Maine Moose and Winter Ticks

Maine’s moose population is stable in its core habitat range. However, the proliferation of winter tick due to climate change is impacting the health and reproduction of the moose population. MDIFW’s goal for managing moose into the future is to have a healthier moose population where reproduction is higher, and parasites (such as winter tick) are lower.

MDIFW biologists have been closely monitoring and studying the impact of winter tick since 2014. Although adult moose survival remains high, winter tick is the leading cause of death for moose less than one year of age. Winter tick also reduces adult cow reproduction. For the next phase of the ongoing study, the Department increased the number of cow permits in a small section of the state (6% of Maine's core moose range) to reduce the moose population density. This will help determine if lowering moose densities can break or lessen the winter tick cycle ultimately improving the health of Maine’s moose.

Read on to learn more about winter tick, how it impacts moose, what works and what doesn’t, and the next steps of the Adaptive Management Study.

What is the status of Maine’s moose population?

Maine has the highest moose population in the lower 48 states thanks to abundant habitat, no predators, and successful wildlife management. Over the last ten years Maine’s core moose population has remained stable, but this comes after a century of fluctuation. Maine’s moose population experienced a severe decline to an estimated 2,000 individuals in the early 1900s. However, with improved hunting restrictions regulated by MDIFW and forest regeneration, the moose population experienced significant growth into the 1990’s to an estimated 29,000 and likely reached their highest population by the year 2000. Pioneering new techniques MDIFW estimated moose at around 76,000 in 2012. Since the recovery, climate change has begun to influence the success and proliferation of winter tick, leading to poor reproduction and low calf survival through the first winter. Without intervention, biologists believe that the population will likely destabilize, but MDIFW’s biologists are working to implement adaptive methods to keep our moose population healthy and stable for the future.

What are winter ticks, and why are they a problem?

The winter tick is a small, external parasite which, like all of Maine’s 15 tick species, survives on the blood of animals. Though they are capable of feeding on many different species, winter tick infestations are most commonly found in moose. Unlike other ticks, winter ticks are not known to spread disease. However, they can be deadly to moose less than one year of age and lower cow reproductivity.

Bites by the thousands

In late spring, fully-fed adult female winter ticks drop off their host to seek leaf litter, where they lay up to 4,000 eggs. The eggs hatch in late summer or early fall, at which point the larvae climb vegetation, “questing” for an organism to be its host. They can sense vibrations and detect CO2 from an animal up to 20 meters away; and as a passing moose brushes by one tick, thousands of others come with it using their interlocking limbs.

In the photo below, MDIFW Moose Biologist Lee Kantar examines ticks on a calf that died in the spring:


A one host tick

To make matters worse for moose, while most other tick species move to a different host for each developmental stage (larvae, nymph and adult), winter ticks feed on a single animal for all three stages, consistently eroding its health from fall to spring.

Winter tick life cycle

Winter Tick Life Cycle text version (PDF)

A product of climate change

Recent climate change in the form of shorter, milder winters has allowed winter ticks to thrive. Despite their name, winter ticks can’t survive cold winters without a host. Late falls give nymphs extra time to find one, and early springs give females abundant leaf litter to lay their eggs in.

A vulnerable host

Unlike deer, elk, and caribou, which scientists believe have evolved with tick populations and can more effectively deal with them, moose are a newer, less-adapted prey. And because the tick questing season coincides with the moose mating season (the time when they are most actively moving around the woods), moose are particularly vulnerable.

How do winter ticks affect moose?

Unlike deer ticks, winter ticks do not spread disease. The problem is that they attach by the thousands and stay attached for five or six months, causing moose to experience significant blood loss, hair loss, and even behavioral change.

Although adult moose survival rates remain high, winter tick is the leading cause of death for moose less than one year of age and reduces adult cow reproduction.

Why are calves so vulnerable?
A calf moose has very little body fat going into the winter and can’t replace blood fast enough. The winter and spring food availability has low nutritional value, preventing calves from replenishing adequate blood supply and fat reserves. This results in calves not only losing blood from winter tick but body weight as well. In fact, a calf may lose nearly 100 pounds (20-30% of their body weight) between the months of January and March/April. By March, with depleted blood volume, caloric deficits, and little hair to keep them warm, tick-infested moose calves (around 10 months old) will often succumb to the elements.  In parts of the core moose range calves under a year old have experienced greater than 50% mortality. Those that make it through the winter are left with openly bleeding wounds when the fully-fed ticks drop off.

Info graphic shows 20-30% body weight loss from Jan to March/April

What is the impact on pregnant cows?
Pregnant cows, who are essentially dealing with two parasites – the tick load and the fetus, are also vulnerable. By spring, pregnant cows and their fetus are often so anemic and malnourished that the newborn calf in May may have a much-reduced chance at surviving.

If a yearling cow does survive, they typically are in poorer condition so that they do not gain enough mass to breed until a later age (three years old). Cows are much less likely to have twins as they get older, resulting in lower reproductive rates.

