Lyme is not decreasing, despite our understanding major factors of transmission since 1981. A study by University of Connecticut has shown a link between thickets of nonnative invasive Berberis and increased risk of Lyme disease. Scientists in Maine find that various nonnative invasive species create a thick underbrush loaded with infected mice. Gardeners should know about these recent findings concerning Japanese barberry and other nonnative invasive plants plaguing northeastern forests.
Japanese barberry (Berberis thunbergii) was brought to America as early as 1875 for landscape use. The species is naturally tidy in appearance, deer resistant, and tough as nails. Japanese barberry is now offered in dozens of cultivars, in a range of foliage colors and overall plant shapes. This shrub is practically universal in home and commercial landscaping in the eastern half of the US.
But Japanese barberry didn't stay in the garden. It escaped from New England gardens within 35 years. Birds carried barberry seeds from gardens into nearby forests. Barberry now grows in thick stands in northeastern US and eastern Canadian woodlands, and many states officially list this plant as invasive. Some states prohibit the sale of Berberis thunbergii. (There is a native American barberry, but it is far less common and naturally inhabits a more southern region. For the rest of this article, when I say barberry, I mean B. thunbergii.)
Connecticut finds itself in prime invasive barberry territory. Connecticut is also famous as the "origin" of Lyme disease. More accurately, Lyme, Connecticut is where a previously unrecognized disease was finally attributed to a bacterium carried by blacklegged ticks.
The blacklegged tick (Ixodes scapularis) has been nicknamed "deer tick" in recent decades. Adult blacklegged ticks do feed on deer, as do some other adult ticks. But young deer ticks prey on small mammals and birds, as do the young of most other tick species found in North America. The bacterium that causes Lyme disease was identified in 1981, and young deer ticks carry the bacterium and transmit it. Deer ticks carry other diseases as well, and other ticks also spread diseases that affect humans and pets.
Confirmed cases of Lyme disease in the US have ranged from 20,000 to 30,000 for over a decade. It is the "most commonly reported vectorborne illness," according to the CDC. New Hampshire had the highest rate of infection last year, with neighboring states close behind. Many studies look for the most effective means of control of deer or deer ticks in order to reduce Lyme infections. Scientists in Maine found a correlation between thick growths of various invasive plants (bittersweet, barberry, buckthorn, etc. ) and higher numbers of ticks carrying Lyme disease. Connecticut researchers studied barberry in particular and found it created a moist habitat that fostered both mice and deer ticks. It now seems clear that rodents are a major factor in deer tick populations. And the story of Lyme disease is really a complex tale involving forest fragmentation, invasive plant species, shifts in populations of deer, mice, and their predators, and suburban development that integrates natural areas. We love to live close to nature, but Nature doesn't always play nice.
Where do we go from here?
It's become clear that the increase in Lyme disease is a complex problem. The Connecticut study sucessfully demonstrated that removal of wild barberry stands greatly reduced the level of Lyme-carrying ticks. The Maine project mentioned above implies similar results with the elimination of other invasive plants that clog forest edges in northeastern US. Overpopulation of deer is linked to increased Lyme disease, but an unseen overabundance of mice and the presence of other small rodents and birds are important factors in the tick disease scenario. Forest fragmentation eliminates the predators that keep small rodents under control.
Gardeners can play a part in the effort to reduce Lyme disease. Home gardeners can choose native plants over nonnatives, especially those known as invasive. Knowledgeable gardeners can help spread awareness of nonnative invasive plants, and their role in our rapidly changing suburban ecosystem. Concerned citizens can participate in park projects aimed at controlling invasives, and feel good about reducing the health risks to park visitors.
Ticks are most abundant in the simulated "forest edges" that surround many backyards. Ticks do not travel far into lawn or open areas. General "tidyness" and good landscape care are recommended to reduce rodent, and thus tick, habitat. However, a 2009 study found no evidence that landscape alterations effectively reduced Lyme disease. Gardeners should practice Lyme disease prevention hygiene, even after a few hours spent in one's own yard. Use personal insect repellants, shower after gardening, and check for ticks thoroughly within 36 hours. Use good tick prevention hygiene after spending time in any outside setting where forest meets lawn, especially where nonnative invasive plants are found in abundance.
Image of deer tick, Photo by Scott Bauer. (USDA ARS) [Public domain], via Wikimedia Commons
Barberry image courtesy of PlantFiles
! recommend this presentation for further reading and sources:
Landscape Planning & Tick Management, A Residential Perspective
Kirby C. Stafford III, Ph.D. Vice Director, State Entomologist
The Connecticut Agricultural Experiment Station
Another lengthy description of the problem and attempts at control, though I am unable to identify exact source:
CDC webpage about Lyme disease: