1. A tale of two vectors: spatial distribution of Ixodes scapularis
and Amblyomma americanum
Robert J. George
U.S. Forest Service Fellowship
Disease Ecology
Dr. Vincent D’Amico
Solny Adalsteinsson

2. 2
Abstract
Here I review the role of host density, habitat type, and the local microclimate on the
abundance and spatial distribution of two epidemiologically relevant ticks (Acari:
Ixodidae), Amblyomma americanum and Ixodes scapularis. These two species were
chosen because (i) they are both widely distributed across North America, (ii) their
estimated ranges overlap, (iii) they appear to be differentially affected by local
environmental conditions, and (iv) they are both organisms of great importance to public
health because they are vectors for emerging and established infectious diseases. While
there are numerous factors that could potentially influence species distribution, host
dynamics, habitat type, and microclimate are recognized as prominent influences on food
acquisition, development, and survival of individuals. Gaining a greater understanding of
the biological, ecological, and climatic processes that affect the distribution of vector tick
populations is important for natural resource managers and public health officials to
evaluate and mitigate disease transmission.

3. 3
Introduction
The lone star tick (A. americanum) is an obligate hematophagous arthropod that is
broadly distributed across the United States: spanning the Atlantic Coast states, much of
the south (including southern Florida and as far west as eastern Texas), and the southern
Plains states [Figure 1](Springer, Eisen, Beati, James & Eisen, 2014). White-tailed deer
(Odocoileus virginianus), wild turkeys (Meleagris gallopavo), and raccoons (Procyon
lotor) have been identified as preferred hosts, but A. americanum is a generalist during
each life stage and will readily parasitize other vertebrates it encounters during questing
(Jacobson & Hurst, 1974). Questing is an active host-seeking behavior of hard ticks
(Acari: Ixodidae) in which a specialized chemoreceptor, the Haller’s organ, is used to
detect acute changes in CO2 concentration to locate homeotherms (Steullet & Guerin,
1992). This behavior is most prevalent during the afternoon in spring and summer
months, when human presence and activity in the forest is also on the rise (Schulze and
Jordan, 2003). Thus, aggressive questing behavior concurrent with human presence and
activity in the forest results in many human-tick encounters. Lone star ticks have been
long known as nuisance biters and recent research has recorded the emergence of this
species as a primary vector for etiological agents that cause zoonotic diseases such as
Ehrlichiosis, southern tick-associated rash illness (STARI), and Heartland virus
(Anderson et al., 1993; Masters, Granter, Duray, & Cordes, 1998; Savage et al., 2013). In
addition, A. americanum bites have been shown to stimulate production of IgE antibodies
with an affinity to alpha-gal, a carbohydrate commonly found in mammalian tissue,
resulting in delayed anaphylaxis following red meat consumption (Commins et al., 2011).
Because of its wide distribution across North America and the suite of pathogens that can
be potentially disseminated, natural resource managers and health professionals are
carefully monitoring the ascendency of the lone star tick as a major zoonotic disease
vector.
The blacklegged tick (I. scapularis) is a common tick widely distributed across the
United States [Figure 2]. Similar to A. americanum, I. scapularis is an important vector
for etiological agents responsible for tick-borne zoonosis; most notably it is the primary
vector for Borrelia burgdorferi, the causative spirochete of Lyme disease (Burgdorfer et

