Road mortality potentially responsible for billions of pollinating insect deaths annually

Abstract

Pollinating insects are vital to the survival of many primary producers in terrestrial ecosystems, as up to 80–85 % of the world’s flowering plants require pollinators for reproduction. Over the last few decades however, numerous pollinating insect populations have declined substantially. The causes of these declines are multifaceted and synergistic, and include pesticides, herbicides, monoculture, urbanization, disease, parasites, and climate change. Here, we present evidence for a generally understudied yet potentially significant source of pollinator mortality, collisions with vehicles. Negative impacts from roads have been observed for the majority of vertebrate groups but studies of the effects on invertebrates have remained largely absent from the scientific literature. We documented road mortality of pollinating insects along a 2 km stretch of highway in Ontario, Canada and used our findings to extrapolate expected levels of road mortality across a number of landscape scales. Our extrapolations demonstrate the potential for loss of hundreds of thousands (on our studied highway) to hundreds of billions (generalised across North America) of Lepidopterans, Hymenopterans and pollinating Dipterans each summer. Our projections of such high levels of annual road mortality highlight the need for research to assess whether the mortality levels observed are contributing to the substantial declines of pollinating insects occurring on a global scale, thus putting the ecological functioning of natural areas and agricultural productivity in jeopardy.

Full text

SHORT COMMUNICATION
Road mortality potentially responsible for billions of pollinating
insect deaths annually
James H. Baxter-Gilbert
1,2
•Julia L. Riley
1,2
•Christopher J. H. Neufeld
1
•
Jacqueline D. Litzgus
1
•David Lesbarre
`res
1
Received: 26 May 2015 / Accepted: 29 September 2015
ÓSpringer International Publishing Switzerland 2015
Abstract Pollinating insects are vital to the survival of
many primary producers in terrestrial ecosystems, as up to
80–85 % of the world’s flowering plants require pollinators
for reproduction. Over the last few decades however,
numerous pollinating insect populations have declined
substantially. The causes of these declines are multifaceted
and synergistic, and include pesticides, herbicides, mono-
culture, urbanization, disease, parasites, and climate change.
Here, we present evidence for a generally understudied yet
potentially significant source of pollinator mortality, colli-
sions with vehicles. Negative impacts from roads have been
observed for the majority of vertebrate groups but studies of
the effects on invertebrates have remained largely absent
from the scientific literature. We documented road mortality
of pollinating insects along a 2 km stretch of highway in
Ontario, Canada and used our findings to extrapolate
expected levels of road mortality across a number of land-
scape scales. Our extrapolations demonstrate the potential
for loss of hundreds of thousands (on our studied highway)
to hundreds of billions (generalised across North America)
of Lepidopterans, Hymenopterans and pollinating Dipterans
each summer. Our projections of such high levels of annual
road mortality highlight the need for research to assess
whether the mortality levels observed are contributing to the
substantial declines of pollinating insects occurring on a
global scale, thus putting the ecological functioning of
natural areas and agricultural productivity in jeopardy.
Keywords Biodiversity Diptera Hymenoptera 
Lepidoptera Pollinator declines Road ecology
Introduction
As current losses to biodiversity persist and worsen, it is
imperative that research expands our understanding of the
complexity of threats impacting global ecosystems, so that
some level of mitigation to alleviate the negative ramifi-
cations of an increasingly anthropogenic world can be
achieved (Pimm et al. 2001; Soule
´and Orians 2001;Ehr-
lich and Pringle 2008; McCune et al. 2013). Most impor-
tantly, it is crucial to comprehend the inter-specific
relationships that provide widespread ecosystem functions
(Soule
´et al. 2003), if such mitigation is to be successful.
Pollinators are one group of animals that play a major role
in maintaining ecosystem functions (Biesmeijer et al. 2006;
Potts et al. 2010; Ollerton et al. 2011). Yet, over the last
20 years, numerous pollinating insect populations have
declined massively from threats such as: pesticides, habitat
destruction, monoculture, urbanization, emerging disease,
parasites, and climate change (Brown and Paxton 2009;
Cane and Tepedino 2001; Cameron et al. 2011; Khoury
et al. 2011; Wallis de Vries et al. 2012; Paxton et al. 2015).
