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Urban Warming Drives Insect Pest Abundance on Street
Trees
Emily K. Meineke
1
*, Robert R. Dunn
2
, Joseph O. Sexton
3
, Steven D. Frank
1Department of Entomology, North Carolina State University, Raleigh, North Carolina, United States of America, 2Department of Biology, North Carolina State University,
Raleigh, North Carolina, United States of America, 3Department of Geography, University of Maryland, College Park, Maryland, United States of America
Abstract
Cities profoundly alter biological communities, favoring some species over others, though the mechanisms that govern
these changes are largely unknown. Herbivorous arthropod pests are often more abundant in urban than in rural areas, and
urban outbreaks have been attributed to reduced control by predators and parasitoids and to increased susceptibility of
stressed urban plants. These hypotheses, however, leave many outbreaks unexplained and fail to predict variation in pest
abundance within cities. Here we show that the abundance of a common insect pest is positively related to temperature
even when controlling for other habitat characteristics. The scale insect Parthenolecanium quercifex was 13 times more
abundant on willow oak trees in the hottest parts of Raleigh, NC, in the southeastern United States, than in cooler areas,
though parasitism rates were similar. We further separated the effects of heat from those of natural enemies and plant
quality in a greenhouse reciprocal transplant experiment. P. quercifex collected from hot urban trees became more
abundant in hot greenhouses than in cool greenhouses, whereas the abundance of P. quercifex collected from cooler urban
trees remained low in hot and cool greenhouses. Parthenolecanium quercifex living in urban hot spots succeed with
warming, and they do so because some demes have either acclimatized or adapted to high temperatures. Our results
provide the first evidence that heat can be a key driver of insect pest outbreaks on urban trees. Since urban warming is
similar in magnitude to global warming predicted in the next 50 years, pest abundance on city trees may foreshadow
widespread outbreaks as natural forests also grow warmer.
Citation: Meineke EK, Dunn RR, Sexton JO, Frank SD (2013) Urban Warming Drives Insect Pest Abundance on Street Trees. PLoS ONE 8(3): e59687. doi:10.1371/
journal.pone.0059687
Editor: Ben Bond-Lamberty, DOE Pacific Northwest National Laboratory, United States of America
Received January 16, 2013; Accepted February 16, 2013; Published March 27, 2013
Copyright: ß2013 Meineke et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by a grant from the USGS Southeast Regional Climate Science Center to RRD and SDF. RRD was also supported by NASA
Biodiversity Grant (ROSES-NNX09AK22G) and an NSF Career grant (0953390). SDF was also supported by grants from USDA Southern Region IPM (2010-02678),
North Carolina Nursery and Landscape Association, the Horticultural Research Institute, and the USDA IR-4 Project. EKM was also funded by the NCSU Department
of Entomology and an EPA STAR Fellowship. (URLs: http://www.epa.gov/ncer/fellow/; http://ir4.rutgers.edu; http://www.doi.gov/csc/southeast/index.cfm; http://
cce.nasa.gov/cce/biodiversity.htm; http://ww w.nsf.gov/funding/pgm_summ.jsp?pims_id = 503214; http://www.cals.ncsu.edu/entomolog y/; http://www.csrees.
usda.gov/funding/rfas/ipm_southern.html; http://www.hriresearch.org; http://www.ncnla.com). The funders had no role in study design, data collection and
analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: ekmeinek@ncsu.edu
Introduction
Urban areas are generally hotter than surrounding rural areas
[1]. This ‘‘urban heat island effect’’ results from the presence of
less vegetation cover [2] and greater impervious surface cover [3]
in cities compared to rural or natural areas [1]. Although urban
warming was first noted in 1833 [4], the effects of heat on animal
abundance and community characteristics in cities remain largely
unknown. Instead, studies have emphasized the roles of habitat
connectivity [5], [6] and resource availability [1], [7] in shaping
urban animal communities. The effects of temperature deserve
further attention because urban warming is becoming more
extensive and more extreme as cities grow larger and is now
coupled with global warming [8].
High urban temperatures should have the most pronounced
effects on ectotherms, because thermal accumulation drives
development in many ectothermic species [9]. Insects are of
particular interest as the most diverse ectothermic taxon and
because of their ecological and economic importance as pollinators
[10], disease vectors [11], and plant pests [12]. Herbivorous insect
pests are often more abundant in urban than in rural areas, though
the proposed mechanisms for this pattern–changes in host plant
quality [13], [14] and natural enemy efficacy [15]–do not
consistently explain higher herbivorous insect pest abundance
[16]. We hypothesize that the urban heat island effect is the most
important driver of higher insect pest abundance in cities.