4. 4
al., 1982). Lyme disease is the most common arthropod-borne disease in the United
States infecting approximately 300,000 people per year (Kuehn, 2013). The deleterious
impact of Lyme disease on public health has motivated researchers to extensively study
population dynamics of the blacklegged tick; their findings will be reported in this
review.
These parasites require a blood meal acquired from a host to complete each life cycle and
reproduce, thus, the availability and accessibility of local host populations is an important
consideration. High host densities increase the likelihood of an opportunity where a
questing individual can obtain food. In addition, hosts can serve as vehicles that
disseminate populations into suitable habitats. Therefore, host density is a key biotic
parameter when examining tick distribution.
Because ticks only spend short periods of time on-host, environmental factors, like forest
type and microhabitat preference, are important when considering off-host survivability.
By characterizing habitat preferences, researches try to gain a complete understanding of
which particular factors are predictive of tick distribution. The inherently dynamic nature
of ecosystems coupled with seasonal and yearly variations can confound simple
conclusions formed from research employing a correlation approach; however, the utility
of past habitat preference analyses are useful when comparing interspecific differences in
distribution. Forest type strongly influences the processes that occur at the micro-level,
which determine the survival and persistence of individuals. Because A. amblyomma and
I. scapularis have different tolerances to these processes, they will likely be differentially
affected by each habitat.
The impacts of microclimate on tick survivability are the final factor this review will
examine. Here I define microclimate as the sum of two parameters (temperature,
humidity) that constitute the climatic conditions at a scale relevant to ticks (i.e. leaf litter
and upper soil layers). Survival of individuals at each life stage is dependent on many
contributing factors, but these conditions influence development, questing behavior, and
fecundity of individuals and hence are the final consideration for localized abundance and
distribution of each respective species.

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Effects of host density on tick abundance and distribution
Ixodes scapularis
Ixodes scapularis is an exophilic parasite that requires blood meals to provide the
sustenance needed to progress to each succeeding life stage. It has four life stages: egg,
larva, nymph, and adult. White-tailed deer (Odocoileus virginianus) are the predominant
host for adult I. scapularis (Spielman et al. 1985). In order to confirm and qualify the link
between deer density and tick abundance, two approaches have been used: firstly, deer
have been experimentally excluded by deploying deer-proof fencing around a forest plot
and secondly, researchers have measured the impact of deer culling events on tick
abundance within large study areas.
Employing the first method, Stafford (1993) collected larvae, nymphs, and adults in deer
exclosures over time. Within these exclusions, a marked decrease in host-seeking larvae
(-50%) and nymph (-80%) abundance was observed. The adult population did not
significantly decline, suggesting that larva and nymphs were being fed by alternate hosts,
which allowed them to successfully molt into the adult stage. Later studies reported
similar declines in the subadult population within exclosures (Daniels, Fish, & Schwartz,
1993; Daniels & Fish, 1995). A review performing meta-data analysis on investigations
using deer exclusions reported that large deer exclosures (>2.5 ha) resulted in a drastic
reduction of questing tick density, while small deer exclosures (<2.5 ha) increased
questing tick density (Perkins et al., 2006). The authors suggested that within exclosures
ticks shift from deer to smaller vertebrates. In the small exclosures, ticks are able to
diffuse to the interior via rodent importation. These findings illustrate the importance of
large-scale studies that consider complete host density on I. scapularis abundance.
Because of the impracticality and inefficiency of methods for removing and preventing
recolonization of deer in large natural areas, few investigations have had the opportunity
to scrutinize large-scale deer removal and its impact on tick ecology. Two notable studies
in New England took advantage of local deer removal- attempts to control Lyme disease
under the assumption that deer removal will prevent the tick life cycle from being
completed. On Cape Cod, Massachusetts, researchers monitored the adult, nymphal, and