In this study, we address yet another significant, but
underappreciated, threat to pollinating insects—road
mortality. Recently there have been drastic increases in
both the size and density of road networks worldwide,
with a 35 % increase in road surface area over the last
decade alone (Dulac 2013). Since its inception, the field
of road ecology has examined a wide variety of threats
&David Lesbarre
`res
dlesbarreres@laurentian.ca
1
Department of Biology, Laurentian University, 935 Ramsey
Lake Road, Sudbury, ON P3E 2C6, Canada
2
Present Address: Division of Brain, Behaviour and Evolution,
Department of Biological Sciences, Macquarie University,
209 Culloden Road, Marsfield, NSW 2122, Australia
123
J Insect Conserv
DOI 10.1007/s10841-015-9808-z

posedbyroadnetworksonamyriadofspecies,commu-
nities, and ecosystems; however, this field of research is
generally dominated by studies on species that are a direct
threat to human health from vehicle collisions (e.g.
ungulates and carnivores; Forman et al. 2003;Trombulak
and Frissell 2000) or endangered taxa (e.g. Florida pan-
ther, Puma concolor coryi, Johnson et al. 2010; reptiles at
risk, Baxter-Gilbert et al. 2015) as opposed to that of
overall ecosystem health (e.g. pollinating insects; De la
Puente et al. 2008;Sko
´rka et al. 2013). The lack of
research on insect road ecology is exemplified in the
recent review by Mun
˜oz et al. (2015) which describes
only 50 articles on the topic published in the last 46 years
(an average of only 1.08 articles/year since 1969), and
only 7 articles examined road mortality of pollinating
insects.
With such lack of fundamental knowledge regarding the
relationship between roads and a faunal group as ecologi-
cally important as pollinating insects, the need to examine
whether road mortality is a substantial threat becomes
evident, and begins with the basic question, ‘‘How many
pollinating insects are dying on roads?’’ Furthermore, it is
imperative to determine whether road mortality levels
observed are sustainable, or if they could be contributing to
pollinating insect population declines (Cane and Tepedino
2001; Brown and Paxton 2009; Potts et al. 2010). In this
study, we extensively surveyed the asphalt surface and
gravel shoulders of a section of highway to determine how
many pollinating insects were dying from collisions with
vehicles during their active season.
Methods
Collection of road killed insects
The study site was located within Magnetawan First Nation
along a 2-km transect of Highway 69/400, a portion of the
Trans-Canada Highway and major thoroughfare connecting
central and southern Ontario, Canada. This section of
Highway 69 has a maximum speed limit of 90 km/h, and
moderate daily traffic flow (averaging 9700 vehicles per
day during the summer months; Ministry of Transportation
Ontario 2012). The habitat bisected by the highway con-
sists of a continuous natural mosaic of wetland patches
separated by upland areas of mixed forest and rock out-
crops (i.e., Boreal Shield ecozone), with the only major
anthropogenic disturbance in the area being the highway
itself. The lack of other anthropogenic structures in the area
provided the opportunity to control for other confounding
variables associated with urbanization (e.g. habitat degra-
dation, monoculture crops, use of pesticides and herbi-
cides; Kearns 2001; Potts et al. 2010).
Daily walking surveys alongside the northbound and
southbound lanes of the highway occurred at 10:00 each
morning from 1 May to 31 August of 2012 and 2013 (two
active seasons), resulting in 235 transect surveys (Baxter-
Gilbert et al. 2015). Surveys were conducted by 2–3 indi-
viduals, and occurred daily regardless of weather. All
insects found on the road surface (9 m wide), road-adjacent
asphalt shoulder (1.5 m wide), and asphalt shoulder-adja-
cent gravel shoulder (2 m wide) were collected by hand,
stored in labelled plastic bags, and frozen for future iden-
tification. Collected insects were identified to Order with
dichotomous keys (Marshall 2006) and counted at Lau-
rentian University, Sudbury, Ontario. Incomplete speci-
mens (e.g., legs, wings, and body segments) were clustered
to represent a single whole specimen whenever possible, to
ensure conservative counts were made.