To test this hypothesis, we investigated the effects of urban
warming on the biology of the soft scale insect Parthenolecanium
quercifex. As a group, scale insects are among the most important
pests of forest and landscape trees and are closely related to many
other pests such as aphids and whiteflies. They are also sedentary
and, thus, subject to the full effects of urban warming. We
therefore selected P. quercifex, a common scale insect pest of oaks,
as a study organism to test four specific hypotheses. First, we
expected urban warming to increase P. quercifex abundance. Our
approach to testing this hypothesis differs from that of other
studies because we sampled scale insects on warm and cold trees
within the city rather than comparing urban to surrounding rural
areas [17], [7]. Second, we hypothesized that urban warming
increases P. quercifex abundance by decreasing parasitism. To test
this hypothesis, we measured percent parasitism [18] of P. quercifex
in hot and cold sites. Third, we tested the hypothesis that urban
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1
warming increases P. quercifex abundance by increasing P. quercifex
fecundity. This is a common physiological response to warming in
ectotherms [19], [20], at least when warming pushes them toward
their thermal optimum rather than beyond it [21]. Finally, we
hypothesized that P. quercifex response to warming depends on
thermal origin, such that P. quercifex from warmer areas have
a physiological or adaptive advantage over individuals from cooler
areas when placed in hot conditions. To test this hypothesis, we
collected P. quercifex from warmer and cooler urban environments
and placed them in warmer and cooler greenhouses. Because this
common garden experiment provided trees with equal water and
nutrients, we controlled for host plant quality, the other most
common hypothesis for why herbivorous insect pests are more
abundant in urban than in rural areas.
Methods
Study Organism
Soft scale insects (Hemiptera: Coccidae) are phloem-feeders on
perennial plants [22]. They are commonly more abundant in cities
than in rural areas [15,16]. Parthenolecanium quercifex is an oak pest
that has one generation per year and is native to North Carolina
and much of North America [22]. Adults produce eggs in the late
spring, usually in May [23]. Gravid females lay a dozen to several
thousand eggs in an ovisac [22]. First instars migrate from ovisacs
to leaves and feed on phloem throughout summer [22], [23]. In
fall they molt and migrate back to tree stems [23]. Second instars
overwinter and undergo development into adults in the early
spring [23].
Study Location
Raleigh has a humid subtropical climate, and the city center is
located at 35.772096uN 78.638614uW. The average long-term
winter temperature is 5.8uC. The average long-term summer
temperature is 25.6uC. The average annual rainfall is 116.9 cm.
Climate data were retrieved from the NOAA National Climatic
Data Center (NCDC) (www.ncdc.noaa.gov) from the North
Carolina State University weather station as 1981–2010 station
normals.
Hypothesis 1) Urban Warming Increases P. quercifex
Abundance
We used thermal maps overlaid with maps of willow oak
locations in ArcMap (ArcGIS Desktop 10, Redlands, CA) to locate
study sites. To create thermal maps, winter and summer
temperature measurements of the study area were extracted from
the 120-m thermal band (Band 6) of Landsat-5 World Reference
System 2 (WRS-2) path 16, row 35 images acquired on December
12, 2005 (winter) and August 18, 2007 (summer). The summer and
winter multi-spectral images were geometrically rectified by
polynomial transformation with nearest-neighbor resampling to
1-meter resolution, panchromatic digital orthorectified photo-
graphs acquired in March and April 1993, archived by the North
Carolina Department of Transportation. The thermal-band
images were then converted from 8-bit storage values to at-
satellite brightness temperature (uC). Clouds and snow were
identified visually using combinations of all seven spectral bands
and removed manually.
We identified 20 of the hottest (‘‘hot’’) and 20 of the coldest
(‘‘cold’’) sites with at least two willow oak trees (Figure 1) in
Durham, NC (1 site) and Raleigh, NC (39 sites). All sites were
located in urbanized locations to minimize habitat related
differences in natural enemy communities and host plant quality
that might affect scale abundance. Each site was at least 200
meters away from any other site. This study was approved by the
Raleigh Parks and Recreation Department, and all sites were
located on public land except one site, which was located at
a residence. Here, sampling was permitted by the homeowner.
