Land Management for Mitigation
By Tim King
Maine is among a cluster of eastern seaboard states, trailing south to Virginia, that are considered “high incidence” Lyme disease states according to the Center for Disease Control’s measurement of confirmed cases. For 2017 to 2019, New Hampshire, which is included in the cluster, averaged 73.6 cases of Lyme disease per 100,000 people, and Vermont had an average of 90.8. Among these high incidence states — and all states in the United States — Maine had the highest incidence of the tick-borne disease for the three-year window: an average of 103.6 cases per 100,000 people.
Recognizing Lyme disease as a growing problem, in 2008, the Maine Legislature passed a law requiring the Maine Center for Disease Control and Prevention to annually report to the Legislature on the state of the disease in Maine. In March of 2021 the agency reported that in 2020 cases declined to 1,118 total cases, or 83.2 cases per 100,000. Maine CDC speculated that the decline was caused in part by people’s reluctance to visit doctors during the COVID-19 pandemic and in part by dry weather. The total reported for 2021, as of March 2022, was higher: 1,508 cases. (The full report on 2021’s cases was delayed due to COVID, with an anticipated release in the summer of 2022.)
In the mid-1980s, Lyme disease was concentrated in Southern Maine. In the last decade or two it has moved into the Midcoast.
“Twenty-eight percent of reported cases were from the southern counties of Cumberland and York, and twenty-seven percent of reported cases were from the Midcoast counties of Knox, Lincoln, Sagadahoc, and Waldo,” Maine CDC reported to legislators regarding 2020 cases.
Maine CDC is tasked with tracking the incidence of six other tick-borne diseases, including five that are currently in the state. Their report of cases for 2020 enumerated 468 cases of non-Lyme tick-borne diseases including 385 cases of anaplasmosis, an infection caused by the bacterium Anaplasma phagocytophilum which can also be transmitted by the blacklegged deer tick (Ixodes scapularis) known to transmit the bacterium Borrelia burgdorferi commonly responsible for Lyme.
Early signs and symptoms of anaplasmosis can be similar to that of Lyme, including fever, headache, fatigue and body aches.
The CDC reported that 2020 cases of anaplasmosis had declined from 685 in 2019 for the same reasons that reported cases of Lyme disease may have declined from 2019 to 2020.
Still, the disease has been increasing dramatically in the last decade, according to the federal CDC. The agency reports that eight states (New York, Massachusetts, Maine, Vermont, Minnesota, Wisconsin, New Hampshire and Connecticut) account for nearly nine in ten of all reported anaplasmosis cases.
“In order for a deer tick to be infected with either of the bacteria that cause anaplasmosis or Lyme disease, the [tick] larvae or nymph need to feed on an infected wildlife host,” said Megan Porter, infectious disease health educator with Maine CDC.
She added, “Once a host animal, such as a white-footed mouse, is infected by the bacteria that causes Lyme disease they are likely to be infected for the rest of their life and can infect other ticks with the bacteria.”
Anaplasmosis bacteria are not as persistent. “For anaplasmosis, the animals that get infected are only able to pass the bacteria on to a tick for a limited time – maybe weeks or a month,” Porter continued. “This means that there are fewer opportunities for juvenile ticks to become infected, so there end up being fewer ticks infected with anaplasmosis than with Lyme disease. I think this may be the major reason why we see fewer cases of anaplasmosis than Lyme disease.”
Proper habitat is critical to I. scapularis surviving and thriving. And Maine, which is around 90% forest, has more prime tick habitat than just about anywhere in the country. Not surprisingly, habitat that is good for ticks is also good for its hosts.
“The deer tick’s spread is possible because of … several factors,” Porter said. “First, Maine has an abundant deer population, which are the key host for the adult stage of the deer tick. Maine is covered in forests, which is the perfect place for the tick to live. Maine has also been experiencing warmer winters and relatively warmer and more humid summers. That means that ticks are better able to survive the winter and find good hosts to feed on in the spring and summer.”
Studies have been done by forestry researchers to see if land management on a moderately large scale can affect tick populations. Dr. Allison Gardner, a medical entomologist and an assistant professor in the University of Maine’s school of biology and ecology, is the principal investigator at the University’s Vector Biology Lab which studies landscape management to reduce vector-borne disease transmission.
“Your personal tick protection strategies don’t reduce the quantity of ticks in the environment, but rather only your likelihood of being bitten. We think through land management there’s potential to reduce the quantity of ticks at larger spatial scales,” said Gardner.
In 2019 Gardner and graduate student Christine Conte gathered data from privately held mixed hardwood forests in Southern Maine. They wanted to compare recently harvested plots to plots that had been harvested more than 20 years earlier to see if there was a difference in tick densities.
The recently harvested plots had about a 40% percent forested canopy and the control plots (those harvested 20-plus years prior) had roughly a 90% canopy, Gardner said.
They also wanted to investigate if either of the plot types had a higher number of ticks infected with Lyme or other pathogens. To do so, the researchers used trail cameras to photograph large mammals visiting the plots. They also trapped small mammals, conducted tick counts and vegetation surveys, and collected ticks not on hosts.
