Winter ticks: the pest hammers moose population as warming climate moves in



Published: 05-09-2017 4:45 PM

Winter ticks, it seems, live by climate change and die by climate change.

The parasitic blood-suckers have hammered the New England moose population for years. Shorter winters mean a greater likelihood tick larvae live long enough to grab onto a host. And in April, when the then-adults finally detach, they are less likely to fall onto snow and die.

“Of course we’re all aware of the warming climate,” said Cedric Alexander, Vermont’s moose project leader. “That’s beneficial to winter ticks, because when they drop off in April, if they fall onto snow, their survivorship is much lower.”

Combine mild winters with higher moose densities and, come April, higher body counts among the four-legged icons are more likely, scientists believe.

More recently, however, there is another factor to consider: drought conditions toward the end of summer can end up killing tick eggs, easing the tick loads moose end up carrying.

“A really dry summer can kill those eggs, but a really wet end of the summer is really pretty good for ticks,” said Lee Kantar, Maine’s moose biologist. “So there’s this combination of things that all have to do with climate changes that can really affect how well ticks do.”

Biologists in Maine, New Hampshire and Vermont are conducting a multi-year moose collaring project in an effort to assess the problem. Preliminary results from this year’s collaring indicate last year’s drought conditions may have spared some moose.

Still, the parasites are taking a toll.

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“It’s obviously a sad sight to see,” Kantar said. “Parasites typically are organisms that don’t outright kill an animal. The phenomenon of a parasite like a winter tick being a primary driver of killing a moose is a pretty significant event.”

Collaring study

Kristine Rines, moose project leader for New Hampshire Fish and Game, said May is when it becomes clear how bad the outgoing winter was for moose. April is when the adult ticks tend to take their last blood meal, and when the moose succumb to weight loss and anemia.

“April is the month of death, is what we say,” Rines said.

Calves are the most vulnerable, and scientists believe ticks are a primary factor in their death.

In New Hampshire, this was the seventh winter the state has collared a sampling of moose, and the preliminary results indicate a less-brutal winter than previous years.

“As of now, mortality rates are 24 percent for calves, which is greatly reduced from the last few years,” Rines said. She pointed to research from University of New Hampshire graduate student Kyle Ball, suggesting lack of moisture toward the end of summer can kill tick eggs.

“That helped things considerably,” Rines said, adding heavy tick loads contributed to eight of 37 collared calf deaths as of April 27. One died crossing a river.

In Maine, there are two study areas — one in the north and one in the west of the state, providing different climate, habitat and perhaps different moose densities, Kantar said.

This year, 19 of 36 collared calves died in the western zone (or 53 percent) and 8 of 37 died in the north (or 22 percent). In addition, three cows died in the western zone, a 5 percent mortality rate.

That may seem high, but the death rates were down 21 percent in the west and 26 percent in the north, respectively, he said.

“Keep in mind that many species of wildlife have difficulties in winter, especially first-year animals,” Kantar said. “So in a sense losses this year are more in line with what happens elsewhere, ticks or not.”

In Vermont, which participated in the collaring study for the first time this year, 11 of 30 collared calves died, Alexander said. He added two of 30 collared cows died.

Scott Darling, of Vermont Fish and Wildlife, said the northeastern corner of the state, where the testing zone is, saw snow in “shoulder” months, and drought conditions last summer.

Because of that, “I had anticipated a lower mortality,” Darling said Tuesday.

Massachusetts does not participate in the collaring.

There are still an estimated 1,000 moose in Massachusetts, and David Stainbrook, the state’s moose and deer project leader, said at least two moose calves likely succumbed to winter ticks this year — one near the Quabbin Reservoir and one near the Peru State Forest west of Worthington.

“Anecdotally we have heard some more reports of higher tick loads on moose in more recent years,” Stainbrook said.

Attaching, detaching

The bloodsucking starts in the fall.

“When a moose walks by some brush that has ticks on it, it’s usually — it can be thousands of ticks on one bush,” Kantar said. “The moose does not react or sense that the ticks are on the animal.”

The larvae launch into a “very small blood meal,” Kantar said.

“The ticks basically lay dormant until about January when they” molt into nymphs, he said. At that point, the ticks start to have a noticeable health effect.

“If you have 30,000 nymphal ticks on you in January, they’re taking a blood meal at a tough time,” Kantar said. There is some mortality at that point.

In March and April, the ticks reach adulthood.

“Right now, what we’re starting to see — literally right now — is the adult winter ticks are feeding,” Kantar said in early April. “They are much larger ticks than the nymphal stage, and they take a huge amount of blood volume.”

Unlike deer, which can sense larvae when they attach and can groom themselves, moose can’t. The moose burn energy trying to scratch off the ticks — and in a grisly way.

“In a normal day, 15 percent of the time, that moose is scratching or rubbing or being bothered by the ticks,” Kantar said.

“The moose start vigorously trying to scratch them off,” Alexander said, “by rubbing against trees.”

The adult ticks detach in April.

Scientists assess the damage to moose population.

In the fall, a new batch of larvae climb brush, and scope out a host.


Why are winter ticks having such an impact only in recent years?

Alexander said parasites such as winter ticks and brain worm historically lived off of deer.

“Because of land-use change after European contact, the deer populations — really throughout the country — spread northward,” he said, adding deer were drawn to New England and New York by agricultural crops and the reduction of predators. “They ended up bringing — as they expanded into moose range, northward — they ended up bringing a couple of major diseases. Now you have these moose as a species impacted by these deer parasites.”

Over-hunting drove the moose range northward. In 1896, the Vermont Legislature banned moose hunting, but the population did not move south until decades later. Alexander said the first time he saw a moose was in 1979, a year before he started working for Vermont Fish and Wildlife.

“The moose was at a low-enough density that the probability of a winter tick larvae attaching to a moose the next fall was low,” he said, “because you wouldn’t necessarily have a moose walk by.”

Until about 2002, Alexander said, when there was the first documented case of high tick numbers on a moose. Around that time, the moose population in Vermont peaked at about 5,000. Officials noticed scattered evidence of high tick numbers during the following years.

Then, in 2011, “The tick numbers were extremely high on a number of individual moose,” Alexander said, adding there were tick counts ranging from 30,000 to 70,000 on any one moose.

The moose population now hovers at around 2,200, after more hunting permits were issued, which Alexander and Vermont Fish and Wildlife believe could have alleviated some of the toll winter ticks have taken.

Kantar, the Maine biologist, hopes the collaring project will inform best management strategies going forward.

“Moose in a state like Maine are an icon and we want to see moose do well now and into the future,” he said. “And that’s very critical. And parasites are just a drain on the population.”

Jack Suntrup can be reached at