Sampling at all other sites was approved by the Raleigh Parks and
Recreation Department.
We sampled 2
nd
instar scale insects by collecting terminal
30.5 cm branches from each cardinal direction of study trees in
January and February 2011 using a pole pruner. In the laboratory
we counted 2
nd
instar P. quercifex using a dissecting scope. We
calculated mean scale insect abundance per branch on each tree.
We then summed these values and divided them by the number of
trees at each site (2) to generate a single insect-per-branch
abundance value for each site. We compared mean scale
abundance hot and cold sites with a t-test in SAS (SAS 9.1, Cary,
NC).
Between April 20th and 29th, 2011, we sampled P. quercifex
ovisacs by collecting the terminal 30.5 cm of one branch per tree
at 6 hot sites and 5 cold sites (12 hot trees and 10 cold trees). To
choose our study trees, we randomly selected individuals from the
subset of trees occupied by 2
nd
instar P. quercifex in our first sample.
We selected trees occupied by P. quercifex to be sure higher
abundance was due to differences in population growth rather
than differences in colonization between hot and cold sites. Data
did not meet ANOVA assumptions, even after log transformation
with log(x+1), so we compared ovisac abundance per 30.5 cm
between hot and cold sites with a Kruskal-Wallis Test in SAS (SAS
9.1, Cary, NC).
Between May 20th and 25th, 2011, we sampled 1
st
instar scales
on the same trees from which we sampled ovisacs by counting
individuals on 10 leaves per study tree. We calculated mean
abundance per 10 leaves on the two trees at each site. We
compared log(x+1) transformed mean 1
st
instar abundance on 10
leaves between hot and cold sites with a t-test in SAS (SAS 9.1,
Cary, NC).
To measure temperature differences between hot and cold sites,
we placed ibutton thermachrons (Dallas Semiconductor of Dallas,
TX) that recorded temperature 6 times per day at a subset of sites
(5 hot, 6 cold). We placed thermachrons in ibutton wall mounts
(Dallas Semiconductor of Dallas, TX) inside a 2.54-cm deep
plastic cup to protect them from precipitation and direct sun.
Thermachrons were in place from May until August 2011. We
calculated daily mean and maximum temperatures in each
treatment. We then compared average mean and average
maximum daily temperatures at hot and cold sites using a repeated
measures ANOVA in SAS (SAS 9.1, Cary, NC).
Hypothesis 2) Urban Warming Increases P. quercifex
Abundance by Decreasing Parasitism
To test for the influence of warming on parasitoids and
subsequent effects of parasitism on P. quercifex abundance, we
collected one branch with 20 or more P. quercifex individuals from
the same trees from which we sampled 1
st
instars and ovisacs on
five sampling dates while the scale were developing and laying eggs
(March 7, April 22, April 29, May 20, and May 27, 2011). We
dissected 20 individuals per branch for parasitoid larvae and
marked each individual as parasitized or not parasitized. We
calculated mean percent parasitism at each site on each date. We
compared mean percent parasitism between hot and cold sites
using a repeated measures ANOVA in SAS (SAS 9.1, Cary, NC).
To identify parasitoids that attack P. quercifex in Raleigh, we
clipped P. quercifex infested branches, removed all other arthro-
pods, and placed them in cotton-plugged vials on each date. We
reared out parasitoids from March to August 2012 in an incubator
Urban Warming Drives Abundance of an Insect Pest
PLOS ONE | www.plosone.org 2 March 2013 | Volume 8 | Issue 3 | e59687
at 23uC, 50% humidity, and a 12 hr/12 hr light-dark cycle. We
point-mounted each parasitoid that emerged and identified it to
genus or species.
Hypothesis 3) Urban Warming Increases P. quercifex
Abundance by Increasing P. quercifex Fecundity
To determine whether P. quercifex fecundity differed between
hot and cold sites, we collected 2 ovisacs from the same trees
used to assess ovisac and 1
st
instar abundance on April 29
th
,
2011. Ovisacs were returned to the laboratory in a cooler
within 2 hours of collection. We emptied the eggs from each
ovisac into a separate petri dish filled with 10 ml of 80%
ethanol. We took a picture of each petri dish containing eggs
using a Canon EOS DS126071 Rebel XT camera with a Canon
EF-S 60-mm Macro lens. We used ImageJ (ImageJ 1.45 m,
Bethesda, MD) to count the particles (eggs) in each image and
the total area of those particles. To avoid counting multiple eggs
as one, we used Image J to calculate the areas of ten eggs,
found the mean of those areas, and divided the total egg area in
each petri dish by the mean area of a single egg to get an egg
count for each ovisac. We calculated mean egg counts for each
ovisac at each site. Then we calculated mean egg count per tree
and mean egg count per site. We compared mean egg count
per ovisac between hot and cold sites with a t-test in SAS (SAS
9.1, Cary, NC).