6. 6
larval population on deer and white-footed mice (Peromyscus leucopus) before, during,
and after experimental deer removal. They demonstrated that the removal of deer resulted
in a significant decline of larval ticks the following year due to the failure of adults to
feed and reproduce (Wilson, Telford, Piesman, and Spielman, 1988). Later on
Monhegan- a small island (240 ha2) located off the coast of Maine- intensive hunting
from 1996-1999 completely eliminated the deer herd. In 2000, one year after deer
extirpation, adult tick density peaked at 17 ha-1, which was the highest density recorded
during the study. In 2001 the questing adult tick population plummeted to <1 ha1. The
authors suggested that the collapse was a result of the prior adult generation’s inability to
feed on deer and hence was responsible for the lag observed. Since then, tick abundance
has remained depressed and has not normalized [Figure 3](Rand, Lubelczyk, Holman,
Lacombe, & Smith, 2004). These studies support the general conclusion that where deer
and ticks are both present, immature I. scapularis abundance is indirectly and moderately
regulated by fluctuations in deer density because deer are an important maintenance host
for adult ticks. However, as demonstrated by aforementioned examples (Perkins et al.,
2006), ticks, especially larvae and nymphs, are generalists and can change their feeding
behavior in response to very low density or absence of deer. Thus, in order to fully
understand the complete impact of host density on tick abundance and distribution, other
hosts must be considered, especially woodland rodents, which tend to occur at very high
population densities.
While I.scapularis subadults are opportunists and have been documented parasitizing
opossums, raccoons, foxes, black bears, and other hosts in the eastern United States, it is
widely accepted that white-footed mice and the Eastern chipmunk (Tamias striatus) are
key maintenance hosts for larvae and nymphs (Anderson & Magnarelli, 1980; Donahue,
Piesman, & Spielman, 1987; Anderson, 1988; Schulze, Jordan, & Schulze, 2005).
Further, densities of P. leucopus and T. striatus have been found to be the best predictor
of nymph density (Ostfeld, Canham, Oggenfuss, Winchcombe, & Keesing, 2006). The
suggested mechanism of this regulation are three-fold: (i) these rodents occur at relatively
high densities within tick-infested habitats, (ii) in the summer live trapped rodents
typically harbor 20-30 ticks, and (iii) larvae that fed on P. leucopus and T. striatus
achieve a high molting success (91 and 75%, respectively) (LoGiudice, Ostfeld, Schmidt,

7. 7
& Keesing, 2003; Brunner, LoGiudice, & Ostfeld, 2008; Brunner et al., 2011).
Populations of woodland rodents vary in response to mast production, particularly acorns
(McCracken, Witham, & Hunter, 1999). Thus, evidence detailing a regulatory cascade
linking acorn production, rodent abundance, and tick abundance has been proposed
(Ostfeld, Canham, Oggenfuss, Winchcombe, & Keesing, 2006). In order to further
evaluate this pathway, examination of forest type and its effect on I. scapularis
distribution is necessary.
Amblyomma americanum
There are three parasitic life stages of Amblyomma americanum: larva, nymph, and adult.
While birds (Carolina wren, bobwhite quail, wild turkey) and medium to large mammals
(gray squirrels, eastern cottontail rabbits, raccoons, opossum, red fox, coyotes) have been
found to support high abundances of a particular life stage of A. americanum, white-
tailed deer consistently harbor high relative abundances of all three life stages (Kollars,
Oliver, Durden, & Kollars, 2000). Some evidence suggests that secondary host
associations may differ across the range of the lone-star tick, but deer have been
repeatedly confirmed as a universally preferred host (Atwood, Lamb, & Sonenshine,
1965; Zimmerman, McWherter, & Bloemer, 1988). For example, four studies spread
across the tick’s distribution (Missouri, Arkansas, Tennessee, South Carolina) have found
prodigious burdens of all life stages on deer- in one case a total of 4,800 on an ear
(Bishopp & Trembley, 1945; Bloemer, Zimmerman, & Fairbanks, 1988, Goddard &
McHugh 1990; Kollars, Oliver, Durden, & Kollars, 2000). In addition to serving as a key
food source for A. americanum, deer are also a vehicle that can transport and introduce
ticks.
Attempts to elucidate the dependence of A. americanum on white-tailed deer have used
the previously described deer exclusion strategy. Unlike I.scapularis, exclusion of white-
tailed deer from large and small plots has resulted in drastic reductions of questing A.
americanum. In a large hardwood-forest exclosure (71 ha) significant mean reductions in
larvae (-88%), nymphs (-53%), and adults (-51%) were achieved (Bloemer et al., 1990).
Comparable reductions were observed in two small (~1 ha) exclusions, where deer were

8. 8
locally post treated with acaracide (Ginsberg et al. 2002). To supplement these
experimental approaches, Mount, Haile, and Barnard (1993) constructed a computer
model by inputting known development rates, survival rates based on climatic conditions,
fecundity, and host finding success. Simulations confirmed empirical observations that
demonstrated a link between deer density and A. americanum in forested habitats.