Generalising road morality rates and extrapolating
across landscape scales
Using the number of individuals collected, we calculated
the road mortality rate by dividing the total number of dead
insects collected by the transect size and by the number of
daily surveys. The resulting number of dead insects/km/day
could then be used to calculate a seasonal yield per kilo-
metre (i.e., # of road-killed insects/km/summer). The sea-
sonal road mortality rate was then used to estimate levels of
road mortality for the full length of our studied highway
(i.e., the total length of Highway 69/400 is 388 km;
Transport Canada 2012), for the geo-regional road network
(i.e., Southern Ontario has 40,800 km of public road;
Transport Canada 2012), for the eco-regional road network
(i.e., the road network in the Boreal Shield Eco-region is
236,615 km long; Transport Canada 2012), and lastly, for
roads across the continent (i.e., continental North America
has 7,600,000 km of road network (Federal Highway
Administration 2012; Transport Canada 2012)).
Results
Number of road-killed insects
During our surveys, many insects collected exhibited signs
of impact trauma, and numerous vehicle-insect collisions
were directly observed. The number of insects we collected
from the highway after being struck and killed by traffic
(N =117,675 individuals) represented substantial diver-
sity, encompassing 15 Orders (Table 1). These casualties
were predominantly pollinators (96 %), such as bees,
wasps, butterfly, moths, and flies. Insects from the Order
Hymenoptera comprised 11 % (N =12,639) of the total,
J Insect Conserv
123

and predominately included yellow-jacket wasps (Vespi-
dae), bumblebees and honeybees (Apidae; Fig. 1). Insects
from the Order Lepidoptera comprised 4 % (N =4763) of
the total, and consisted of a wide assortment of local but-
terfly and moth species. Dipterans made up 81 %
(N =95,094) of the total collected insects, and consisted
primarily of horse flies (Tabanidae), blow flies (Cal-
liphoridae), and bibionid flies (Bibionidae). In 2013, there
was a substantial bloom of bibionid flies (Bibio sp.) in
central Ontario, Canada, which largely contributed to the
increased number of Dipterans collected in May 2013
(removing the May 2013 Dipterans the total falls to
N=4378). Though, overall, the general seasonal trends of
insect road mortality appeared similar in both sampling
year (2012 and 2013; Fig. 2). The remaining 4 %
(N =5179) consisted of insects from the Orders Blattodea,
Coleoptera, Dermaptera, Ephemeroptera, Hemiptera,
Mantodea, Megaloptera, Neuroptera, Odonata, Orthoptera,
Plecoptera, and Trichoptera (Table 1).
Road mortality rates
At our site, for the three most commonly collected insect
groups, we calculated that Lepidopterans are struck and
killed by traffic at a rate of 10.1 individuals/km/day,
Hymenopterans experienced a road mortality rate of 26.8/
km/day, and Dipterans were struck and killed at a rate of
202.3/km/day (or 10.4/km/day excluding the May 2013
data). By using our calculated road mortality rates we
extrapolated the number of individual insects killed at four
spatial scales (the entire length of Hwy 69/400, the geo-
region, the eco-region, and across all of North America).
The estimates of road mortality ranged from hundreds of
thousands on the studied highway, to millions of individ-
uals per group across the geo-region and eco-region, and up
to billions and hundreds of billions when we extrapolated
across the entirety of the North American road network
(see Table 2).
