Study Finds Invasive Honeysuckle Harbors Disease-Transmitting Ticks

Invasive plants have long been cast as exotic villains of a community, snuffing out native resident plants and establishing roots, so to speak, that are difficult to eradicate. Recently scientists in the Midwest have discovered that one such nuisance plant—invasive honeysuckle—also increases the risk of tick-borne diseases.

This past month, an article by scientists at Washington University in St. Louis and the University of Missouri at St. Louis concluded that areas infested with Amar, or bush, honeysuckle had greater populations of the lone star tick than areas populated with native plants. But the honeysuckle is not attracting the ticks. It’s attracting the deer that bring the ticks.

Lonestar tick, left-male, right, female

The lone star tick (Amblyomma americanum) is found in the southeastern and south central United States. The tick is named because of a single white splotch on its back (not because it hails from Texas). It lives in low-growing brush and feeds on vertebrates, including deer, squirrels, dogs, humans and other warm-blooded mammals. It has been found to transmit ehrlichiosis, a disease affecting the white blood cells, and southern tick-associated rash illness (STARI). Unlike other ticks, the lone star tick does not transmit Lyme disease (source: Centers for Disease Control). However, the CDC and other health organizations are concerned about the transmission of STARI, also called Masters disease. The lone star tick is the only tick species that transmits STARI.

So far scientists do not know what causes STARI; however, the disease seems to respond well to oral antibiotics.

Dr. Brian F. Allan, who led the study on honeysuckle and tick populations, began with several questions: are areas invaded by honeysuckle more likely to have high tick populations than areas with no honeysuckle? Why would ticks be more prevalent in honeysuckle-invaded areas? What about the honeysuckle is attracting the deer that bring the ticks?

To answer the first question, Allan and his colleagues set up several plots with coolers containing dry ice and laced with double-sided carpet tape. Ticks are attracted to carbon dioxide; in fact, they pursue their victims based on reading the carbon dioxide signature. Lone star ticks are very aggressive and will run several feet before attaching to their target.

He also wanted to know how many ticks carried disease pathogens, testing for Ehrlichia chaffeensis, the bacteria that causes ehrlichiosis.

When Allan and his team compared the proportion of ticks captured by the traps in honeysuckle-invaded areas versus the proportion in native vegetation areas, Allan found that the density of nymphs infected with E. chaffeensis was ten-fold higher in invaded areas. A separate survey revealed that the honeysuckle plots were five times more likely to support a white-tailed deer population than native plots. The honeysuckle plots also were 18 times denser than native plots.

Allan and his team also tested the DNA of the ticks to find out what animal species they were using as hosts. Ticks feed only once during their lifetime, so sometimes host DNA can be hard to find. However, Allan was able to trace deer DNA back to the disease-carrying ticks.

But why would deer prefer areas with non-native vegetation to those with native plants? The thick, low-growing honeysuckle provided deer with a good resting place and shelter from enemies, according to results of another experiment. To distinguish whether deer were attracted to the plant or the berries, Allan and his staff established three types of plots: one with honeysuckle plants intact, one with honeysuckle plants that had been stripped of berries, and the third with plants removed and berries sprinkled on the ground.

Allan’s paper closes on a philosophical question: if honeysuckle is capable of harboring carriers of human disease, what about other invasive species? Although hundreds of studies have pointed to ways in which invasive species alter habitats by changing the ecological cycle of the geographical area, this study is the first that has studied the role of invasive plants in human disease transmission.

Perhaps, as Allan alludes to in his last sentence, removing invasive plants could not only aid the recovery of the native ecosystem but also protect the public from disease. That would, as he states, be a “win-win” scenario for both nature and people.

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