Fine-Scale Mapping Could Make for Better-Targeted Tick Management
By Ed Ricciuti
Geographic information systems mapping has confirmed what was already known to suburban parents who bundle up offspring in homemade hazmat suits before releasing them into backyard wilds: The ‘burbs are thick with ticks that spread Lyme disease and other illnesses. A new technique using GIS mapping of landscapes that promote density of disease-bearing ticks confirms that they can thrive—and thus risk of exposure to them is high—in wooded spots with the patchwork of trees and grass typical of suburbs. Importantly, this knowledge can be used to improve tick control in those very neighborhoods.
“We provide a blueprint for mapping predicted tick density at a scale which is operational for tick control,” says David Allen, Ph.D., assistant professor of biology at Middlebury College, in Vermont. Allen and his colleagues published their study of GIS mapping of tick exposure risk in October in the Journal of Medical Entomology. Some of the most important tick hosts, notably the white-tailed deer, “thrive in fragmented, human-dominated landscapes where small forest patches are surrounded by other land cover types,” say the researchers. And that landscape is the type created by suburbs, especially at the interface of habitats where lawn meets trees, where ticks are especially abundant.
The new method, says Allen, may give worried suburban residents—not to mention neighborhood associations, public health authorities, and pest control contractors—a break. It allows targeting of “real-world” control strategies on smaller areas where people and ticks are cheek by chelicerae, making it easier to pinpoint existing control measures that are otherwise unfeasible because they are not cost effective on a per-acre basis. Typically tick control is done by blanketing large areas with acaricides, as tick-killing chemicals are called, or targeting them at hosts such as deer and white-footed mice. And it’s not a one-shot deal but rather must be done repeatedly, thus making it pricey and time consuming.
A word about geographic information systems is in order. As explained by the United States Geological Survey, GIS is a computer system that analyzes and displays geographically referenced information, using data attached to a unique location. If, for example, a rare plant is observed in three different places, GIS analysis might show that the plants are all on north-facing slopes that are above an elevation of 1,000 feet and that get more than 10 inches of rain per year. GIS maps can then display all locations in the area that have similar conditions, so researchers know where to look for more of the rare plants.
The method for mapping predicted tick exposure risk has a fine resolution, of 200-by-200 meters, appropriate for public health intervention and, it turns out, appropriate for use on small areas suited to tick control measures. Previous studies have used more coarse resolution over large areas. “The resolution of our map, 200 meters by 200 meters, is an area over which it is feasible to apply the tick control methods,” says Allen. The buffer, or area, that performed best in the study spanned 100 meters.
Field research for modeling the method was carried out in wooded habitat of western Vermont with various stages of forest fragmentation, elevation, and alteration by human activity. Samples of blacklegged ticks (Ixodes scapularis) were collected biweekly during June and the first half of July, when tick nymphs, main culprits when it comes to human infection, are most active and people are recreating outside. They determined the density of nymphs as a measure of disease exposure risk. To construct the method, researchers took into account a plethora of environmental variables that impact ticks and their hosts, such as leaf litter, canopy coverage, and soil moisture. The model was applied and tested by mapping predicted tick density across Addison County, Vermont.
The research points to some tantalizing ecological relationships that suggest there is more to factors contributing to tick density than meets the eye. “We wanted to understand what habitat, climate, land cover, and physical characteristics best explained tick density” says Allen. “Tick density should be driven by the habitat needs of not only the tick itself but also of the tick’s hosts. We thought that tick density might be influenced by these predictor variables in a large area surrounding the actual tick collection location, because some tick hosts, like deer, have large home-range sizes.”
It turned out, however, that as buffer size was increased, the ability of the model to explain tick density decreased. This relationship suggests that ticks concentrate in areas of closed cover, which retain moisture, and thick leaf litter, within the larger context of fragmented landscapes.
Commenting on the study in which he was not involved, Scott C. Williams, Ph.D., of the Connecticut Agricultural Experiment Station says it aligns with studies he and his colleagues have done on high humidity under plants such as Japanese barberry that foster tick density. When both forest canopy and understory are intact, conditions are even more ideal for ticks, he notes.
There is a seeming paradox here. White-tailed deer carry ticks but, where deer are numerous, they thin the understory that ticks love. Interestingly, when Allen and his associates expanded the buffering zone to 2,000 meters, attuned to the larger home ranges of tick hosts such as deer, the method’s ability to explain tick density decreased.
“We think that tick density is largely driven by the tick’s own habitat needs and that of small-mammal hosts with relatively small home-range sizes, such as mice and chipmunks, rather than that of larger hosts with larger home range sizes, such as deer,” says Allen.
Journal of Medical Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.