The Block Island Times

Warm weather means more active, not more, deer ticks

Spring humidity is what increases tick numbers
By Judy Tierney | Mar 17, 2012

Will the warm winter we have had this year increase the population of the disease-bearing deer ticks? Common sense might lead us to say it will, but the results of just completed scientific studies reveal that when it comes to tick numbers, it’s the weather of the next few months that will matter, not the weather of the last few.

Dr. Thomas Mather, Director of the University of Rhode Island’s Center for Vector-Borne Disease, says that while the warm weather increases tick activity in winter, it does not create more of them.

It is another weather condition, the relative humidity, that affects their numbers.

“Spring ticks,” Mather said, “are just leftovers from the summer. When winters are cold, they slow down and they can’t climb a stem to host seek.” Which is why last winter your pets, and you, probably stayed tick-free during winter months.

When the winters are warm ticks are more active, climbing stems and then descending to re-hydrate, but they wear out their stored energy earlier. This winter, Mather reported, the ticks were active almost every day. Hence there was no respite from tick checks this winter.

Next up come the nymphs that hatch in late May through mid June. They might hatch a week or so earlier in a warm spring, Mather said, but the number of hatchings won’t increase based on recent warm weather.

What matters for ongoing population numbers is the number of nymphs that survive, and that’s all about the relative humidity when the nymphs hatch. Mather and a graduate student have just completed a 14-year research study using humidity monitors on mainland Rhode Island. “An 82 percent humidity rating is optimal for them,” Mather said. If the humidity drops below 82 percent for more than eight hours, the nymphs die. They like the atmosphere wet.

Block Island, Mather suggested, probably has a more welcoming environment for ticks than the mainland because of humidity from the ocean.

Humidity is also why piles of leaves and brush are good habitats for the pesky ticks. Dense vegetation lets in less sunlight and is cooler than sunny areas. The dewpoint changes and water from the air condenses there.

The warmer winter could have some effect two years down the road, but only indirectly, Mather said. In this case, we’re talking about host species, in particular deer.

If more deer survive the winter and reproduce more, then there will be more of them to become feeding sources for future ticks. A recent article in the Cornell Daily Sun addresses this phenomenon, calling it “The Jummanji effect: Warm winter disturbs ecosystem.” Cornell university researchers suggest fewer fatalities among non-hibernating animals, such as deer, can cause a population boom in the herd which could increase the tick population.

The deer ticks are, in fact, spreading further north into new habitats, which some researchers attribute to the warming climate. The Yale University research team led by Dr. Marie Diuk-Wasser, which has been counting ticks on Block Island, recently published results of their deer tick counts across the northeastern United States. The story was picked up by the Huffington Post.

Along the eastern seaboard, the Huffington Post reports, the researchers documented high concentrations of the tick from New Hampshire through Delaware. They have spread inland, with parts of New York, Vermont and Pennsylvania showing high volumes, and they are on the march northward, as well, in Maine.

Diuk-Wasser’s group also found high number of the ticks in Wisconsin and eastern Minnesota. Though lower in number, they are increasing in parts of Michigan, Ohio, Illinois and Indiana.

The ticks have crossed the Canadian border as well.

Researchers led by Patrick Leighton of the University of Montreal recently reported in the Victoria Time Colonist, “rapid growth” of the tick population in areas where they were “almost non existent in l990.” Now they can be found east of Sasketchewan in southern Quebec, southern and eastern Ontario, and southern Nova Scotia.

Reforestation and increasing deer populations along with warming temperatures are thought to be the reasons for the spread of the ticks. However, it is possible that with the warmer weather, the relative humidity in those areas is also changing.

Block Islanders concerned about the coming tick season can learn more about these arachnids and how to protect themselves from the illnesses they spread by visiting the URI Center for Vector-Borne Disease website at Once there, click on “Top ten things everyone should know about ticks these days and stay disease free.”

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