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Urban Warming Drives Insect Pest Abundance on Street Trees

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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.
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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
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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
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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
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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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Urban Warming Drives Abundance of an Insect Pest
PLOS ONE | www.plosone.org 7 March 2013 | Volume 8 | Issue 3 | e59687
... Near-ground temperature had no effect on taxonomic or functional diversity, but was negatively associated with the number of trapped individuals (24%). This result Fig. 4 Relative variance explained independently by landscape-and local-scale predictors according to GLMM models for spiders' a number of trapped individuals, b species diversity (orders Q = 0, 1 and 2) and c functional diversity indices (orders Q = 0, 1 and 2) is in line with several recent studies that identified urban near-ground temperature as a prevailing driver of arthropod abundance (Hamblin et al. 2018;McGlynn et al. 2019;Meineke et al. 2013). Yet, unlike several studies reporting an increased abundance of phytophagous arthropods with temperature increase Frank 2014, 2018;Meineke et al. 2013), our results show a negative relationship between the number of trapped individuals and near-ground temperature. ...
... This result Fig. 4 Relative variance explained independently by landscape-and local-scale predictors according to GLMM models for spiders' a number of trapped individuals, b species diversity (orders Q = 0, 1 and 2) and c functional diversity indices (orders Q = 0, 1 and 2) is in line with several recent studies that identified urban near-ground temperature as a prevailing driver of arthropod abundance (Hamblin et al. 2018;McGlynn et al. 2019;Meineke et al. 2013). Yet, unlike several studies reporting an increased abundance of phytophagous arthropods with temperature increase Frank 2014, 2018;Meineke et al. 2013), our results show a negative relationship between the number of trapped individuals and near-ground temperature. Although phytophagous arthropods and predator spiders display opposite patterns to urban thermal conditions, our result is supported by recent studies on flying arthropods (Hamblin et al. 2018;McGlynn et al. 2019), showing that temperature is associated with decreased abundance (but see Geppert et al. 2023). ...
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Urbanisation is an ongoing process associated with multiple environmental changes affecting ecosystems worldwide. Temperature and habitat are main drivers of animal communities within cities, but quantifying their relative weights remains a challenge, as urban heat islands (UHI) often co-vary with land-cover. This study aims to disentangle the effects and relevant scale of temperature and habitat on ground-dwelling spider communities. Based on an original sampling design, we collected 20,761 spider individuals belonging to 137 species at 36 sampling sites in the city of Rennes (northwest France). We characterised communities by assessing the number of trapped individuals at each site, as well as calculating several metrics to estimate taxonomic and functional diversities. Temperature metrics were obtained from two sensor networks monitoring UHI (100-m resolution) and near-ground temperature (1-m resolution) independently. Land-cover and isolation were used to describe landscapes, and vegetation structure to describe local habitats. We used generalized linear mixed models to disentangle the effects of temperature from those of habitat at the landscape and local scales, and identified relationships between community descriptors and predictors. We show that temperature-related metrics are important predictors of spider communities, and that the landscape and local scales have independent effects. Near-ground temperature alone explained 24% of the number of trapped individuals, whereas UHI explained 20% of taxonomic diversity. Local vegetation height and cover were significant predictors of functional diversity, and explained 22% and 25% of variance, respectively. We conclude that locally applied planning measures could mitigate the loss of taxonomic diversity induced by the atmospheric UHI and promote the establishment of more diverse communities.
... The manifestation of the luxury effect would also depend on the background ecosystem where a city develops. A strong relationship between economic wealth and biodiversity exists in regions with water limitations, where the main factor driving biodiversity differences is irrigation due to water being a limiting variable for vegetation growth [23][24][25]. In cities located in Mediterranean and semi-arid climates, where vegetation is limited and there are higher urban heat effects, increased water availability leads to higher biodiversity [25]. ...
... A strong relationship between economic wealth and biodiversity exists in regions with water limitations, where the main factor driving biodiversity differences is irrigation due to water being a limiting variable for vegetation growth [23][24][25]. In cities located in Mediterranean and semi-arid climates, where vegetation is limited and there are higher urban heat effects, increased water availability leads to higher biodiversity [25]. As a result of climate change these low precipitation areas will become drier [26], which could intensify the luxury effect in Mediterranean and semi-arid areas in the future. ...
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Urban trees enhance biodiversity, provide ecosystem services, and improve quality of life in cities. Despite their benefits, trees are not distributed equitably, and many cities exhibit a “luxury effect”. Given the importance of public green space for providing access to urban tree benefits, we investigated the relationship between socioeconomic level and tree diversity and structure in 60 green areas in Santiago de Chile. Species richness and total tree abundance did not significantly vary among socioeconomic levels; however, a differential effect was found according to species origin. Introduced tree species exhibited similar abundance and species richness across socioeconomic levels, but native tree species were more abundant and richer in higher socioeconomic level areas compared to lower ones. Tree cover was higher in the high and medium socioeconomic level areas than in the low socioeconomic level area. A higher average DBH was found in the medium socioeconomic level area, which may be explained by older neighborhoods and a legacy of the luxury effect. Our findings reveal that socioeconomic groups are associated with differences in tree cover, width, and the number of native species in public green areas. Consequently, urban residents have different provisions of ecosystem services and opportunities to interact with natural heritage. Increasing the amount of tree cover and native species available to vulnerable groups will reduce disparities.