How does winter tick impact adult moose?
Adult moose survival remains high – with a nearly 90% survival rate of adult cows in the core moose range. However, a winter tick infestation has major physiological effects on a moose. During the winter, when moose should be focused on staying safe, warm, and fed, those with winter tick infestations will instead spend over two hours a day grooming (by contrast, those without infestations spend less than five minutes). This burns excess energy, causes insulative hair loss, steals time and attention from the task of finding food, and makes moose distracted, disoriented, and vulnerable to disease.

Solving the winter tick problem: what works, and what doesn’t

Often, people suggest various methods of killing off ticks to improve moose health. Unfortunately, many have been tried with little success. These include:

  • Providing feed with an insecticide (Ivermectin):

    In theory: Ingested ivermectin does eliminate ticks from moose.

    In practice: It is difficult to get wild moose to eat the right dose of Ivermectin frequently enough to make an impact. During the late summer and early fall, when ticks are looking to attach, natural food is prevalent, so moose ignore bait food. And even if the animal is successfully baited (as in Texas, where this approach has worked with deer), ticks can develop immunity – and not only to the ingested insecticide, but to applied ones as well.
  • Fitting moose with tick collars:

    In theory: Tick collars are effective on pets, and should work the same way on moose.

    In practice: For this solution to work, 40-90% or a region’s moose population would have to be captured and given a fresh collar with the appropriate dose of pesticide for their weight every three to five weeks from early September to late December. At the cost of $1000 to capture one moose one time in good weather, the fiscal barrier makes this option a nonstarter. Additionally, other areas have attempted to use feed or collars on wildlife for tick deterrents, but because ticks reproduce quickly and is vast amounts, their genetics change rapidly and become accustomed to repellents (just like tick and flea medication is constantly changing for dogs and cats).
  • Releasing guinea hens and opossums to the core moose range:

    In theory: Guinea hens and opossums eat ticks and would help reduce the winter tick population.

    In practice: Guinea hens clean themselves and others of ticks and might consume a tick when found, but their diet naturally consists of several different foods. Guinea hens require heated and/or insulated coops in the winter and would not survive a Maine winter in the wild.

    Opossums are on their northern range in central Maine and moose exist far more north. Studies have shown that while opossums clean themselves of ticks, there is not enough nutritional value for opossums to use ticks as an adequate meal.
  • Burning the vegetation to kill the ticks:

    In theory: A long, slow, and hot burn of tick habitat would kill off the winter tick population.

    In practice: Winter ticks cover hundreds of thousands of acres. For many reasons, burning that much vegetation is not a realistic option, nor a viable option for private landowners who comprise the majority of the Maine woodlands
  • Reducing moose densities:

    In theory:Increased moose density leads to increased presence of ticks. By reducing moose densities, you give tick populations less food and therefore less of a chance to survive.

    In practice: Moose research in other parts of North America shows that moose that live at low population densities have fewer ticks.  Reducing current moose population densities in some areas of Maine may be the most realistic and effective way to break the winter tick cycle. Lower moose densities should lessen winter tick abundance, ultimately improving the health and reproductive efficacy of Maine’s moose population.

What has the Department already done?

To carry out the right solution, we first needed to know exactly what was happening, how, why, and where. Over the past few years, MDIFW has been laying that groundwork, and as of 2020 we have successfully:

Confirmed the correlation – Following reports of increased moose mortality throughout the 2000s, MDIFW moose biologists launched a GPS collar study to assess calf and cow moose survival throughout their core range from 2014 to 2020. The results were clear: winter tick loads significantly impact calf recruitment (the amount of calves living to be one year old) and cows’ reproductive success.

Documented Infestations - Since then, the Department has kept track of instances where heavy tick infestations (40k to 60k ticks per moose) have led to high juvenile mortality and chronic loss of physical condition in adult cows, leading to reduced fecundity (number of potential offspring over a lifetime) or birth of underweight calves.

The Next Step: Adaptive Management Study (2019-2025)

Moose research in other parts of North America shows that moose that live at low population densities have fewer ticks. To determine if moose density reduction can solve the winter tick problem in Maine, MDIFW is currently conducting a multi-phase adaptive management study. For the purpose of this study, which began in 2019 and is expected to run through at least 2025, a 2,000 square mile management zone in northwestern part of the state (Zone 4) has been divided into two similarly sized sections.

In phase one, aerial helicopter surveys assessed current population size and composition.  Sixty moose calves (30 in each section of the unit) were captured and fitted with GPS collars to monitor their survival. 

For phase two, MDIFW increased the number of cow hunting permits in the western half of zone 4 (6% of Maine's core moose range) in fall 2021 to reduce the moose population density. In the eastern half of zone 4 as well as all other zones open to moose hunting, moose permits will remain at normal levels. Biologists will collect several data points from all moose harvested in either subsection of Zone 4, including canine teeth (for aging), antler spreads, winter tick counts, corpora lutea, and carcass weights. In addition, biologists will continue aerial surveys and monitoring calves with GPS collars.

After five years of increased permits in the western half of zone 4 if we do indeed find that lower moose density leads to lower winter tick levels and healthier moose, that information may allow for increased permit allocations in other areas of the state.