The results? The tick infection rate, at about 20%, was the same in both of the plots. However, the recently harvested forest plots hosted fewer small mammals and tick nymphs, and also had reduced densities of adult ticks, compared to the control plots. Temperatures were higher and humidity was lower in the recently harvested parcels: Conte and Gardner speculated these conditions may not be congenial to ticks and their mammalian hosts.
“Our team has led a citizen science project the past few years where we’ve also seen this pattern,” Gardner said. “Any forests harvested in the last 20 years are less likely to have ticks than forests that were harvested 20-plus years ago.”
Other recent studies, led by University of Maine research associate Elissa Ballman and graduate student Stephanie Hurd, have suggested that the percent of the forest canopy may not be as important for good tick habitat as a brushy understory.
Gardner says that forest fragmentation also has an impact on tick populations. “One of the strongest associations that’s been established for several decades is that highly fragmented forests with small patch sizes tend to have a higher density of infected ticks compared to more contiguous forests,” she said. “This is because you have more deer in those fragmented forest environments because they like to browse the edges. Also, smaller patches can’t support natural predators of mice, which tend to have large home ranges. So you often have a larger mouse population in those fragmented habitats because of predation relief.”
Gardner points out that the highest level of tick disease, as well as the highest levels of forest fragmentation, are in southern and coastal Maine. That part of Maine also has a high concentration of private woodland owners.
“We have many small forest landowners in the southern part of the state where there is a high level of tick-borne diseases,” she said. “That area is 80% private land ownership, so we have landowners making decisions about how to manage their land that could have implications for tick-borne disease.”
Gardner’s work is focused on the effects logging may have on ticks and tick-borne disease and may not translate into how to manage residential lots. But there are several lessons from Gardner’s, and other researchers’, work that may be applicable for homeowners.
Acorn management is one example.
“Scientists in New York have seen a relationship between oak mast and tick-borne disease,” Gardner said. “The reason is because in years when there is mast you’ll get a lot of mice in the area eating the acorns. The mice are important because they are the principal reservoir host for pathogens like Lyme disease. You’ll also get a lot of deer in the area as well and they will bring in adult ticks that are ready to lay eggs.”
Ticks, mice, deer and acorns are all part of a complex web of life that can bring infection to humans.
“During the year after the mast you’ll start to see high tick densities and high mouse populations because of all the acorns the previous year,” Gardner continued. “The eggs will have hatched and the larvae will encounter the infected mice. In the following year or two after the mast, when nymph populations are especially high, you’ll end up seeing a lot of Lyme disease transmission among wildlife and into humans compared to previous years.”
The mitigation strategy for residential lot owners may be simply to keep acorns raked up as they fall, thereby discouraging rodents and deer.
Prompt raking of acorns is not one of the recommendations listed for controlling ticks on the website operated by the University of Maine’s Tick Lab in Orono. The lab does, however, recommend not planting invasive shrubbery such as Japanese barberry and honeysuckle. Research at the University of Connecticut has indicated that controlling, or eliminating, Japanese barberry reduces tick densities.
“Blacklegged ticks tend to lose moisture fairly quickly and thus need to remain in habitats with relatively high humidity in order to survive,” Griffin Dill, an integrated pest management (IPM) professional and the Tick Lab coordinator for the University of Maine Cooperative Extension, said. “Open areas, like mowed lawns, and areas exposed to direct sunlight maintain lower humidity levels and thus tend to harbor fewer ticks. Invasive plant species, like Japanese barberry, can quickly take over the understory and create dense thickets. Within these thickets, high humidity levels are maintained and they provide cover for both ticks and their small mammal hosts. As there is no single solution to controlling ticks, we recommend combining multiple strategies to reduce tick populations around residential properties.”
Some of the Tick Lab recommendations are:
- Keep lawns mowed to 3 inches or less and leaves raked to reduce humidity at ground level.
- Clear leaf litter, tall grasses and brush from around homes, stonewalls, wood piles, and the edges of lawns.
- Discourage rodent activity by removing woodpiles or stacking wood neatly in dry areas away from the home.
The Tick Lab also suggests using the naturally occurring fungus Metarhizium anisopliae which is registered in Maine for tick control. M. anisopliae, however, is a broad-spectrum biological control that has been observed to infect over 200 insect species.
Susan Engel, of Eldredge Lumber and Hardware in York, Maine, recommends a Texas red cedar oil spray to customers looking for natural products to control ticks. Red cedar oil is an essential oil derived from Juniperus mexicana that can be sprayed. It is a fairly broad-spectrum contact insecticide that will kill ticks, fleas, mosquitos, flies and wasps. It can also be used as a repellent by applying to clothing.
Engel also suggests creating a wood chip perimeter around the areas of high activity on residential lots. The perimeter, which can serve as a walkway, may deter ticks especially if it’s partly created from red cedar bark or pellets.
Eldredge Lumber and Hardware is certified by NOFA Organic Land Care, an organization that developed standards in 2001 to present a vision of how organic agricultural principles can be applied to the landscaping profession. The standards do not address ticks specifically. They do, however, recommend mowing your lawn less often, thereby making the soil more absorbent and also allowing grass to go to seed. They also recommend substantially reducing yard sizes. Without intending to, the standards seem to highlight a tension that exists between organic and climate-friendly land care and tick control.
About the author: Tim King is a produce and sheep farmer, a journalist and cofounder of a bilingual community newspaper. He lives near Long Prairie, Minnesota.