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Species-specific habitat suitability and preference
Many attempts have been made to understand the impacts of habitat on tick population
demography (Randolph, 2004; Williams & Ward, 2010; Willis, Carter, Murdock, &
Blair, 2012). While much has been documented, especially in regard to I. scapularis, a
comparison of these two species may be helpful in identifying differences that
characterize species-specific distribution. Site assessments classify habitat types for
analysis, where statistical associations with tick presence and abundance can be
identified. Both species of tick do not display random distribution across forested habitats
and there appear to be species-specific preferences for forest type and canopy cover.
I. scapularis occur across a broad spectrum of habitat types and are abundant in
deciduous/coniferous mixed forests (Schulze, Lakat, Bowen, Parkin, & Shisler, 1984).
Further analysis discovered positive associations between tick presence and dry to mesic
deciduous forests. Absence of I. scapularis was found to correlate with grasslands and
exclusively coniferous forests (Guerra et al., 2002). Conversely, A. amblyomma are most
abundant in canopied forests with brushy understory and grasslands (Schulze & Jordan,
2005). Because of the great diversity of forest types that exist in habitats occupied by
both species of ticks (i.e. represented by the large area of overlap of their ranges), the
ability to definitively ascertain differences in habitat preference when both species are
sympatric is very difficult, especially when intraspecific preferences may exist. For
example, Koch and Burg (2006) found that A. amblyomma nymphs are more resilient to
prolonged sunlight exposure than adults, prompting them to suggest that the behavior of
each life cycle has adapted to differing habitats as to increase the possibility of finding a
host.
One approach to minimize the impact of confounding variables is to study both species
simultaneously in the same habitats and determine if significant differences in habitat
preference exist. Schulze and Jordan (2005) looked at distribution of both species within
forested habitats in New Jersey. Environmental characters were used to describe
interspecific differences. Two characters were associated with I. scapularis abundance:
forested areas where the canopy was composed of hardwood species and fragmented
forests. In areas where pitch pine dominated the canopy, I. scapularis abundance was

10. 10
substantially lower relative to hardwood and fragmented forests. A possible explanation
for the observed disparity is that the pitch-pine canopy promotes specific vegetation types
within the subcanopy; thus, percent canopy cover and the dominant tree species in a
forested area indirectly act on microclimatic conditions. Specifically, data loggers
reported that the microclimate in the pitch pine forests were much hotter and drier than
hardwood forests. In contrast to I. scapularis, the relationship between vegetation type
and abundance of questing A. americanum was inconclusive for all three parasitic life
stages. Habitat type is a common input used in many tick distribution models, however,
as the aforementioned study highlighted, large-scale determinants, like a pitch pine
dominated forest, may only indirectly influence conditions at a small scale more relevant
to ticks. Thus, focusing on the microhabitat and microclimate is pertinent to
understanding off-host tick development and survival.
Ticks spend the majority of their life cycle in leaf litter and the upper layers of soil
(Lindsay et al., 1993). Leaf litter is a critical habitat for hard ticks because it supports
conditions (i.e. high relative humidity) that are conducive to individual survival (Bertrand
& Wilson, 1996). Impacts of experimental alterations of leaf litter on tick populations
have yielded intriguing results. Allan (2009) measured deer and A. americanum
abundance in Oak-Hickory forests that were subject to intensive prescribed burns. Prior
to this report, it had been proposed that prescribed burns could be a useful tool in
managing tick populations via three mechanisms: (i) the burning directly kills ticks, (ii)
destruction of suitable microhabitat would slow reintroduction and lower tick survival
rate, and (iii) hosts may avoid the disturbed area. While population suppression was
exhibited following treatment, peak larval tick densities (>6x control areas) were
observed two years later. At the burned areas, deer densities immediately spiked. While it
is likely that the fire killed many of the ticks present in the substrate, the boon in deer
density could have been responsible for reestablishment of ticks into the burned areas.
This study suggests a lack of dependency of A. americanum on intact leaf-litter habitats
and discredits the perceived effectiveness of prescribed burns as a tool of long-term
population control.
A similar study examined the impact of prescribed burns on nymphal, larvae, and adult

