Journal of the American Mosquito Control Association, 36(1):37–42, 2020
Copyright Ó 2020 by The American Mosquito Control Association, Inc.
SCIENTIFIC NOTE
TRANSFLUTHRIN SPATIAL REPELLENT ON US MILITARY MATERIALS
REDUCES CULEX TARSALIS INCURSION IN A DESERT ENVIRONMENT
S. C. BRITCH,1 K. J. LINTHICUM,1 D. L. KLINE,1 R. L. ALDRIDGE,1 F. V. GOLDEN,1 J. WITTIE,3 J. HENKE,2
K. HUNG,2 A. GUTIERREZ,2 M. SNELLING2 AND C. LORA3
KEY WORDS Integrated vector management, military operational entomology, passive control, residual
pesticide, resistance management
Residual pesticide treatment of US military
materials is important in military integrated vector
management (IVM) because these materials exist
nearly everywhere US military personnel are positioned in the field. Pretreatment of US military field
materials, such as tents (Frances 2007), camouflage
netting (Britch et al. 2011), or the geotextile in blast
protection walls (Britch et al. 2018), with a residual
pesticide could automatically establish the 1st layers
of an effective IVM system when these materials,
already organic to units deploying to austere
environments, are formed into perimeters around
outposts or bivouacs. However, host-seeking mosquitoes may still enter perimeters treated with
standard residuals, such as k-cyhalothrin, without
contacting treated surfaces or may contact treated
surfaces but retain mobility and host-seeking behavior long enough to contact humans (Britch, personal
observations; Viana et al. 2016), and in the long run
this technique could induce the evolution of resistance because the treatment is lethal to target insects
(Hoy 1998).
Fortunately, some pyrethroids exhibit spatial
repellant properties (Nentwig et al. 2017) that could
be leveraged in IVM programs to decrease probability of biased inheritance of protective mutations
(Heckel 2012). Spatial repellents enter the environment posttreatment as a vapor and may disrupt host-
seeking behavior, reducing vector–human contact in
indoor or outdoor protected perimeters or zones
(Achee et al. 2012). One prominent spatial repellent
recently registered in the USA is transfluthrin (TFL;
USEPA 2018). Recent studies indicate that TFL
shows promise as a passive area protection technique
effective outdoors in a hot-arid environment when
applied to natural materials like hessian (Ogoma et
al. 2012). However, little is known regarding the
potential efficacy of TFL applied to artificial
substrates such as US military field materials.
In October 2018 we investigated 2 types of US
military material—desert-pattern radar-scattering Ultra Lightweight Camouflage Netting System (ULCANS; Britch et al. 2010) and brown polypropylene
geotextile material (Müller and Saathoff 2015) from
Hesselden Company (HESCO, North Charleston,
SC) MIL blast protection barriers (Szabó et al.
2011)—treated with TFL and placed in a desert site
in the Coachella Valley, CA (Fig. 1), to reduce
collections of natural populations of disease vector
mosquitoes in small protected areas. This site was
previously described in Britch et al. (2009), and
natural populations of host-seeking medically important mosquitoes were present at the time of the study.
Prior to deployment to the field site we treated the
materials outdoors at the US Navy Entomology
Center of Excellence, Jacksonville, FL, with a Bayer
proprietary formulation of Bayothrine transfluthrin
(EPA REG 432-1588; Bayer Environmental Science,
Research Triangle Park, NC) diluted in water and
applied to the exterior side of each material at the
maximum rate recommended by the manufacturer of
1 g AI/m2 using a 2.5-gal (9.5-liter) hand-pressurized
portable sprayer (no. 91703; H. D. Hudson Mfg. Co.,
Chicago, IL).
1
USDA Agricultural Research Service Center for
Medical, Agricultural, and Veterinary Entomology, 1600
SW 23rd Drive, Gainesville, FL 32608.
2
Coachella Valley Mosquito and Vector Control
District, 43-420 Trader Place, Indio, CA 92201.
3
University of Florida, Department of Environmental
and Global Health, Gainesville, FL 32610.
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ABSTRACT. Standard residual pesticides applied to US military materials such as camouflage netting can
reduce mosquito biting pressure in the field but may contribute to the evolution of resistance. However, residual
applications of a spatial repellent such as transfluthrin could allow mosquitoes the opportunity to escape, only
inducing mortality if insects linger, for example after becoming trapped in a treated tent. In this study we
investigated the capability of transfluthrin on 2 types of US military material to reduce natural populations of disease
vector mosquitoes in a cool-arid desert field environment in southern California. We found that transfluthrin could
reduce Culex tarsalis incursion into protected areas by up to 100% upon initial treatment and up to 45% for at least
16 days posttreatment, showing that this compound could be an effective element in the US Department of Defense
integrated vector management system appropriate for further study.