... L. emarginatus exploits a wide range of resources, which includes hunting or scavenging dead insects as well as consuming plant sap, nectar, and elaiosomes found on seeds (Seifert 2018). Within New York in particular, L. emarginatus forms mutualisms with honeydew producing insects, such as aphids and scale insects, which are common in urban street trees and may provide an abundant a reliable food source (Meineke et al. 2013;Seifert 2018). ...
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approaches to confirm the identity of the Manhat-tAnt and characterize its introduced range and colony structure. Results from genetic analyses placed the ManhattAnt within the Lasius emarginatus species complex, and morphological comparisons ruled out closely related species to identify the ManhattAnt as the European ant L. emarginatus with 93-99% certainty depending on the nest sample. Since its initial discovery in the region, the ManhattAnt has become one of the most common ants in New York City and has spread at a rate of 2 km/yr into New Jersey and onto Long Island. Based on the climate it inhabits within its native range, L. emarginatus could expand to cover much of the eastern United states from Massachusetts to Georgia. Although many successful invasive ants display supercoloniality with little aggression between neighboring nests, we found no evidence that L. emarginatus colonies in New York City are supercolonial. Continued monitoring of L. emarginatus is warranted, as it has been increasingly reported as an indoor pest and is known to form mutualisms with honeydew producing pests of street trees.
... This is why the distribution of this species is mostly linked to temperate areas, such as coasts, or areas where human activity has modified the environment, like urban spots, which are key to its expansion (Touyama et al. 2003;Holway & Suarez 2006;Stringer et al. 2009). Ants are highly adaptable to urban environments (Angilletta et al. 2007;Buczkowski & Richmond 2012) and cities facilitate the expansion of nonnative species (Grimm et al. 2008;Lahr et al. 2018), due to their generalist nature (Ducatez et al. 2018) or pre-adaptations to heat or anthropized environments, as in the case of scale insects (Meineke et al. 2013) or some butterflies (Kaiser et al. 2016). ...
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Urban environments often present environmental conditions that facilitate the introduction and establishment of nonnative and invasive species. These can expand their range into areas with unfavorable climates by taking advantage of the ecological and climatic homogenization of cities, bypassing the ecological barriers presented by the surrounding environment. One way to monitor the expansion of these species is using potential distribution models. We used as a model species the Argentine ant, Linepithema humile (Hymenoptera: Formicidae) whose invasion has caused serious consequences for biodiversity and economic losses worldwide. We used the average result of six different algorithms and used climatic variables and population density as a proxy for the urbanization level in the Western Palearctic to build the predictive model. The model indicates this ant prefers to inhabit areas with Mediterranean and Temperate Oceanic climates and that its suitability depends on two main factors: the continentality (temperature annual range) and the degree of urbanization. The species is predicted to be absent in areas with large temperature contrasts throughout the year, particularly in rural and peri-urban areas (i.e. adjacent to urban areas) of inland regions. Conversely, the species has a predilection for coastal and urban areas where environmental conditions are attenuated by the influence of the sea or the “urban heat island” effect in the case of inland cities. In this sense, cities act as “bioclimatic islands” facilitating the establishment of the Argentine ant as a reservoir, enlarging its distribution into climatically nonoptimal areas, and promoting its future expansion in a scenario of global warming and socioeconomic change.
... L. emarginatus exploits a wide range of resources, which includes hunting or scavenging dead insects as well as consuming plant sap, nectar, and elaiosomes found on seeds (Seifert 2018). Within New York in particular, L. emarginatus forms mutualisms with honeydew producing insects, such as aphids and scale insects, which are common in urban street trees and may provide an abundant a reliable food source (Meineke et al. 2013;Seifert 2018). ...
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An unidentified ant species was discovered in the heart of New York City in 2011, which garnered national headlines and created the memorable nickname “ManhattAnt.” New York City is one of the oldest and largest cities in North America and has been the site of introduction for some of North America’s most damaging invasive pests. Nevertheless, there has been little follow up research on the ManhattAnt since its discovery, and it has yet to be formally identified. Here we use genetic and morphological approaches to confirm the identity of the ManhattAnt and characterize its introduced range and colony structure. Results from genetic analyses placed the ManhattAnt within the Lasius emarginatus species complex, and morphological comparisons ruled out closely related species to identify the ManhattAnt as the European ant L. emarginatus with 93–99% certainty depending on the nest sample. Since its initial discovery in the region, the ManhattAnt has become one of the most common ants in New York City and has spread at a rate of 2 km/yr into New Jersey and onto Long Island. Based on the climate it inhabits within its native range, L. emarginatus could expand to cover much of the eastern United states from Massachusetts to Georgia. Although many successful invasive ants display supercoloniality with little aggression between neighboring nests, we found no evidence that L. emarginatus colonies in New York City are supercolonial. Continued monitoring of L. emarginatus is warranted, as it has been increasingly reported as an indoor pest and is known to form mutualisms with honeydew producing pests of street trees.