Hypothesis 4) Parthenolecanium quercifex Response to
Warming Depends on Thermal Origin
To further isolate the effects of temperature from other biotic
and abiotic effects on P. quercifex abundance and to test how P.
quercifex origin affects response to temperature, we conducted
a common garden experiment with a 2 by 2 factorial design,
wherein we reared scales originating from hot and cold sites in
hot (36uC day–18:00–6:00/32uC night–6:00–18:00) and cold
(32uC day–18:00–6:00/28uC night–6:00–18:00) greenhouses.
When scale matured in April 2011, we collected 4 ovisacs
from a subset of our study trees (10 hot and 10 cold). We
attached two ovisacs to each of 40 willow oak saplings in
greenhouses at the NCSU phytotron facility in the two
temperature treatments. Bare root willow oak saplings
(1.0460.02 m) were purchased from Rennerwood, Inc (Tennes-
see Colony, TX) and grown in 20.3 cm pots in Fafard 2P
potting mix (Agawam, MA). They were fertilized 3 times per
week with nutrient solution (N-P-K 10.2-1-10.7) mixed in the
Figure 1. Thermal image overlaid with
Parthenolecanium quercifex
abundance across the Raleigh, NC urban heat island. 2
nd
instar P.
quercifex abundance across the Raleigh, NC urban heat island. Dots represent relative 2
nd
instar P. quercifex abundance per 30.5 cm stem at each hot
(red) and cold (blue) site (n = 40) in winter 2011. The image is a thermal map of the Raleigh, NC urban heat island created from a Landsat image
acquired on August 18, 2007.
doi:10.1371/journal.pone.0059687.g001
Urban Warming Drives Abundance of an Insect Pest
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NCSU phytotron (http://www.ncsu.edu/phytotron/manual.pdf,
pp. 15–16) and watered once per day. The potting media in
both treatments was kept moist to ensure that high temperature
did not result in water deficiency. Two weeks before infestation,
saplings were treated with Tau Fluvalinate (Mavrik, Aquaflow)
1 mL/L H
2
0 to ensure no other insects were being transported
into the greenhouses.
After egg hatch in April 2011, we counted settled first instar
nymphs on 10 leaves per tree on May 10, 17, 26, and July 15,
2011. We used repeated measures ANOVA in SAS to compare 1
st
instar abundance per 10 leaves among treatments.
Results
Hypothesis 1) Urban Warming Increases P. quercifex
Abundance
We found that overwintering second instars were 13 times more
abundant on hot than on cold trees (t
38
= 2.90, P= 0.006; Figures 1
and 2A). In April 2011, ovisacs deposited by the same generation
were 5.5 times more abundant on hot trees (X
21
= 6.53, P= 0.011;
Figure 2C). In June 2011, the next generation of 1
st
instars was
over 7 times more abundant on hot than cold trees (t
9
= 2.46,
P=0.043; Figure 2B).
There was a significant interaction between site temperature
and time, wherein the extent of the differences in mean average
temperatures (F
112, 1120
= 1.96, P,0.0001) between hot and cold
sites depended on time of year. Similarly, the interaction between
site temperature and time was marginally significant for mean
maximum temperatures (F
112, 1120
= 1.23, P= 0.0583). Mean
average hot site temperatures were between 0–2.4uC higher than
mean average temperature at cold site temperatures (F
1, 10
= 7.90,
P= 0.0185; Figure 3A), and mean maximum daily temperatures at
hot sites were between 0–3.8uC warmer than mean maximum
daily temperatures at cold sites (F
1, 10
= 6.42, P= 0.0297;
Figure 3B).