Table 1 Counts of insects killed by wildlife-vehicle collisions, identified to Order, found during the spring and summer months in 2012 and
2013 on a 2 km section of Highway 69, Ontario, Canada
Insect order Time period
May
2012
May
2013
June
2012
June
2013
July
2012
July
2013
August
2012
August
2013
Total
2012
Total
2013
Lepidoptera 631 98 569 857 615 952 229 818 2044 2725
Hymenoptera 1492 773 826 2604 1346 3307 592 1697 4256 8381
Coleoptera 161 503 148 531 134 319 43 207 486 1560
Orthoptera 15 11 36 70 266 132 166 565 483 778
Odonata 80 24 242 202 109 165 31 170 462 561
Diptera 163 90,716 381 506 880 1751 214 483 1638 93,456
Hemiptera 14 2 7 6 52 51 6 38 79 97
Neuroptera 2 0 5 0 5 0 1 1 13 1
Plecoptera 0 5 2 174 2 70 0 7 4 256
Blattodea 2 12 13 20 11 29 2 24 28 85
Mantodea 0 0 0 0 0 1 1 1 1 2
Trichoptera 13 0 23 3 33 2 1 21 70 26
Megaloptera 0 0 32 38 37 51 2 26 71 115
Dermaptera 0 0 0 0 0 1 0 1 0 2
Ephemeroptera 0 0 0 1 0 0 0 0 0 1
Fig. 1 A single day’s collection of Hymenoptera from a 2 km section
of 2-lane highway in central Ontario, Canada
J Insect Conserv
123

1
10
100
1000
10000
Early May 2012
Early May 2013
Late May 2012
Late May 2013
Early June 2012
Early June 2013
Late June 2012
Late June 2013
Early July 2012
Early July 2013
Late July 2012
Late July 2013
Early August 2012
Early August 2013
Late August 2012
Late August 2013
Log10 Average Daily Number of Collected Individuals
Time Period
Order Lepidoptera Order Hymenoptera Order Diptera
Fig. 2 A fortnightly account of
the daily average number of
individual Lepidopterans (light
grey bars), Hymenopterans
(black bars) and Dipterans
(dark grey bars) collected along
our survey transect (a 2 km
stretch of Highway 69/400,
Ontario) over the summers of
2012 and 2013
Table 2 Seasonal losses of individual insects across four landscape scales
Landscape scale Road length
(km)
Road mortality rate (Individuals/
km/day)
Estimated total losses per
summer
Estimate
accuracy
Lepidopterans
Road specific (Highway
69/400)
388 10.1 478,094 High
Geo-region (Southern
Ontario)
40,800 10.1 50,273,760 Moderate
Eco-region (Boreal Shield) 236,615 10.1 291,557,003 Moderate
Continental North America 7,600,000 10.1 93,64,720,000 Low
Hymenopterans
Road specific (Highway
69/400)
388 26.8 1,268,604 High
Geo-region (Southern
Ontario)
40,800 26.8 133,399,680 Moderate
Eco-region (Boreal Shield) 236,615 26.8 779,977,686 Moderate
Continental North America 7,600,000 26.8 24,848,960,000 Low
Dipterans
Road specific (Highway
69/400)
388 202.3
10.4 (excluding May 2013)
9,576,072 High
492,294
Geo-region (Southern
Ontario)
40,800 202.3
10.4 (excluding May 2013)
10,06,968,480 Moderate
51,767,040
Eco-region (Boreal Shield) 236,615 202.3
10.4 (excluding May 2013)
5,839,800,169 Moderate
30,026,152
Continental North America 7,600,000 202.3
10.4 (excluding May 2013)
187,572,560,000 Low
9,642,880,000
Extrapolations were derived from the taxon-specific road mortality rates observed at our study site multiplied by the length of the active season
(122 days) and the length of the road networks
J Insect Conserv
123

Discussion
The substantial level of insect road mortality ([117,000
individuals across two active seasons) on only a 2-km
section of highway underscores the threat road-networks
pose to insect populations. Furthermore, our findings mir-
ror that of Seibert and Conover (1991) who also described
the three most common insect Orders killed by traffic as
Lepidopterans, Hymenopterans, and Dipterans. Lepi-
dopterans and most Hymenopterans are considered polli-
nators, and about 32.5 % of Dipterans are pollinators
(Larson et al. 2001; Kearns 2001); however, the Dipterans
we collected consisted of mostly horse flies (Tabanidae;
Larson et al. 2001), blow flies (Calliphoridae; Larson et al.
