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Wildlife Society Bulletin 45(2):305–311; 2021; DOI: 10.1002/wsb.1168
From the Field
Evaluating Exclusion Barriers for Treefrogs
in Agricultural Landscapes
DANIEL F. HUGHES ,
1
Department of Biology, Coe College, 1220 1st Avenue NE, Cedar Rapids, IA 52402, USA
MICHELLE L. GREEN, Department of Biological Sciences, University of South Florida Saint Petersburg, 140 7th Avenue South, Saint Petersburg,
FL 33701, USA
JONATHAN K. WARNER, Texas Parks & Wildlife Department, 10 Parks and Wildlife Drive, Port Arthur, TX 77640, USA
PAUL C. DAVIDSON, Department of Agricultural and Biological Engineering, University of Illinois Urbana‐Champaign,
1304 West Pennsylvania Avenue, Urbana, IL 61801, USA
ABSTRACT Agricultural co‐management aims to promote environmental sustainability while maintaining
food‐safety standards. Amphibians, especially treefrogs, are known to enter fields of fresh produce intended
for human consumption, which raises concerns for food safety and quality. We evaluated the effects of
modifications to reduce the scalability of exclusion barriers on the fence‐crossing behavior of Pacific
treefrogs (Hyliola regilla) from lettuce fields in the Salinas Valley of California during January to May 2019.
From small‐scale field experiments, we found that fences modified with a horizontal lip at the top prevented
all treefrogs from successfully crossing over. Fences incorporating sandpaper were also effective at deterring
treefrog crossings compared to fences made of fiberglass square mesh (window screen) and polypropylene
fabric (silt fence). The fence design that inhibited the most treefrog crossings—aluminum flashing with a
horizontal lip—was a modification to commercially available material that is feasible to install at large
scales, is durable over long periods of time, and can help to exclude additional nuisance wildlife, such as
small mammals and lizards. Our findings provide quantitative evidence on the efficacy of an exclusion
barrier constructed with off‐the‐shelf material that can be easily modified to improve the co‐management of
amphibians and agriculture. © 2021 The Authors. Wildlife Society Bulletin published by Wiley Periodicals
LLC on behalf of The Wildlife Society.
KEY WORDS Amphibian, California, co‐management, exclusion barrier, Hyliola regilla, modified fence, Pacific
treefrog, Salinas Valley.
When applied appropriately in agriculture, co‐management
techniques provide methods for growers to maintain food
safety and quality standards while simultaneously promoting
wildlife conservation and sustainability (Wild Farm
Alliance 2016). Following a 2006 outbreak of Escherichia coli
O157:H7 in spinach (Jay et al. 2007), the leafy‐green in-
dustry sought methods to protect consumers from future
risk of contact with microbial pathogens (Nguyen‐the
et al. 2016). Although growers and producers considered
consumer safety as top priority, the majority of growers also
feel personal responsibility for the protection of the water,
environment, and biodiversity on their farms (Beretti and
Stuart 2008, Lowell et al. 2010). Co‐management offers an
overall approach that growers can use to minimize microbial
hazards and conserve wildlife and other natural resources
through actions that are “science‐based, adaptive,
collaborative, commodity‐specific, and site specific”
(Lowell et al. 2010: 50). The leafy‐green industry continues
to promote co‐management science because wildlife that use
agricultural areas can pose a threat to food safety
(Mandrell 2009, 2011; Langholz and Jay‐Russell 2013).
The diversity of animals linked to food‐borne illnesses
(Jay‐Russell 2013, Erickson 2016) makes it difficult for
growers to mitigate wildlife contaminations because there is
no single solution to exclude all taxa from agriculture fields
(Rice 2014). Therefore, science‐based co‐management that
is site‐and commodity‐specific can provide innovative ways
to effectively reduce contamination risks while protecting
wildlife and natural environments.
The irrigation used for leafy greens in California’s Salinas
Valley (Hansona et al. 1997), which is the largest growing
region for leafy green vegetables in the United States
(United States Department of Agriculture [USDA], 2014),
provides moisture for local amphibians in an otherwise arid
Received: 19 June 2020; Accepted: 4 February 2021
Published: 8 April 2021
This is an open access article under the terms of the Creative Commons
Attribution‐NonCommercial‐NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly
cited, the use is non‐commercial and no modifications or adaptations
are made.
