- July 6, 2017
Ticks are ecologically complex Arachnids. Researchers are continuously discovering new dimensions to the elaborate interplay between ticks, their ecosystems, and human health. Understanding what makes ticks, tick may help individuals and communities make management decisions to minimize tick populations. In this article we’ll set the stage for these recommendations by elaborating on the life of a tick. Then, in a sister article we provide land management recommendations based on current understandings of tick ecology.
Media attention has focused on the blacklegged tick (Ixodes scapularis) and western blacklegged tick (Ixodes pacificus) because these species transmit the Lyme disease-causing bacterium, Borrelia borgdorferi . Lyme is a growing human health concern with over 300,000 reported cases per year.open_in_new The blacklegged tick (Ixodes scapularis) is found throughout the northeast, southeast, and in portions of the midwest. The western blacklegged tick (Ixodes pacificus) is found predominantly in California, Oregon, Washington, and parts of New Mexico, Arizona, and Utah. While all ticks are capable of carrying pathogens that can cause disease, not all ticks that bite are carriers of pathogens or can transfer disease. To learn more about the diversity of ticks and the diseases they carry, visit the Center for Disease Control and Prevention. For purposes of this article we will focus on the blacklegged ticks.
Blacklegged ticks are native to the United States. They are not an introduced invasive species. Changing environmental conditions, however, may be affecting the percentage of ticks harboring the Lyme disease-causing bacterium (B. burgdorferi) and increasing in their populations by enabling the expansion of tick ranges.open_in_new
These conditions include, but are not limited to:
We will examine each of these conditions as they relate to the complex ecological needs and life cycle of ticks.
Ticks have long life cycles. Adult female ticks generally feed on a mammalian host and mate in the fall before overwintering in the leaf litter or soil (see image below, the engorged female in the bottom of the image just laid the shiny yellow eggs). In spring the females lay a batch of eggs (on average an egg mass is approximately 3000 eggs), which hatch into larvae in the summer (larva is depicted as the small tick to the right, above the egg mass). These larvae seek a blood meal, drop off the host, and then molt into the nymphal stage (nymph is center, above the egg mass, looks like a small adult tick). The nymphs then become inactive and overwinter in the leaf litter or soil.
The next spring, nymphs emerge and seek a blood meal. In early summer they molt into an adult (the female is the largest tick to the left, male is the darker, smaller large tick to the right) and go in search of a blood meal and mate in preparation for laying eggs the next spring.open_in_new
There can be some variation to this cycle depending on weather and availability of hosts. Under pressure, ticks can remain in larval, nymphal, and adult developmental stages for longer periods of time.open_in_new On average, however, their life cycle spans two years, but can span up to four years for the blacklegged tick (I. scapularis) and three years for the western blacklegged tick (I. pacificus)open_in_new
The length of a ticks’ life cycle is one of the greatest challenges in efforts to study and minimize tick populations. As ecologist Solny A. Adalsteinsson articulates, ticks have long lags in their developmental phasesopen_in_new and climatic or environmental pressures that may minimize the density of ticks at one developmental stage may not impact ticks at other developmental stages. For instance, heavy rainfall may flood forests in the spring and summer while nymphs are questing for blood meals. This flooding, however, may not affect ticks at other developmental stages. A set back one year, at one developmental level, is not likely to have a major long-term impact in overall populations of ticks.
Blacklegged ticks require a somewhat particular set of environmental conditions to survive. Like Goldilocks, they need environments that are not too wet and not too dry, not too hot or too cold, and with weather that is relatively stable.open_in_new
Ticks have some tolerance to cold temperatures. In laboratory studies conducted on fed vs. unfed larvae and nymphs there is between 50-100% mortality of the blacklegged tick (I. scapularis) exposed to eight hours or more of temperatures in the -10°C (14°F) to -17°C (1.4°F).open_in_new Those with blood meals in their system, can tolerate colder temperatures for longer periods of time.
In native habitat, ticks may be more hardy in colder temperatures depending on the depth of leaf litter or availability of insulating snow.open_in_new This may be one reason for a slower expansion of ticks in the higher altitudes and northern latitudes of their natural ranges.open_in_new Some research points to the abundance of tick predators in ecosystems as being more significant than snow pack or cold temperatures, such as this Lycosidae family of wolf spiders pictured above.open_in_new
In temperatures 32°C (89.6°F) and above ticks exhibit a reduced ability to lay eggs, less questing (searching for host to attach to for bloodmeal), and some mortality.open_in_new Temperatures above 40°C (104°F) result in significant tick mortality.open_in_new Heat equals desiccation (dryness). Heat, especially in the absence of humidity, can be a killer of ticks.