2001), and bibionid flies (Bibionidae; Free 1993; Fitzgerald
2005; Larson et al. 2001; Woodcock et al. 2014), all of
which include known pollinators. Because the three most
common Orders of insects killed in our study were polli-
nators, and because they were killed in high numbers, there
is the potential for roads and traffic to be a substantial
threat to insect pollinator populations. Furthermore, due to
the high level of ecological importance of pollinating
insects, the removal of pollinators by road mortality may
pose a much larger risk to ecosystem health if these pol-
linator populations are decreased below a sustainable
threshold (Biesmeijer et al. 2006; Potts et al. 2010; Ollerton
et al. 2011).
It is important to note that the use of our observed road
mortality rates to produce an estimate of widespread pol-
linator losses is nested in the assumption that our road
mortality rates are representative of a broader trend. If we
compare the road mortality rates for Lepidopterans
between our study and previously reported research, we see
the same rate (*10/km/day) as those reported by Yamada
et al. (2010) along a coastal road in northern Japan. In
contrast, our Lepidopteran road mortality rates were higher
than those seen in southern India (0.5–3/km/day; Rao and
Girish 2007) and notably lower than those in both central
Spain (80/km/day; De la Puente et al. 2008) and southern
Poland (47/km/day; Sko
´rka et al. 2013). It is important to
note that variation in sampling methods, climate, traffic
flow, habitats, and insect diversity and density would create
variability among reported averages (Mun
˜oz et al. 2015).
Nevertheless, we see that in a global context, the level of
road mortality we report for Lepidopterans would be con-
sidered low to moderate (Mun
˜oz et al. 2015). It is therefore
within reason to make the assumption that the road mor-
tality rates recorded for Dipterans and Hymenopterans
could also be considered moderate, and that they follow
similar trends described in previous literature (e.g., Seibert
and Conover 1991). Furthermore, it is also important to
consider that our predicted rates of pollinating insect road
mortality may be underestimates, as our sampling method
was limited to the road and shoulder. Thus, we did not
collect insects that adhered to the vehicle during the col-
lision, any individuals that were scavenged after collision,
or were ricocheted far off the road and shoulder after the
insect-vehicle collision. As well, our estimates of polli-
nating insect road mortality rates are based on the number
of dead insects collected along a highway with moderate
traffic (Ministry of Transportation Ontario 2012) bisecting
a boreal habitat with moderate insect diversity (Ricketts
et al. 1999). Therefore, the road mortality rates we report
should be a suitable approximation for use in landscape
scale extrapolation. Overall, because the actual road mor-
tality rate is likely much higher, and our observations are
likely underestimates of insect death on roads, our study
clearly highlights that road mortality of pollinating insects
is occurring at high levels, but whether these levels are
extensive enough to cause population declines is yet to be
seen, and requires future research.
It is also important to note that as the landscape scale
used in our extrapolations became larger and more gener-
alised, the levels of reliability and accuracy of our
extrapolation were reduced. Different landscapes will
experience variability in factors effecting road mortality
rates. Certainly, the estimate of the seasonal expected level
of road mortality along Highway 69/400 is likely quite
reliable and accurate, as the amount of traffic, road size,
speed limit, habitat, climate, insect diversity, and road side
vegetation are generally similar along the road’s entirety
(Sko
´rka et al. 2015). The level of predicted road mortality
likely became less accurate and reliable as the landscape
scale was increased to geo-regional, eco-regional, and
continental landscape scales as variation in the aforemen-
tioned variables (Sko
´rka et al. 2015) becomes larger. With
this in mind, it is clear that our predictions of the annual
loss of hundreds of billions of pollinating insect across
North America do rest in the notable assumption that our
reported road mortality rates represent a North American
average. Yet, it is important to consider the reality that our
predictions may be relatively accurate, and if this is the
case much more research on the impact roads may be
having on pollinator population is crucial.