Hypothesis 2) Urban Warming Increases P. quercifex
Abundance by Decreasing Parasitism
We reared six parasitoid species from P. quercifex:Coccophagus
lycimnia Walker (Hymenoptera: Aphelinidae), Pachyneuron altiscutum
Howard (Hymenoptera: Pteromalidae), Eunotus lividus Ashmead
(Hymenoptera: Pteromalidae), Encyrtus fuscus Howard (Hymenop-
tera: Encyrtidae), Blastothrix sp. Mayr (Hymenoptera: Encyrtidae),
and Metaphycus sp. Mercet (Hymenoptera: Encyrtidae). Percent
parasitism did not differ between P. quercifex from hot and cold sites
(F
1, 6.45
= 0.21, P= 0.6631; Figure 4).
Hypothesis 3) Urban Warming Increases P. quercifex
Abundance by Increasing P. quercifex Fecundity
The number of eggs in ovisacs from hot and cold sites did not
differ (t
9
= 1.87, P= 0.094).
Hypothesis 4) P. quercifex Response to Warming Depends
on Thermal Origin
The effect of greenhouse temperature on scale abundance
depended on scale origin, such that P. quercifex collected from hot
trees reared in hot greenhouses were over twice as abundant as P.
quercifex in any other treatment (F
1, 134
= 11.57, p-value = 0.0009;
Table 1, Figure 5). P. quercifex from cold trees did not become more
abundant when reared in hot greenhouses. In the cold greenhouse,
P. quercifex from hot trees were significantly more abundant than P.
quercifex from cold trees; still, they were less than half as abundant
as in hot greenhouses.
Discussion
We found urban warming directly leads to higher P. quercifex
abundance. While the two most common hypotheses for elevated
pest abundance in cities are changes in host plant quality and
natural enemy efficacy [16], we found no evidence that either of
these factors contribute to P. quercifex abundance patterns across
the Raleigh, NC urban heat island. We also found no evidence
that urban warming directly affects P. quercifex fecundity. Instead,
we found evidence that P. quercifex populations may be locally
Figure 2.
Parthenolecanium quercifex
abundance across the
Raleigh, NC urban heat island. Abundance of P. quercifex on hot
and cold urban trees. Bars represent the mean (6SEM) abundance of (A)
2
nd
instars in winter 2011 (n = 40); (B) 1
st
instars in June 2011 (n = 11);
and (C) ovisacs in spring 2011 (n = 11) on 30.5-cm terminal branches of
hot (red) and cold (blue) urban trees in Raleigh, NC.
doi:10.1371/journal.pone.0059687.g002
Urban Warming Drives Abundance of an Insect Pest
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adapted, or individuals acclimatized, to the temperature of the
urban habitat patches in which they reside.
Urban trees are frequently stressed due to lack of water and
nutrients [24], [25]. In some cases, stress can reduce tree defenses,
leading to higher herbivore abundance [26]. Because our study
sites were all in urban habitats, we have no reason to believe that
nutrient levels available to trees covaried with temperature. It is
conceivable that warm trees are more water stressed, and such
a possibility deserves study. However, water stress tends to lead to
decreases in the abundance of piercing-sucking herbivores [27],
[28], which suggests that water stress should lead to lower P.
quercifex abundance in hot urban areas. We observe the opposite
pattern. Additionally, in our common garden experiment, we
watered trees daily and provided equal nutrients to all trees to
minimize any effects of water or nutrient stress. It is unlikely that
differences in tree stress or quality account for the difference in
scale abundance between hot and cold sites.
Natural enemies are often less abundant and diverse in urban
than rural habitats. This difference has been cited to explain
higher pest abundance in cities [16], [15]. All our study sites were
within urban habitats, so–given that natural enemies tend to be
relatively good dispersers [29], [30] –natural enemy communities
should be similar among trees. As such, it is not surprising that we
did not find a difference in percent parasitism between P. quercifex
from hot and cold sites. Differences in parasitoid efficacy do not
account for greater P. quercifex abundance on hot trees, as percent
parasitism of P. quercifex on hot trees was equal to that of cold trees.
Additionally, P. quercifex was more abundant in hot chambers in
our greenhouse experiment, which excluded natural enemies.
Thus, reduction of biological control by parasitoids does not
explain high scale abundance at hot sites.