11. 11
populations of I. scapularis (Stafford, Ward, & Magnarelli, 1998). Two experimental
plots were treated with fire in the spring. The first plot was only partially burned (67% of
leaf litter consumed). The second plot was completely scorched (100% of leaf litter
consumed). Compared to the control plot, nyphal abundance decreased by 74% within the
partially burned plot and 97% at the intensely burned site. A later study reported declines
ranging from 73-100% in nyphal abundance at sites where leaf litter was mechanically
removed during the spring (Shulze, Jordan, & Hung, 1995). Unlike A. americanum,
marginal population growth was observed in the years following microhabitat alteration.
This evidence supports the notion that A. americanum can occur in broader habitats and
are more resilient to microhabitat fluctuations than I. scapularis.

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Influence of microclimate on behavior and off-host survival
Temperature
Field studies of I. scapularis adults have established a minimum limit for activity at 5°C
[Figure 4](Lindsay, 1995). The empirically established temperature range for immatures,
who are most active during the spring and summer months, is 11-30°C, with an optimal
questing temperature of 26°C (Vail & Smith, 2002). Outside of these limits tick
populations cannot exist because the extreme conditions can kill individuals or inhibit
questing behavior (Ogden et al., 2004). Within this range, temperature influences
developmental rates and it has been shown that lower temperature results in longer
development and higher tick mortality (Ogden et al., 2005). In addition, lower
temperature thresholds have been established for oviposition (6.5°C) and hatching (9°C)
(Peavey & Lane, 1996). Thus, it has been hypothesized that low temperature and early
winter arrival set the northern range limit in North America. Recent observed northern
expansion into Canada associated with climate change supports this hypothesis (Ogden,
Lindsay, Morshed, Sockett, & Artsob, 2009). While local temperature is an important
environmental condition to consider when examining I. scapularis’ range, persuasive
evidence exists that suggests relative humidity as a more important limiting factor
(Stafford, 1994; Schulze, Jordan, & Hung, 2002).
Similar to I. scapularis, temperature strongly influences questing activity of adult A.
americanum (Robertson, Patrick, Semtner, & Hair, 1975). Development time of all life
stages is directly dependent on average ambient temperature (Koch, 1984). Further,
experimental studies have shown that manipulation of temperature alters the time when
nymphs and adult females feed (Barnard, Morrison, & Popham, 1985). In addition to
development and behavior, reproductive behavior and success is also influenced by local
temperature. For example, ovipositon, hatching success, incubation time, and molting
time are all associated with higher temperatures (Semtner, Sauer, & Hair, 1973; Patrick &
Hair, 1979). Thus, the life cycle of A. americanum is heavily reliant on optimal
temperature.

