journal of Medical Entomology. The research struck a personal chord for lead author and entomologist Ilia Rochlin, Ph.D., whose Long Island home sits in one of the heavily infested areas. (Photo courtesy of Ilia Rochlin, Ph.D.)

Why were there so many ticks back then? People helped, Rochlin says. Kalm’s report notes that lone star ticks were not a problem in the forests of the region before European colonization began in earnest, and it suggests a main reason was a change in how forests were managed. Ticks, Rochlin explains, do best in forests with a thick understory and leaf litter, along with lots of white-tailed deer, their favored host animal, but native Americans had historically kept both in check through a combination of regular controlled burns of the forest and extensive hunting.

“By the end of the 1600s and into the early 1700s, however, Native American populations had declined precipitously—in New Jersey, for example, by more than 95 percent—and they weren’t hunting deer or burning forests any longer,” Rochlin says. At the same time, colonial governments issued restrictions on forest burning so that thick undergrowth would be available for cattle grazing, and European settlers did little deer hunting compared to their native predecessors. As a result, Kalm arrived to find densely undergrown and deer-filled forests that were ideal for lone star ticks to thrive, “and that’s why he saw them all the way from New Jersey into upstate New York north of Albany,” Rochlin says.

The ticks finally met their match when the logging boom of the late 19th and early 20th century came to northern forests. Lumbermen leveled the land from the Atlantic coast to the Great Lakes, deer populations decreased with both the transition to a vast landscape of farmland and greatly increasing hunting pressure, and tick habitat disappeared. “With their habitat and main host both gone, lone star ticks retreated south into the areas where there were still forests and deer,” Rochlin says.

Back on the Upswing

Not long after the logging era, the tide began turning again. Despite high hopes for agriculture, the climate and soils in northern states weren’t as amenable to crops as hoped, so much of that land slowly began reverting to forests,” Rochlin says. “By the mid-20th century, second-growth forest had come back, and that was followed by a humongous increase in the deer population. And that was then followed by the lone star tick, because they had the habitat and the host.”

Besides the return of their hosts and habitat, changes in the forest species may be helping the ticks re-enter northern states. American chestnuts, which were the dominant forest tree in the 1700s, have now been replaced by oaks, which have leaves that decompose much more slowly and therefore contribute to more substantial leaf litter, Rochlin says. Additionally, invasive plants are now prominent in the understory, but deer often avoid eating them, so the understory is also becoming thicker. He adds, “On Long Island, for instance, the forest nowadays is full of briars, such as Japanese barberry, and so dense that you can’t walk through it, but ticks love that kind of environment.”

A timeline illustrates estimated changes in northeastern forest cover, while tailed deer density, and trends in lone star tick (Amblyomma americanum) populations. (All trends are descriptive rather than quantitative.) Arrows indicate various time periods: A) Active management of northeastern forests and deer hunting by Native Americans. B) Precipitous Native American decline in 1600s, leading to regrowth of forests and deer populations. C) Peak of the dense primary forest in 1700s. D) Rapid decline in forested land in mid-1800s due to agriculture, and later clear cutting; deer extirpated in many areas. E) Reforestation (accompanied by invasive understory) and deer herd restoration in 20th century. (Image originally published in Rochlin et al 2022, Journal of Medical Entomology)

The larva, nymph (shown here, engorged), and adult of the lone star tick (Amblyomma americanum) have piercing mouthparts that they use to draw blood from animals. A bite to a human can cause itching and burning welts, as well as other allergic reactions, and may also transmit disease-causing pathogens that can cause illness and, if left untreated, death. (Photo by Ilia Rochlin, Ph.D.)

So far, lone star ticks are causing the biggest problems in certain regions, such as Long Island and other Atlantic Coast islands, but they are starting to make their presence known from New York and Pennsylvania to Michigan. Part of the reason for the high numbers in some areas, as well as their expanding range, is the changing climate, especially in the winter, according to Rochlin. A long period of cold weather can kill ticks, but short cold snaps are not enough. As climate change ushers in more winter temperature swings, tick populations can not only soar but can also move farther and farther north.

“So, there’s good news and bad news,” Rochlin says. “The bad news is we can expect lone star ticks to show up in new areas because of climate change, and, at this point, there’s not much we can do about it, but the good news is that we know which land-management practices work. We know that controlling the deer population, getting rid of invasive species, and getting back to the more natural cycle of periodic burning of the forest all work. So, now we have to have the political will to do it.”

These are steps worth taking, Rochlin says. “Lone star ticks are a real problem, because they carry disease-causing pathogens, many people are allergic to them and react to their bites, and there’s also the nuisance factor in that people are unable to use their own backyards in the summer.” He adds, “What Kalm described 250 years ago about the lone star tick applies 100 percent to what we’re experiencing now. And it’s exactly how I feel about the ticks in my backyard today on Long Island. I definitely see the connection.”

Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.