1
E‐mail: dhughes@coe.edu
Hughes et al. •Treefrog Fence Modifications 305
region. Frogs, consequently, are frequently observed in the
above‐ground reservoirs and drainages associated with
agriculture‐based irrigation in the region. Even though
amphibians may not pose a serious contamination risk, they
have not been entirely ruled out as a potential vector or
reservoir for some human pathogens, such as Salmonella
bacteria, near crop fields (Jay‐Russell and Doyle 2016).
Amphibians can also cause significant economic losses to
farmers by ending up in prepackaged produce in the hands
of the consumer (Hughes et al. 2019). Furthermore, if they
are detected by food‐safety or food‐quality professionals
during inspections, large areas of produce fields can be
labeled unsellable, and the product will often be destroyed.
Additionally, the 2013 foreign material manual from the
USDA considers frog parts or whole animals found in food
products as foreign material that is “highly objectionable,
harmful, and unfit for consumption”(USDA 2013: 4).
In response, growers may use a co‐management barrier
strategy in an attempt to exclude amphibians from sensitive
growing areas. Because previous barriers were nonspecific
and temporary (e.g., silt fences), they proved inadequate to
mitigate intrusions by treefrogs, which have been observed
climbing directly over silt fences shortly after installation
(D. Hughes, Coe College, personal observation).
Because of the myriad of risks that amphibians pose to
leafy‐green production in the Salinas Valley (e.g., Hughes
et al. 2019), there is a need to explore novel co‐management
practices. Modified fences could be a powerful exclusion
measure for reducing intrusions into crop fields that
improves the co‐management of treefrogs and agriculture.
Here, we evaluated the effectiveness of fence modifications
to impede fence‐crossing behavior in Pacific treefrogs
(Hyliola regilla), the most commonly identified species in
consumer‐based incidents (Hughes et al. 2019). We tested
4 fence designs in microcosm field experiments with the aim
to provide a more effective co‐management practice to
mitigate treefrog intrusions into fields that contain fresh
produce intended for human consumption.
STUDY AREA
We studied Pacific treefrogs (Anura: Hylidae: Hyliola
regilla) in the Salinas Valley, Monterey County, California,
USA. Known as America’s salad bowl (Anderson 2000), the
Salinas Valley was the largest region for growing leafy green
vegetables in the United States, producing more than 70%
of the nation’s lettuce annually (USDA 2014). Agriculture
in the region consisted of large, industrialized monocultures
with advanced irrigation systems, high fertilizer input, and
contract‐based farm labor (FitzSimmons 1986). The region
contained a unique combination of fertile soil and suitable
climate for year‐round cultivation (Stuart 2010). In coor-
dination with leafy green industry cooperators, we selected
two reservoirs in the southern area of the Salinas Valley
(near King City, Monterey County, California) that had
histories of treefrog intrusions. The above‐ground wetlands
were reservoirs that stored water to irrigate the nearby
lettuce fields.
METHODS
Frog Sampling
We captured individuals for fence trials with passive and
active trapping methods as part of a larger study on treefrog
intrusions into produce fields across the Salinas Valley
(Hughes et al. 2020). To intercept treefrogs, drift fences
made of silt fencing were installed with paired pitfall
traps around the two wetland reservoirs (Gibbons and
Semlitsch 1981, Todd et al. 2007). Pitfall traps consisted of
5‐gallon buckets embedded in the soil. A hole was cut in a
series of lids to create a rim (8 cm) at the top of each bucket
to deter treefrogs from climbing out. A separate series of
whole lids were used to cover buckets between sampling
bouts. Holes were drilled into the bottom of buckets for
drainage and a sponge was placed in the bucket for mois-
ture. The buckets were opened during monthly 5‐day in-
tervals from January to May 2019. When a sampling session
was over, whole lids were secured on the buckets. We de-
ployed 25 polyvinyl chloride (PVC) pipes (3.81 cm diameter
and 60 cm long) as treefrog retreats (Boughton et al. 2000),
which were elevated offthe ground by fastening them
to a standalone stake or a post along the fence
(Myers et al. 2007). The bottom of the pipe was sealed to
allow water to fill and a small hole was drilled 6 cm from the
bottom of the pipe for drainage. We deployed 5 wooden
cover boards (0.9 m ×0.9 m) flat on the ground around
each reservoir. We spaced all traps (pitfalls, pipes, and
boards) at approximately even intervals around reservoirs
dependent upon their size. During sampling bouts, all
traps were checked once every 24 hr. Treefrogs were
captured during daily trap visits and held in plastic con-
tainers (22 cm ×17 cm ×10 cm) with moist paper towels
and air holes until fence trials, which began in the evening
of each capture day.