13. 13
Off-host survivability and reproduction is dependent on the temperature for both species,
which are tolerant to fluctuations of temperature if the changes occur within acceptable
ranges respective to each behavior (i.e. oviposition, questing, etc).
Humidity
During host attachment, feeding ticks imbibe a large volume of blood resulting in
engorgement. Excess ions and water are excreted back into the host via the salivary
glands (Bowman, Coons, Needham, & Sauer, 1997). During off-host periods, which
compose the majority of an Ixodid tick’s life cycle (i.e. ~97% for I. scapularis), great
pressure is placed on the tick’s water balance. Immatures are especially susceptible to
desiccation because of their high surface area to mass ratio. Two adaptations that occur in
hard ticks to assist in osmoregulation include: (i) an exoskeleton composed of chitin that
acts as an osmotic barrier and (ii) water can be ingested from saturated air to avoid
dehydration (Stafford, 1994; Yoder & Benoit, 2003). Thus, ticks are dependent on
moisture-rich environments where the relative humidity (RH) is consistently high.
Comparisons of both species have found interspecific differences in the ability to exist in
suboptimal climactic conditions.
I. scapularis nymps are physiologically sensitive to dry conditions and quickly perish
(<48h) when exposed to RH <85% (Adler, Telford, Wilson, & Spielman, 1992; Stafford,
1994). On a population scale, persistent drought in Illinois resulted in drastic declines in
larvae and nymphs (Jones & Kitron, 2000). While subadults are most sensitive to RH,
adult survival is also affected. For example, adults suffer higher mortality rates in open
fields, where RH is low, than in edge and forested habitats (Bertrand & Wilson, 1996). In
addition to mortality, RH influences I. scapularis’ behavior. Vail and Smith (2002)
demonstrated a significant relationship between the mean questing height of nymphs and
RH. This behavior is in response to RH gradients that occur within the leaf litter. In other
words, when the RH is >95% in the upper layers of the leaf litter, nymphs will actively
migrate to this area from the lower layers to engage in questing. Conversely, when RH is
<75% nymphs will remain in the lower layers, where RH is much higher to avoid

14. 14
desiccation. This behavior suggests that survival of I. scapularis is largely dependent on
RH.
A. americanum has a low tolerance for dehydration and water loss results in desiccation
in environments where the RH is <75% (Yoder & Benoit, 2003). In environments with
RH >75% ticks can absorb moisture from the ambient environment to balance its internal
water budget. Another adaptation used to osmoregulate is secretion of lipids that coat the
cuticle to limit water diffusion. During host attachment, cuticular wax is not secreted;
thus, water excretion through the exoskeleton is facilitated. However, immediately after
host drop off, a three-fold increase in cuticle lipid presence was observed (Yoder, Selim,
& Needham, 1997). Moreover, “fed nymphs could discriminate between low and high
relative humidity, enabling adults to conserve lipid that would otherwise be lost with the
exuvia and feces. This conservation strategy likely adds to the lipid pool needed by the
tick to survive in a dry environment and complements the tick's behavioral abilities for
seeking out optimum conditions for water conservation and host location.” Another
adaptation to maximize the viability of eggs is an impermeable chorion that limits
transpiration in suboptimal RH conditions (Yoder, Benoit, & Opaluch, 2004).
Schulze, Jordan, and Hung (2001) monitored field populations of both species to examine
the impact of climate on questing behavior. A. americanum were found to be more
tolerant of relatively dry conditions (<90% RH) because they were more abundant and
persisted in locations that were prohibitive for I. scapularis. In addition, I. scapularis
adults were crepuscular and were more abundant when RH was very high. In contrast, A.
americanum were more active in late morning and early afternoon when temperatures
where higher and RH was lower. These findings suggest that while both species prefer
warmer moisture-rich environments, RH is a more limiting factor for I. scapularis while
temperature is more limiting for A. americanum.

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Discussion
Many processes govern the abundance and spatial distribution of I. scapularis and A.
americanum in the United States. The objective of this review is to highlight interspecific
differences that may explain the current ranges of these two species. The literature
suggests that host dynamics, habitat type, and microclimatic conditions are the three most
important factors that regulate Ixodid tick range.
Host dynamics influence questing activity, blood meal acquisition, and molting success
of individuals. Moreover, migration and movement of hosts facilitate establishment of
tick populations in novel habitats. White-tailed deer are an important host for both
species. Deer are responsible for feeding many adult female I. scapularis, who require a
blood meal to reproduce. Thus, immature I. scapularis abundance is indirectly dependent
on deer density. Where deer are absent or declining, larvae and nymphs parasitize other
available hosts, especially woodland rodents, which can occur at very high population
densities. The prevailing belief that deer are critical to supporting I. scapularis population
has been challenged by a more nuanced explanation proposing a mast, rodent, tick
cascade. In contrast, A. americanum, abundance and distribution are strongly correlated
with deer abundance. Deer support exceptionally large lone-star tick burdens of each life
cycle across their range. Further, both correlational and experimental strategies have
yielded reproducible results that suggest A. americanum abundance is regulated by deer
density. In addition, deer appear to be largely responsible for dissemination of ticks into
novel habitats. It should be noted that A.americanum has not received the same intensity
of investigative inquiry that I.scapularis has and it is possible that a more complex
picture will emerge after additional review. But the available evidence asserts that A.
americanum distribution and abundance is more tightly coupled with deer density than
I.scapularis.
These two species are sympatric over a large area of their respective ranges. At a finer
scale, localized preference for differing microhabits offers insight into habitats that are
suitable for each species. Both species occur in diverse habitat types, but I. scapularis is
relatively more abundant in deciduous/coniferous mixed forests and are exist at low
levels or are absent in grasslands and coniferous forests. A. amblyomma thrive in