Counterintuitively, there have been a number of studies
that demonstrate that insect populations can benefit from
anthropogenic disturbance and habitats (Bolger et al. 2000;
Threlfall et al. 2012), however much more research is
required to understand if beneficial roadside vegetation
(Munguira and Thomas 1992) can outweigh costs of road
mortality (Mun
˜oz et al. 2015). Research into the population
viability of pollinating insects when considering various
levels of road mortality is an important next step in
understanding whether the levels of road mortality being
J Insect Conserv
123

recorded at our site, and the expected levels across larger
landscape scales, present significant threats to global pol-
linator populations. It is worth consideration that all the
cumulative effects of currently established causes of pol-
linator population declines (e.g., pesticides, habitat
destruction, monoculture, urbanization, emerging disease,
parasites, and climate change; Brown and Paxton 2009;
Cane and Tepedino 2001; Cameron et al. 2011; Khoury
et al. 2011; Wallis de Vries et al. 2012; Paxton et al. 2015)
coupled with a major annual population loss due to road
mortality may potentially represent a serious threat; if not
now then in the future as road networks expand and traffic
densities increase (Dulac 2013).
Unlike some of the threats that can be regulated (e.g.,
use of harmful pesticides; Rundlo
¨f et al. 2015), design of
mitigation to reduce road mortality of pollinating insects is
not as clear. Mitigating the threat of road mortality to
flying insects is clearly a more difficult task than miti-
gating roads for other larger taxa (e.g., ecopassages and
fences; Lesbarre
`res and Fahrig 2012). This has already
been recognized for the very specific situation of the
Alkali Bee (Nomia melanderi), a species that is the most
efficient pollinator of alfalfa crops in Washington, United
States (Vinchesi 2013), and whose low-flying nature puts
it at great risk in populations that are bisected by roads
(Vinchesi 2013). In an attempt to reduce the negative
effect of road mortality on Alkali Bees, tall screens were
placed adjacent to roads which forced bees to fly at an
increased height. Unfortunately, this method was found to
be ineffective as after the bees manoeuvred over the
screen, they reverted back to their initial, and risky, low-
flying height to cross the road (Vinchesi 2013). Although
not effective for Alkali Bees, the use of tall screens has
been effective for Hine’s emerald dragonflies (Soma-
tochlora hineana) on simulated small roads (6 m wide),
but was not effective for large simulated roads (12 m
wide; Furness and Soluk 2015). Ultimately, we can see
that much like mitigating roadways for other taxa (Les-
barre
`res and Fahrig 2012), creating an effective means of
preventing insect road mortality requires a great deal of
further research, and likely a host of species-specific
adjustments and modifications from the current screening
method being tested.
Our study is one of only a handful that examined road
mortality of pollinating insects, and our observations and
extrapolations suggest that road mortality may pose a sig-
nificant threat to populations. Future research should
examine whether observed road mortality rates of polli-
nating insects are sustainable. The continued existence of
many natural ecosystems depends on pollinators, and even
from an anthropocentric point of view, approximately three
quarters of global human food crops depend on pollination
(Tylianakis 2013). Ultimately, the continued survival of
humanity depends on pollinating insects, and as roadways
bring food to our tables, they may also be jeopardizing the
future production of those crops.
Acknowledgments The authors thank J.E. Baxter-Gilbert, S. Boyle,
G. Hughes, D. Jones, R. Maleau, L. Monck-Whipp, and K.
Tabobondung and many volunteers for their field assistance, C.
Beckett-Brown, B. Hewitt, K. Marchand, M. McGee, J. Montgomery,
R. Morin, B. Squirrell, and J. Woolley for their laboratory assistance.
We would also like to thank the two anonymous reviewers for their
insights and suggestions during the preparation of this article.
Financial support for this research was provided by Magnetawan First
Nation, Laurentian University, the Ontario Ministry of Natural
Resources, and the Ministry of Transportation Ontario (MTO). All
research was authorized by Magnetawan First Nation’s Chief and
Council. Opinions expressed in this paper are those of the authors and
may not necessarily reflect the views and policies of the MTO.
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