Fence Trials
To assess the effectiveness of fence designs for hindering
fence‐crossing behavior in treefrogs, we constructed ex-
perimental pens using PVC pipes and window screening
(1.5 m ×1.5 m ×1.5 m) (Fig. 1A,B). The tops of the pens
were covered with wood boards and rocks were placed in the
outer areas of the pens as refuge for treefrogs to prevent
desiccation (Boyle et al. 2019). We temporarily installed
two outdoor enclosures near one of the study reservoirs.
Each experimental fence design was built in a square
(0.9 m ×0.9 m) and placed in the center of the screened‐in
arena for trials (Fig. 1A,B). All free‐standing test fences
were initially constructed at the same height (0.9 m tall). To
run overnight trials, we placed treefrogs in the center of an
experimental fence shortly after sundown (1700–2000 hr)
and a wood cover was placed on the top of the outer pen.
The following morning (0600–0900 hr), we counted the
number of treefrogs in the outer test arena (escaped) and
those that remained in the experimental fence (did not
escape). Each treefrog was tested once in these escape‐room
fence experiments and all individuals were held for less
than 24 hr. After testing, treefrogs were marked using a
306 Wildlife Society Bulletin •45(2)
unique toe‐clip combination for later recognition (Donnelly
et al. 1994) and released at their site of capture.
Fence Designs
We compared 4 fence designs for reducing the fence‐
crossing behavior in treefrogs. We used 2 aluminum‐based
fences (more permanent) and 2 mesh‐based fences (more
temporary). First, we tested sandpaper (320‐grit) glued to
aluminum flashing (Fig. 1C) because laboratory experi-
ments demonstrated that the asperity size (i.e., surface
roughness) of this material disrupts the liquid‐adhesive
properties of treefrog toepads and can induce slippage at low
angles while climbing (Crawford et al. 2016). Second, we
tested the effect of adding a 10‐cm horizontal lip at the top
of aluminum flashing (Fig. 1D) because previous studies
have shown that an overhanging edge at the top of a bucket
can limit amphibian escapes (Mazerolle 2003). Third, we
tested a fence made of window‐screening material (fiber-
glass square mesh) supported by galvanized poultry wire
(Fig. 1E) because these fencing materials are often em-
ployed as barriers for deer and pigs in the Salinas Valley.
Fourth, we used a contractor‐grade silt fence made of
polypropylene fabric (Fig. 1F) as a control because it is the
most commonly installed barrier for treefrogs around
Figure 1. Outdoor pens for microcosm exclusion experiments and experimental fence designs: (A,B) Field enclosures made of PVC pipes and window
screening, (C) Aluminum flashing affixed with 320‐grit sandpaper, (D) Aluminum flashing with a 10‐cm horizontal lip at the top, (E) Window‐screen
material with chicken‐wire support, and (F) Polypropylene silt fence used as a control. Fence trials were conducted from January to May 2019 in the Salinas
Valley, Monterey County, California, USA.
Hughes et al. •Treefrog Fence Modifications 307
agricultural water sources in the Salinas Valley and has been
used in research as an inexpensive, temporary drift fence for
amphibians (Zug et al. 2001, Malone and Laurencio 2004).