Our common garden experiment shows that P. quercifex is locally
acclimated or adapted to urban thermal conditions and that this
directly leads to higher abundance. P. quercifex from hot urban
areas became almost 4 times more abundant than those from cold
urban areas when placed in hot greenhouses. This effect is likely
due to differences in survival, because we found no differences in
fecundity between P. quercifex from hot and cold sites. We suggest
that P. quercifex may locally adapt in response to urban warming, as
other studies provide evidence for local adaptation in scale insects
[31], [32]. The scale insect life cycle, which is often parthenoge-
netic and highly localized, inhibits gene flow [33], and evidence
suggests this could lead to differentiation at small spatial scales
Figure 3. Average and maximum temperature differences
between hot and cold sites. Temperatures recorded on ibuttons
at ‘hot’ and ‘cold’ sites in Raleigh, NC May 2, 2011- August 23, 2011.
Dots represent mean (6SEM) a) average daily temperature (uC) and b)
mean maximum daily temperature at hot and cold sites. Average daily
mean temperatures were significantly higher at hot sites (F
1, 10
= 7.90,
P= 0.0185), as were mean daily maximum temperatures (F
1, 10
= 6.42,
P= 0.0297). The extent of the difference between average (F
112,
1120
= 1.96, P,0.0001) and maximum daily temperatures (F
112,
1120
= 1.23, P= 0.0583) depended on time of year.
doi:10.1371/journal.pone.0059687.g003
Figure 4. Percent parasitism of
P. quercifex
on hot and cold
urban trees. Bars represent the mean (6SEM) percent of dissected 2
nd
instars, adults, and ovisacs that had been parasitized on hot (red) and
cold (blue) urban trees in Raleigh, NC on four dates in 2011.
Temperature treatment had no significant effect on percent parasitism
(F
1, 6.45
= 0.21, P= 0.6631, n = 11).
doi:10.1371/journal.pone.0059687.g004
Table 1. Statistics for repeated measures ANOVA of P.
quercifex abundance in common garden experiment. (An *
denotes an interaction.).
Effect Ndf, Ddf
FP
Date 3, 134 0.35 0.7867
Source temp. 1, 134 46.57 ,0.0001
Date* Source temp. 3, 134 0.04 0.9891
Greenhouse 1, 134 31.65 ,0.0001
Date* Greenhouse 3, 134 0.67 0.5698
Source temp.* Greenhouse 1, 134 11.57 0.0009
Date* Source temp. * Greenhouse 3, 134 0.01 0.9987
doi:10.1371/journal.pone.0059687.t001
Urban Warming Drives Abundance of an Insect Pest
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[34]. Further, at least one other scale insects species has been
shown to adapt to thermal conditions within its introduced range
[35]. However, we cannot eliminate the possibility that observed
abundance patterns resulted from maternal effects [36] or
phenotypic plasticity of offspring leading to acclimation [37],
rather than from genetic differences between P. quercifex from
hotter and colder areas [38]. While the specific mechanism by
which warming increases P. quercifex abundance warrants further
investigation, our findings show that P. quercifex are primed to
survive better in response to warming, be it urban or global.
For more than a century, scientists have documented that
arthropod pests, including scale insects [39], are more abundant
on urban trees than rural trees [16]. We provide evidence that
urban heat may explain this effect, and we show that small
temperature differences predict changes of an order of magnitude
in pest abundance. We observed this effect over a temperature
gradient common in many urban heat islands [1], indicating that
urban warming poses a broad and immediate threat to urban trees
and the services they provide, including cooling and carbon
sequestration [2]. The adaptation or acclimation of herbivorous
pests to warm environments may represent an ecological tipping
point after which arthropod pests can overwhelm plant defenses
and escape natural enemy control. Furthermore, temperature
increases of similar magnitude are predicted under global climate
change [40]. If rising global temperatures trigger an herbivore
response similar to the one we observed in the city, then both
urban and rural trees may be threatened by greatly increased
herbivory in the future.
Acknowledgments
Elsa Youngsteadt provided comments on the manuscript. Sally Thigpen
provided tree maps and other assistance finding study sites. Adam Dale
assisted with data collection and management. David Stephan assisted with
scale identification. This study was approved by the Raleigh Parks and
Recreation Department.
Author Contributions
Conceived and designed the experiments: EKM RRD SDF. Performed the
experiments: EKM SDF. Analyzed the data: EKM SDF JOS. Contributed
reagents/materials/analysis tools: EKM SDF JOS RRD. Wrote the paper:
EKM SDF JOS RRD.
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doi:10.1371/journal.pone.0059687.g005
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