Moose are an iconic species across North America playing an important role in ecosystem health, indigenous and First Nation cultures, and subsistence hunting. In recent years, moose populations have experienced unprecedented impacts in the Northeast due to winter tick infestations that can cause lower reproduction rates, anemia, and even death in calves and adults. Climate change and warmer winters can increase the threat of ticks by creating more favorable conditions for the arachnid pests to thrive. To address the growing winter tick problem the U.S. Fish and Wildlife Service’s Wildlife and Sportfish Restoration Program has supported work at the University of Vermont to research fungal spores as a possible control method for ticks.
Winter ticks, also known as moose ticks, typically feed on the blood of large mammals and cause stress and mortality for moose populations across the U.S. and Canada. A single moose can carry over 40,000 ticks and winter ticks have caused over 90 percent of moose calf mortalities in Vermont in recent years. To fight these pests, researchers are turning to a biological control technique that uses microbial entomopathogenic fungi (insect-killing fungi) to invade the tick body. Several species of fungi have been developed commercially for pest control worldwide, but few have been developed for ticks. “These fungi are naturally occurring in the soil and have evolved to kill ticks and other insects,” said Cheryl Frank Sullivan, Research Assistant Professor at the University of Vermont’s Entomology Research Laboratory. “The fungal spores penetrate the outer shell of the tick, cause pressure on the internal tissues, and produce chemicals that help kill the pest."
The key to entomopathogenic fungi designed for pests is finding the right species and strain and the right delivery method. Spores can be delivered via combination of oil-based substances, solid granules, or powders. Once the spores attach to the tick, the fungi get to work penetrating the exoskeleton. “Our research has had a variety of funding, people have an interest in understanding and managing ticks,” said Sullivan. “The Wildlife and Sport Fish Restoration Program funded a recent project examining mortality rates of various-age larval winter ticks following contact with entomopathogenic fungi.” Studies like this are expanding knowledge for winter tick management and are informing new methods of suppression.
Chemical treatments for ticks exist and can help control them in the environment in a smaller area like a residence or backyard or on domestic animals and livestock, however, the unique life cycle of winter ticks could make them more likely to develop a resistance to chemical treatments. Pockets of winter tick infestations are also typically located in remote locations where moose reside making chemical treatments difficult. “Ticks are very adaptable and are a successful parasite in a variety of conditions,” added Sullivan. “All tick species can present problems for hosts, but the winter tick’s biology makes it especially difficult to manage.” Winter ticks are a one-host tick species, meaning that the entire feeding portion of their life cycle is spent on the same host. While other species like the blacklegged tick move to larger hosts throughout the pest’s multiple year life cycle, winter ticks have a shorter one-year life cycle and feed on the same animal with peak blood meals occurring in early spring as the females prepare to lay eggs. These blood engorged females will drop off the moose host in search of leaf litter to lay thousands of eggs. After a brief resting period following hatching, these larvae ticks will climb vegetation in fall waiting to hitch a ride on a host animal walking by in a process known as questing. These larvae have interlocking limbs allowing one tick that manages to attach to a passing moose to bring along hundreds or thousands of ticks looking for a host.
When the engorged females drop off looking for leaf litter to lay eggs the historically cold New England weather presents a challenge, snow covered ground. The snowpack makes the egg-laden female ticks easier for predators to spot and feed on and the snow and ice can also cause mortality from freezing. “A warming climate creates more opportunities for winter ticks to thrive, less snow cover allows the females to come in contact with more suitable ground conditions, increasing the chances of them laying eggs adding to the next generation of ticks.” Less severe winters may also bring more rain to areas that traditionally saw snow. Ticks require a climate with high humidity to stay hydrated while off-host, free-living in the environment. Decreased snow cover and more humid conditions could cause tick populations to increase, expand further north, and cause a variety of challenges for moose.
Sullivan stresses that fungal biopesticide treatments would be just one aspect of an “integrated tick management” program. Federal and state agencies, researchers, landowners, and hunters all have a part to play in reducing tick populations. “There is no silver bullet for tick management, we will need a variety of approaches and activities to control this adaptive parasite that can have widespread impacts on moose populations,” said Sullivan. Integrated tick management includes activities like landscape management, documenting tick hot spots across moose habitats, and sustainable hunting for moose as lower population densities limit the reproductive success of ticks thereby increasing the overall health of the moose population.
Looking forward Sullivan and her research team will continue studying tick management methods and the potential for entomopathogenic fungi to expand as part of integrated tick management programs in New England. Further research is still needed in fungal strain development and application rate. However, using fungi that naturally occur in the environment as part of a holistic approach to pest management could prove a valuable tool as colder regions experience more impacts from a changing climate.