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Warm summers spur more winter ticks for moose

The Isle Royale study used 20 years of data to track winter tick impacts.

A bull moose on Isle Royale with extensive hair loss due to winter ticks. A new study found that warmer summer temperatures lead to more ticks the next winter. Contributed / Sarah Hoy

Pity the poor Isle Royale moose in winter, losing weight daily with little of nutritional value to eat, wolves closing in for the kill and sometimes thousands of blood-sucking winter ticks clinging to their body.

Each tiny tick may not suck much blood on its own, but imagine tens of thousands of ticks on a single moose. That loss of blood can cause anemia and weakness, and make moose easier prey for wolves.

But the worst part is the itch, and moose are scratching so hard to get rid of winter ticks that they can remove 50%, even up to 80% of the hair on their bodies by the end of an average winter. It’s that thick hair or fur that makes moose suited to survive cold winters, but not if they rub their skin bare.

Unlike other ticks, like wood ticks, which may spend just a few days on a host, winter ticks hang on all winter, transforming from larvae to nymph to adult on the moose while consuming blood.

A cow moose with extensive hair loss caused by scratching as the moose tries to rid itself of huge winter tick infestations. A new study by Michigan Tech researchers found warmer summers lead to more ticks the next winter. Contributed / Sarah Hoy free


“The hair loss doesn’t kill them, but it makes them more susceptible to everything else going on around them, at a time when they are essentially starving without much to eat,” said Sarah Hoy, a Michigan Technological University biologist who studies moose on Isle Royale and the lead researcher on moose and tick study on the island.

In a study published Monday, Nov. 22, in the journal Frontiers in Ecology and Evolution, Hoy and other Michigan Tech researchers found that warmer summers are the biggest factor in how many ticks are on moose the next winter. It’s summer when female ticks lay eggs, and those eggs survive better when it's warm, so there are more of them to grab onto moose and hitchhike in the winter.

“It’s the biggest correlation we could find between ticks and hair loss and climate change,” Hoy said. “It’s the summer before winter that matters most.”

Temperatures in July appear to be the most critical, the study found. The study used 20 years of data on moose hair loss and then checked weather data to compare. The study seems to indicate that longer, warmer summers, which are expected as climate change intensifies, may mean more problems for moose, especially in places like Minnesota and Maine already on the southern edge of their range. (Duluth just had its warmest summer in 147 years of records. Five of the top 10 hottest summers have come since 2000. Seven of the warmest 10 summers have come since 1983.)

The Isle Royale study did not find much correlation between the length or severity of winters and the amount of hair loss due to ticks, Hoy noted. That’s contrary to some previous assumptions that shorter winters might spur more ticks. That may be happening in some areas, but not on Isle Royale where Lake Superior-influenced winters remain pretty tough.

The new study could help explain why in some areas of North America, such as the Northeastern U.S., winter ticks appear to be a growing factor in the decline of moose populations. It may be the region’s rapidly warming summer temperatures. (In Minnesota, it’s another parasite, brainworm spread through deer, that appears to be the biggest problem for moose.)

Across North America, deer and elk also get winter ticks, but they tend to rub them off gradually, over the fall and early winter, as the ticks jump on. For whatever reason, moose don't start scratching until late winter, when tick numbers have reached epic proportions. By then it’s too late.

John Vucetich, who leads the now 63-year study of moose and wolves on Isle Royale and who was part of the tick study, said that may be because moose, which came to North America about 14,000 years ago over the Bering land link, are relative newcomers. There were no ticks where they came from in Europe and Asia. Deer and elk, natives of North America, appear to have figured out how to deal with ticks over the millennia.


“No parasite wants to totally destroy their host, so they have this balance worked out with deer and elk. … But that hasn’t happened yet with moose,” Vucetich said.

Winter ticks on a moose. A study published by Michigan Technological University scientists found that warmer summers produce more winter ticks, which can accumulate to tens of thousands on a single moose, spurring the moose to scratch off its protective coat of hair. Contributed / John Vucetich free

Eating less on warm summer days

Next up for the Isle Royale moose and wolf researchers is a study on the impacts of summer heat on moose nutrition. It turns out moose eat far less on warm days, and don’t eat at all when it’s very hot. Too many hot days in any one summer may mean moose head into winter with less fat, and are less suited to survive hard winters.

“I don't know if you have experienced this, but when it’s really hot out, you often don’t feel like eating much … because digestion creates even more heat," Hoy noted. “It’s the same way with moose. When it gets too warm on a summer day, they are going to seek out thick conifer cover, or maybe some inland waters, to move into, and they aren’t going to be thinking about eating.”

That nutritional study is expected to be published soon, Hoy said.

A young bull moose on Isle Royale that scratched off large amounts of its hair due to an infestation of winter ticks. A new study found that warmer summers produce more winter ticks on moose and more hair loss. Contributed / Sarah Hoy free


Winter survey on for 2022

After being canceled for the first time in 63 years in 2021 due to COVID-19 concerns for the researchers, the annual Isle Royale winter moose and wolf survey is back on for 2022, Hoy said, at least as of now.

Each winter, scientists fly over the island and conduct a thorough search to determine an estimate of moose and wolf numbers. That last happened in 2020, when there were an estimated 1,500 moose and about a dozen adult wolves on the island.

Since then moose, which had been eating themselves out of house and home, almost certainly have decreased in number, with many starving over the past two winters.

Wolves, now all transplanted to the island from other areas in recent years, or offspring of those transplants, appear to be doing well, feasting on moose and sorting out their pack relationships, Hoy said.

One issue, however, may be that most of the wolves now on the island appear to have come from the same group of wolves transplanted from Michipicoten Island on Lake Superior. That could mean they share genetic backgrounds, and that could eventually spur the same genetic problems that doomed Isle Royale’s native wolf population in recent decades.

At 45 miles long, Isle Royale is the largest island on Lake Superior, sitting about 14 miles off Minnesota's North Shore from Grand Portage. The island is a national park and mostly designated wilderness with few human visitors in summer and none in winter. There are no other major predators on the island, no hunting is allowed and moose are the only large prey species, making it a unique wild laboratory for the ongoing wolf-moose study.
Moose came to the island around 1900, peaking at 2,445 in 1995 and hitting bottom at just 385 in 2007. Wolves are relatively new to the island, having crossed the ice from the North Shore in 1949. Their numbers reached a high of 50 in 1980, and 24 wolves roamed the island as recently as 2009.

Climate change, spurring far fewer years of ice bridges between the island and the mainland, reduced the number of new wolves venturing to the island in recent decades and reduced the pack's genetic diversity. With no new wolves coming to the island, the animals inbred and developed genetic deformities that doomed their survival, spurring the National Park Service's dramatic wolf reintroduction effort in recent years aimed at maintaining some natural limit on the island's moose.

Wolves were trapped in Minnesota, Michigan and on Ontario islands of Lake Superior and moved to Isle Royale in 2017 and 2018, the basis for the island's current wolf revival.


John Myers reports on the outdoors, environment and natural resources for the Duluth News Tribune. He can be reached at jmyers@duluthnews.com .

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at jmyers@duluthnews.com.
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