Moisture and humidity are ticks’ best friends. As small, hard-bodied animals, they rely on their environment to maintain adequate hydration. In a tick’s long life cycle, they will only feed up to three times–spending the rest of the time trying to stay alive between moltsopen_in_new by hiding in the thick, moist, leaf litter to avoid desiccation. This is why most ticks, in all stages of their life cycle, favor forested or semi-forested conditions where there is substantial leaf litter and soil organic matter.open_in_new
In research comparing tick densities in deciduous vs. coniferous forests, tick nymph’s populations were greater in deciduous forests. This bias is speculated to be correlated with the movement of bloodmeal hosts, specifically white-tailed deer (Odocoileus virginianus) who prefer feeding in the rich understory of deciduous forests.open_in_new
Blood meals are essential for ticks to survive at the larval, nymphal, and adult phases. Mammals and birds are blacklegged ticks preferred targets. Larval and nymphal ticks generally feed on North American rodents such as mice, chipmunks, and squirrels. They will also feed on various other mammals such as opossums, raccoons, and deer. Ticks, unbeknownst to some, are not born with the Lyme disease-causing bacterium, B. borgdorferi. They acquire this bacteria during a blood meal on an infected animal.
White-footed mouse (Peromyscus leucopus), which thrive in close habitation to human-built environments, is the most likely reservoir host of the Lyme disease-causing bacterium (B. burgdorferi).open_in_new White-footed mice are carriers of the bacteria but are asymptomatic for Lyme. A tick, usually during the larval or nymphal stage, will feed on a white-footed mouse and, if the mouse is infected with (B. burgdorferi), the tick can become a carrier of the disease.
Ticks in the nymphal stage are the most dangerous for transmitting the Lyme disease-causing bacterium because they are a small and hard to see which makes them more likely to attach long enough to transmit bacteria into the host.open_in_new There is no consensus among scientists about the minimum amount of time this takes, but, generally, the tick has to be attached +24 hours for B. borgdorferi transmission.open_in_new Ticks can transfer this bacteria to any animal they feed on once infected. The eggs laid by an infected adult, however, do not contain the bacteria–the new offspring would need to feed on an infected host to acquire Lyme disease-causing bacterium, Borrelia borgdorferi.
White-tailed deer (O. virginianus) are a favorite vertebrate host for adult ticks. Look closely at the image above, what appear to be sores on this deer, are actually engorged ticks. By the time an adult tick feeds on a deer, they are either infected with Lyme disease-causing bacterium, B. borgdorferi, or not.
Deer populations are growing in suburban and urban areas.open_in_new Deer have few remaining predators in areas where laws forbid hunting. This allows deer populations in some regions to skyrocket. And, accompanying an increase in deer populations is an increase in ticks populations.open_in_new
The why of this relationship is fascinating. Aside from being a site for blood meals and reproduction–deer, mice, and ticks all thrive in patchy ecosystems created by forest fragmentation (depicted above in image of Ithaca, New York). Forest fragmentation describes the patches of forest that result when larger forested areas are divided by development. The resulting patches are often pushed-up against human-created habitats such as fallow fields, abandoned lots, fields of invasive shrubs, a suburban or urban neighborhoods, etc.open_in_new Do ticks thrive in these environments because that is where their preferred hosts thrive, or for other ecological reasons? Researchers are trying to understand this phenomena more thoroughly; but, the fragmentation theory does help explain the rise of tick populations and increasing Lyme disease transmission to humans.open_in_new Ticks follow their hosts. If mice and deer can live near humans, so, too, can ticks.
To date, however, none of the observed changes in risk of exposure to I. scapularis or B. burgdorferi can be conclusively linked to climate change even though this may be a very likely cause or cofactor.Rebecca J. Eisen
Climate Change adds another layer to the complex ecological landscape of blacklegged ticks. Projections are hard to make given the complexities of the tick life cycle; but, speculative studies indicate that as temperatures warm at higher latitudes the northern and western range of the blacklegged tick could expand. As temperatures in its southern range increase and rainfall decreases, populations may contract in the south. Ticks will continue to be limited by dry regions lacking humid conditions or regions that become too hot or too cold for ticks to thrive. The exact range expansion or contraction will depend on how climate change continues in North America.open_in_new
The short story of the long story is ticks are tricky. Ticks’ long life cycles, coupled with their ability to ride out fluctuations in temperatures, their flexible feeding schedule, and their ability to thrive in human-created landscapes make them hard to pin down. Using this complex network of information, we’ve made specific land-management recommendations for individual homeowners to implement to minimize tick populations. There are no silver bullets. There rarely are in complex, ecological systems. Addressing this issue using multiple strategies is our best bet in minimizing tick populations in yards and community green spaces.
Continue to article: Managing Yards and Green Spaces to Minimize Tick Populations