Statistical Analysis
We compared the percentage of treefrogs that escaped per
trial between the 4 fence designs using ANOVA. We used
Z‐tests for 2 samples to compare frequency distributions for
the proportion of treefrogs that escaped per trial between
different fence designs. Mean proportions of escaped
treefrogs are presented with ±1 standard deviation and
sample ranges. We recognized significance at P<0.05. We
used Excel 2016 (Microsoft Inc., Redmond, Washington,
USA) to organize data and R (v.3.6.2, R Core Team 2019)
to generate graphics and conduct analyses.
RESULTS
From 32 independent trials ( ̅
x
=8±2 trials per fence type,
range =7–11 trials per fence type) using 257 individuals
(̅
x
=8±3.2 treefrogs tested per trial, range =1–13 treefrogs
tested per trial), we found that the proportion of treefrogs
that escaped differed by fence type (F
3,28
=9.95, P<0.001).
Fences modified with a 10‐cm horizontal lip at the top were
the most effective design at preventing successful fence
crossings, with 0 treefrogs escaping during all trials (Fig. 2).
Fences that integrated 320‐grit sandpaper were also effective
at reducing fence crossings, with just 5 treefrogs escaping
across all trials ( ̅
x
=7.9% ±20.9%, range =0–55.6% es-
caped per trial). The other two fences were less effective at
inhibiting crossings: window‐screen fences with 36 treefrogs
that escaped ( ̅
x
=55.8% ±35.3%, range =0–83.3% escaped
per trial) and control silt fences with 37 treefrogs that
escaped ( ̅
x
=55.1% ±30.9%, range =0–100% escaped per
trial).
DISCUSSION
Wildlife exclusion fencing is rapidly becoming a standard
tool employed by conservation practitioners, ranchers, and
land managers to redirect animal movements (Glista
et al. 2009, Knight et al. 2011, Markle et al. 2017), but there
is no one‐size‐fits‐all solution, so each fence needs to be
designed with the biology of the target species in mind
(Baxter‐Gilbert et al. 2015, Dillon et al. 2020). For
amphibians, there have been few experiments that test the
efficacy of different fence designs for their exclusion, with
most fences developed for redirecting non‐climbing species
into ecopassages (e.g., tunnels and culverts) to reduce
road mortality (Dodd et al. 2004, Grilo et al. 2010,
Beebee 2013). A previous study on barrier effectiveness
tested against two non‐climbing frog species (Ranidae:
Lithobates clamitans and L. pipiens) found that plastic fences
with heights of 0.9 m prevented all but one frog from
crossing over, whereas a height of 0.3 m blocked nearly 80%
of frogs and 0.6 m blocked more than 95% (Woltz
et al. 2008). In a series of laboratory experiments, Dodd
(1991) found that 3 treefrog species (Hylidae: Acris gryllus,
Hyla femoralis, and Pseudacris ocularis) successfully crossed
over a 0.35 m tall piece of aluminum flashing deployed in-
side a small container, with 180 individuals of 370 tested
(48.6%) crossing the barrier. We found that the addition of
a horizontal lip to the top of a fence made of aluminum
flashing prevented all tested treefrogs from crossing
over in microcosm field experiments. This fence design may
be generally useful across taxa because a recent study, using
a similar experimental protocol, also found that barriers
Figure 2. Comparison of exclusion experiments showing the percentage of Pacific treefrogs (Hyliola regilla) that escaped per fence trial with bars
representing standard errors. An escape was defined as a treefrog found in the main fence arena after an overnight trial. Similar letters above fence types
indicate no significant difference based on pair‐wise comparisons. Sample sizes for treefrogs and trials per fence type are presented below letters. Lip:
aluminum flashing with a 10‐cm horizontal lip at the top. Sandpaper: aluminum flashing affixed with 320‐grit sandpaper. Control: polypropylene silt fence.
Screen: window‐screen with chicken‐wire support. Fence trials were conducted from January to May 2019 in the Salinas Valley, Monterey County,
California, USA.
308 Wildlife Society Bulletin •45(2)
with a lip were the most effective at preventing ratsnakes
from climbing over (Macpherson et al. 2021).