... Predictors Meineke et al., 2013;Gunton and Pöyry, 2016;Yang et al., 2018). As an example, Galfrascoli et al. (2022) observed that seed predation of the shrub Vachellia caven and the parasitism rate of the seed predator (Pseudopachymerina spinipes) were significantly reduced with increasing urbanization. ...
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Premise During the last centuries, the area covered by urban landscapes is increasing all over the world. Urbanization can change local habitats and decrease connectivity among these habitats, with important consequences for species interactions. While several studies have found a major imprint of urbanization on plant–insect interactions, the effects of urbanization on seed predation remain largely unexplored. Methods We investigated the relative impact of sunlight exposure, leaf litter, and spatial connectivity on predation by moth and weevil larvae on acorns of the pedunculate oak across an urban landscape during 2018 and 2020. We also examined whether infestations by moths and weevils were independent of each other. Results While seed predation varied strongly among trees, seed predation was not related to differences in sunlight exposure, leaf litter, or spatial connectivity. Seed predation by moths and weevils was negatively correlated at the level of individual acorns in 2018, but positively correlated at the acorn and the tree level in 2020. Conclusions Our study sets the baseline expectation that urban seed predators are unaffected by differences in sunlight exposure, leaf litter, and spatial connectivity. Overall, our findings suggest that the impact of local and spatial factors on insects within an urban context may depend on the species guild. Understanding the impact of local and spatial factors on biodiversity, food web structure, and ecosystem functioning can provide valuable insights for urban planning and management strategies aimed at promoting urban insect diversity.
... Non-native scale insects, often arriving via imported live plants (Liebhold and Tobin 2008;Liebhold et al. 2012;Meineke et al. 2013;Meurisse et al. 2019), have become some of the most common and damaging pests of trees in North America. Invaders such as oystershell scale (Lepidosaphes ulmi L.) (Griswold 1925;Crouch et al. 2021), beech scale (Cryptococcus fagisuga Lind.) (Morin et al. 2007), Japanese maple scale (Lopholeucaspis japonica Cockerell) (Addesso et al. 2016), European elm scale (Gossyparia spuria Modeer) (Herbert 1924), and others cause damage to urban trees. ...
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... -Resistance to pests Herbivorous pests are often abundant in urban areas, therefore, a higher resistance to pests favors tree survival (Raupp et al., 2010, Meineke et al., 2013. ...
... Arthropods are diverse, abundant, and ecologically prominent, making their community composition indicative of ecological integrity [6]. Urbanization generally decreases native and/or specialist arthropods (e.g., [7,8]), and may enhance nonnative and/or generalist arthropods (e.g., [9]). These effects can spill over through edge-effects onto arthropod communities in preserved areas bordering urbanized areas ( [10][11][12]), but few studies have investigated these dynamics in arid regions. ...
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The 'student of clouds' Luke Howard (1772–1864) published this work of statistics on weather conditions in London in two volumes, in 1818 and 1820. Howard was by profession an industrial chemist, but his great interest in meteorology led to his studies on clouds (also reissued in this series), and his devising of the system of Latin cloud names which was adopted internationally and is still in use. Volume 1 begins with an introduction to the work, explaining his intention to make available in one place consistent records of weather events. He argues that for the benefit of 'agriculture and navigation', a systematic approach is required, and he outlines his methods and equipment in some detail. The tables of observations taken at Plaistow, near London, in the years 1806–9 then begin, and are interspersed with notes and a commentary which includes accounts of similar weather phenomena observed elsewhere.
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Temperature pervasively impacts the phenotypes and distributions of organisms. These thermal effects generate strong selective pressures on behaviour, physiology, and life history when environmental temperatures vary over space and time. Despite this fact, progress toward a quantitative theory of thermal adaptation has lagged behind empirical descriptions of patterns and processes. This book draws on current evolutionary paradigms (optimization, quantitative genetics, and genetic algorithms) to establish a theory of thermal adaptation. It initially focuses on simple models that describe the evolution of thermosensitivity, thermoregulation, or acclimation. Later chapters focus on more complex models describing the coadaptation of traits or the coevolution of species. Throughout the book, various lines of evidence are used to question the major assumptions of these models. Furthermore, the predictions of these models are confronted with experimental and comparative data. Empirical examples represent a wide range of taxa, including bacteria, plants, fungi, and animals. The result is a synthesis of theoretical and empirical studies of thermal biology that offers insights about evolutionary processes.
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Nature is the international weekly journal of science: a magazine style journal that publishes full-length research papers in all disciplines of science, as well as News and Views, reviews, news, features, commentaries, web focuses and more, covering all branches of science and how science impacts upon all aspects of society and life.