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If you’d like to avoid catching Lyme disease from black-legged ticks that may be lurking in the backyard, you need to tread carefully around stone walls and where woods and lawn meet, according to a new study by scientists at the Connecticut Agricultural Experiment Station New Haven.
Gardens, it turns out, are not among backyard tick hotspots pinpointed in a field study of typical suburban backyards in Guilford from May to July 2022, published February 20 in the journal Environmental Entomology. Leaf-littered woodland with scattered shrubs, on the other hand, is a really bad neighborhood as far as ticks are concerned, according to authors Dr. Megan Linske and Dr. Scott Williams. Another minefield is around stone walls, which triple the density of ticks in their vicinity. Surprisingly, woodpiles do not add to tick numbers. Linske, who led the study, said she suspects the difference stems from the fact that stone walls are relatively permanent, making them virtual condominiums for rodents on which ticks feed, while wood piles are temporary, with few critters using them for shelter.
Understanding the ecology of ticks in the backyard is critical because most people catch Lyme disease near human habitation. The research, during which the scientists counted ticks collected by dragging a cloth around 19 different habitats on 42 homesteads, was contracted by the federal Centers for Disease Control. Its goal, says Linske, is to target pest control more precisely, reducing the potential harmful impact of pesticides and alerting people where it is necessary to watch one’s step. An added benefit is that focused control treatment is cheaper than wide broadcasting of chemicals. Nymphal ticks, especially, are not particularly mobile and tend to stay where they drop off hosts. If microclimatic conditions are suitable, they prosper in place. The better the aim with which pest control measures target that place, the better the results.
Connecticut’s agricultural experiment facility is arguably the nation’s premier research center for the ecology of disease-bearing ticks, so a natural for the CDC effort, which takes on increasing importance as cases of Lyme disease and other tick-borne illnesses mount. The new research fleshes out ground-breaking studies by the station scientists in the 1990s, indicating that in a typical wood-fringed yard, by far the most ticks looking for a host were in a meter-wide strip from lawn edge into the trees. The early studies did not consider how differing details of habitat within that band nor other features of the landscape, artificial as well as natural, might influence tick density.
“Earlier studies established increased risk in a one-meter band,” says Linske, “by finding where the highest risk is with than area we can fine-tune treatment based on specific features.”
Although the vegetation from lawn to woodland is somewhat stratified, horizontally, for practical purposes it is a matrix of varying vegetation types, including mown grass, thickets of invasives such as Japanese barberry and multiflora rose, trees with undergrowth and leaf litter, and hardwood with a scattered understory.
Tick density was highest in the forest at the very edge of the lawn above leaf litter but only scattered undergrowth. Mown grass immediately adjacent to the forest had a much higher density of ticks than the majority of lawns, but less than in the woodland margins. Thickets, particularly of Japanese barberry, are known to be tick heaven, because of cover and the humidity they trap near the ground; desiccation spells doom for ticks. The study, however, did not reveal high densities within this vegetation. Linske surmises that thickets thwart the method with ticks are collected, dragging a heavy cloth through vegetation, a method that may seem crude but mimics the way ticks attach to humans. Assuming tick density is higher than detected, the risk to humans remains low in the wall of vegetation formed by thickets, if only because few people like to negotiate a thorny tangle.
Tick abundance where grass meets trees is not unexpected because white-tail deer, the tick’s main host, and white-footed mice, from which they usually pick up the Lyme disease bacterium, thrive there. Edge environments where two biological communities meet, called an ecotone, are species rich because they provide a greater mix of food and cover than either on its own.
One place where the researchers found ticks was something of a shocker, 11 of them on a pea stone walkway immediately next to a home. A stone path is like the Sahara to moisture-depend ticks. So why the large number? It could be because of ticks dropping off deer taking a shortcut, speculates Linske. Whitetails simply may be using the walkway as a readymade deer trail, she says.