Although all the fences that we tested inhibited some
fence‐crossing incidents by treefrogs, the aluminum‐flashing
fence with a horizontal lip was the most successful. The
modified aluminum‐flashing fence is also the most feasible
for scaling up to a larger area among those tested because
the second most successful barrier requires adhering
320‐grit sandpaper to a long section of fence, which is time
consuming, requires maintenance after installation, and
can be cost prohibitive. The horizontal‐lip design is a
straightforward modification to over‐the‐counter aluminum
flashing, a material that has been used in numerous multi‐
year studies of breeding amphibians because it requires little
maintenance, is relatively inexpensive, and can be easily
installed (e.g., Pechmann et al. 1991, Semlitsch et al. 1996,
Todd et al. 2007). For stability and to prevent passage un-
derneath aluminum flashing, we recommend burying 10 cm
into the soil, attaching stakes to the back with metal ties,
and adding wood planks to the face so that the ties will not
wear through the fence (Fig. 3). Similar designs that in-
corporated horizontal lips for exclusion barriers have been
developed for commercial use by wildlife companies
(e.g., Animex: animexfencing.com), and some have even
been deployed in conservation areas (e.g., Gleeson and
Gleeson 2012). However, modified fences do not appear to
have been thoroughly assessed for their effectiveness as an
amphibian barrier. Novel approaches to redirect amphibian
movements (e.g., Buxton et al. 2018) will not succeed
unless an evidence‐based framework is adopted (Schmidt
et al. 2020) in which techniques are quantitatively
and rigorously evaluated to determine if they achieved their
intended outcome.
There is increasing recognition of the importance of
co‐managing wildlife and agriculture to attain a stable co-
existence with biodiversity while promoting sustainable
economic growth (Treves et al. 2006, Karp et al. 2015).
Incidences of foodborne illness linked to the consumption
of wildlife‐contaminated produce, however, can impede
progress in co‐management (Stuart 2009, 2011). Because
the sampling of human pathogens in wild frogs has not been
robust, either geographically or quantitatively (Ferens and
Hovde 2011, Gorski et al. 2013), growers cannot eliminate
frogs as a potential source of microbial disease, thus they
need access to rigorously tested, easy to implement and
maintain, co‐management options on their farms (Olimpi
et al. 2019). Our evaluation of exclusion barriers for tree-
frogs showed that modifications to fences can be applied in
an agricultural context to improve the co‐management of
wildlife and agriculture. Previous attempts to exclude
Figure 3. Specifications of the optimal fence design (aluminum flashing with a 10‐cm horizontal lip at the top) for inhibiting fence‐crossing behavior in
Pacific treefrogs (Hyliola regilla) and photographs of in‐field installations. Fence trials were conducted from January to May 2019 in the Salinas Valley,
Monterey County, California, USA.
Hughes et al. •Treefrog Fence Modifications 309
treefrogs from agricultural fields in the Salinas Valley were
unsuccessful, in part, because amphibian biology was not
considered when designing the barrier, nor was an evidence‐
based framework implemented. Silt fences, for example,
were used widely but had not been tested to make sure that
their installation led to a reduction in treefrog intrusions.
Farmers are now equipped with quantitative evidence that a
modified barrier will impede fence‐crossing behavior in a
common treefrog species and some additional wildlife, in-
cluding small species of rodents, lizards, and other frogs
(Hughes et al. 2020). The widespread adoption of this
barrier design in the Salinas Valley would benefit both
amphibian conservation and the fresh‐produce industry by
providing an evidence‐based solution to reduce treefrog
intrusions in agricultural landscapes.
ACKNOWLEDGMENTS
We thank the Center for Produce Safety for funding and
continued support of our project. Funding for our project
was made possible by the USDA’s Agricultural Marketing
Service through grant AM170100XXXXG011 and the
California Leafy Greens Research Program. Animals were
handled under approved protocols from the California
Department of Fish and Game’s Scientific Collection
Permit (SCP # 13909) and the University of Illinois’s
Institutional Animal Care and Use Committee Protocol
(IACUC # 18081). We would also like to thank D. Donner
(Associate Editor), A. Knipps (Editorial Assistant), and
anonymous reviewers for their helpful comments, which
improved the manuscript.
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