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Moose-killing ticks thrive in shorter winters due to climate change

Moose calves across northern New England are dying at alarming rates, and scientists believe that deadly parasites benefiting from shorter winters are the primary culprits.

Winter ticks have taken a toll on moose across Maine, New Hampshire and Vermont, killing about 70 percent of moose calves. Winter ticks attach themselves to a single moose by the tens of thousands.

“It’s just off the charts; this should not happen with such frequency,” said Pete Pekins, chairman of the Department of Natural Resources and the Environment at the University of New Hampshire (UNH). “This is about a calf carrying 75,000 ticks that are draining it of blood.”

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A female "ghost moose" with severe hair loss is seen in springtime in New Hampshire. (Photo/Dan Bergeron)


Pekins and UNH are at the center of a six-year study in which researchers in Maine, New Hampshire and Vermont are attaching tracking devices to moose as part of an effort to learn how ticks are affecting them.

The moose that survive the tick infestations don’t necessarily go unscathed. According to Pekins, there is a high chance that a surviving female calf won’t breed the following year since so much was taken out of it fighting off the ticks.

In addition, unlike deer and other animals, moose appear to do a poorer job of removing ticks through grooming.

By early spring, moose may be showing large patches of broken or missing hair where they have tried to rub away ticks. Moose with large patches of broken hair are sometimes referred to as “ghost” moose because the white base of the hair shaft is all that remains.

Winter ticks may be thriving in part due to the New England ecosystem being disrupted by global climate change. According to AccuWeather Senior Meteorologist Brett Anderson, the average winter temperature in Maine has climbed 4 degrees Fahrenheit between 1895 and 2015.

“This region of the country is one of the areas that’s warming the fastest in the lower 48 (U.S. states),” Anderson said.

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The mean winter temperature in Maine rose steadily between 1895 and 2015 (Graph/National Oceanic and Atmospheric Association).


According to Michael Bentley, a staff entomologist for the National Pest Management Association (NPMA), the winter tick benefits from a warming climate.

“In the past, snowfall and freezing temperatures in early/mid spring have curbed winter tick populations by killing a percentage of those ticks that dropped off their host,” Bentley said.

Rising air temperatures have led to shorter winters and less snowcover in the spring, allowing tick populations to climb, he added.

According to Pekins, steadily rising temperatures have caused the fall season in New England to be slightly longer, by about a week, while the winters have also been shortened. That extra week in which winter is delayed gives ticks an extended window to latch onto moose for the duration of the winter.

“For a tick that can lay 3,000 eggs, the impact can be dramatic,” he said.

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Winter tick infestations, such as these on the ear of a dead moose in New Hampshire, are driving a decline in the New England moose population. (Photo/New Hampshire Fish and Game)


He added that climate change has the greatest impact on insects like ticks rather than large mammals like moose, which can withstand varied weather conditions. The New England forests are the “best habitat available” for moose, he said, so the tick infestations are driving the moose die-offs.

While researchers have said that New England’s moose population isn’t at risk for extinction, their numbers have dropped in recent years. According to fieldandstream.com, Maine’s most recent population estimate, from 2012, is 76,000 animals.

A decline in the moose population could have a measurable impact on New England’s economy.

Wildlife watching, including moose viewing, contributed nearly $800 million to Maine's economy in 2011, an increase of 82 percent since 2001, according to a report by the U.S. Fish and Wildlife Service and the U.S. Census Bureau.

In New Hampshire, wildlife watchers spent $281 million in 2011.

While winter ticks may benefit from shorter winters, they are not helped by drought conditions. A drought in New England in August/September 2016 led to lower tick populations, Pekins said, but longer-term impacts can’t be predicted at this point.

Another factor in the spread of winter ticks, researchers believe, is the number of moose, which rebounded by the 1990s after they were decimated by human use of their habitat earlier in the century. More moose, researchers say, mean more hosts for ticks.

“You need a high density of moose to drive this system,” Pekins said.

One possible way to control the problem, Pekins said, though counterintuitive, is increased hunting. If the moose population falls, the ticks would have fewer hosts on which to feed and their numbers would fall.

“However, there would be fewer moose and that might not be positive for moose viewing and local socio-economics,” he added.