16. 16
canopied forests with brushy understory and grasslands but one study found that
vegetation type did not strongly influence abundance of questing A. americanum. This
observation coupled with the fact that A. americanum nymphs and larvae are more
tolerant of dry conditions and therefore can persist in locations that are prohibitive for I.
scapularis suggests that A. americanum can tolerate a broader range of microhabitats
than I. scapularis.
Development and questing activity of both tick species is more dependent on the local,
rather than atmospheric, temperature (Clark, 1995; Schulze, Jordan, Schulze, & Hung,
2009). These characteristics of the off-host period of the life cycle are largely responsible
for the survival of an individual. Attempts to understand temperature’s effect at the
microclimatic level on both species have revealed interspecific similarities: comparable
temperature thresholds (minimum, maximum, optimal), association of higher
temperatures with quicker growth and reproductive success. Given the available
literature, each species have similar temperature requirements. The recent northward
expansion of I. scapularis and A. americanum, temperature is a range-limiting factor for
Ixodid ticks in North America.
Off-host Ixodid ticks must balance host-questing with maintaining an adequate water
balance. Adaptations to avoid desiccation are present in both species, however, prolonged
exposure to conditions where RH is low will invariably result in death. I. scapularis
cannot tolerate RH <85% for >48 hours, whereas, A. americanum cannot tolerate RH
<75% for >48 hours. This difference of RH tolerance is of great importance and
influences questing behavior and the ability to occur in drier habitats. I. scapularis are
more active during the morning and dusk- time of day when RH is typically the highest.
A. americanum were more active in late morning and early afternoon when temperatures
where higher and RH was lower. These observations support the hypothesis that I.
scapularis is more sensitive to RH than A. americanum.
The distribution and abundance of epidemiologically relevant ticks, and the establishment
of new populations, is constrained by biotic factors (host densities, habitat type,
microhabitat) and abiotic factors (microclimate). These constraints affect survival rates,
influencing the densities of tick populations and their range across North America.
Progress in understanding the interspecific differences that influence I. scapularis and A.

17. 17
americanum have been made, but more research, aimed specifically at sympatric
populations will likely yield knowledge that is helpful to natural resource managers and
public health officials. Currently, anthropogenic changes (climate change, habitat
fragmentation, invasive species introduction) and their influence on tick distribution and
abundance is poorly understood. Evaluations of these changes must be included in order
to gain a holistic understanding of tick populations in order to mitigate disease
transmission risk.

18. 18
Appendix
Figure 1. Spatial distribution of Amblyomma americanum in the United States (Springer,
Eisen, Beati, James & Eisen, 2014).

19. 19
Figure 2. Spatial distribution of Ixodes scapularis in the United States (Brownstein,
Holford, and Fish, 2003).

20. 20
Figure 3. (A) Adult I. scapularis abundance collected in October relative to rat (circle)
and deer abundance on Monhegan Island. (B) Sub-adult I. scapularis removed from
Norway rats in July (Rand, Lubelczyk, Holman, Lacombe, & Smith, 2004).

21. 21
Figure 4. Relationship between temperature and questing activity of I. scapularis. Adult
data adapted from Lindsay (1995) and immature data adapted from Vail & Smith (2002).
Assembled by Ogden et al (2005).

22. 22
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