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We constructed 3 different experimental setups in
the northwest quadrant of the desert site to represent
3 US military field scenarios to test the 2 types of
treated materials. Scenario 1 represented a small
billeting tent within a modular blast wall inside a
small forward operating base (FOB). Each of the 2
blast wall perimeter setups consisted of a double-high
36-ft (10.9-m) 3 45-ft (13.7-m) perimeter of unfilled
HESCO MIL cells (Fig. 1A). The centroids of the 2
perimeters were approximately 47 m apart. We
situated a current-issue green 11-ft (3.5-m) 3 10-ft
(3-m) Deployable Rapid Assembly Shelter (DRASH)
C-Series (HDT Global, Solon, OH) tent with 1 door
tied open approximately in the center of each
perimeter. Scenario 2 represented a small, expeditionary combat outpost (COP) in open terrain. We
erected a pair of current-issue COMBAT 2-person
tents (Eureka Expeditionary Systems/Johnson Outdoors Gear, Inc., Binghamton, NY) 2.5 m apart with
doors tied open at each of 4 sites situated
approximately 25 m apart (Fig. 1B). Scenario 3
represented individual personnel on bivouac in open
terrain. We identified 4 naturally open areas approximately 25 m apart and each approximately 4-m
diam. surrounded by vegetation and set up a
perimeter of 5 48-in. (1.2-m) plastic tread-in posts
around each area (Fig. 1C).
We designed a simple test at each scenario to
investigate whether the presence of TFL-treated
material in half of the replicates could reduce
collections of mosquitoes compared with the remaining replicates lacking TFL. First we distributed 22
encephalitis virus surveillance (EVS; Rohe and Fall
1979) traps baited with CO2 (approximately 1 kg dry
ice) and no light across the 3 scenarios as shown in
Fig. 1 to simulate the presence of humans and collect
samples of natural populations of mosquitoes overnight October 22–23, 2018. Starting in the afternoon
of October 23 (Day 0) we then placed TFL-treated
materials at half of the replicates in each scenario
where biting pressure had been highest in an effort to
investigate the most conservative capabilities of the
system. In Scenario 1 we placed an 8-in. (20-cm) 3
138-ft (42-m) strip of TFL-treated geotextile at 4 ft
(1.2 m) from the ground around the inside surface of
the HESCO perimeter with the highest biting
pressure to form a hypothesized protective ring
around the DRASH tent. At 1 g AI/m2, this treated
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Fig. 1. Desert plot study location showing the (A) forward operating base (FOB), (B) combat outpost (COP), and (C)
individual on open terrain (Bivouac) scenarios, and locations of weather recorders (N ¼ 4 yellow triangles) and encephalitis
virus surveillance (EVS) traps (N ¼ 22 white stars). Replicates containing transfluthrin (TFL)-treated materials are marked
TFL; control replicates are marked CTR. Brown ovals represent both 2-person COMBAT tents (COP scenario; N ¼ 8) and
Deployable Rapid Assembly Shelter (DRASH) tents (FOB scenario; N ¼ 2). White pentagons (N ¼ 4) represent 5 points
surrounding bivouac areas. Inset FOB image shows inside of perimeter with TFL-treated strip visible mounted at the
midpoint of the wall; inset COP image shows pair of tents with TFL-treated strips mounted at tent openings; inset Bivouac
image shows TFL-treated strips mounted on 5 tread-in posts surrounding an EVS trap.
MARCH 2020
SCIENTIFIC NOTE
total number of mosquitoesðTFL absentÞ
total number of mosquitoesðTFL presentÞ
3 100:
total number of mosquitoesðTFL absentÞ
We collected 7 mosquito species—Aedes dorsalis
(Meigen) (N ¼ 1), Ae. vexans (Meigen) (N ¼ 196),
Anopheles hermsi Barr and Guptavanij (N ¼ 78),
Culiseta inornata (Williston) (N ¼ 146), Culex
erythrothorax Dyar (N ¼ 13), Cx. quinquefasciatus
Say (N ¼ 12), and Cx. tarsalis Coquillett (N ¼ 938)—
over the 6 collection periods across the 3 scenarios.
Aedes dorsalis, An. hermsi, Cx. erythrothorax, and
Cx. quinquefasciatus were collected in low numbers
and not consistently across the study period, and Ae.
vexans and Cs. inornata were collected in higher
numbers but declined sharply after the first 2
collections. We restricted analyses to Cx. tarsalis
because this species was consistently present and in
the highest numbers across all 6 trap-nights and was
the only species present in all 22 traps and was
therefore the best species to compare efficacy of the
presence of TFL across all scenarios and collection
periods. Percent mosquito collection reduction data
for the FOB, COP, and Bivouac scenarios for Cx.
tarsalis are shown in Fig. 2A–C, respectively.
Weather across the 16-day collection period was
generally cool, dry, and mild, with mean overnight
temperatures ranging from 48.9 to 65.78F (9.4 to
18.78C), wind speed from 0 to 5.3 mph (0 to 2.4 m/s;
variable direction), and relative humidity from 17.2
to 69.0% RH.
Percent reduction data for Cx. tarsalis for the
FOB scenario (Fig. 2A) indicated approximately
46–90% reductions inside the treated perimeter and
75–100% reductions inside the DRASH for up to 2
days posttreatment compared with the control. The
percent reductions inside the DRASH were often
higher (Days 0 and 2) than in the perimeter where
the TFL-treated strip was actually located. The
efficacy outside the perimeter was always lower
than inside the perimeter for Days 0–2 but exceeded
the efficacy inside the DRASH on Day 1. On Days 8
and 16 inside the DRASH and inside the perimeter,
efficacy was no different than control. On Day 8
outside the perimeter, the collections were higher
(i.e., bars below the x-axis) than control. We also
observed this phenomenon in Day 8 collections
from the Bivouac scenario (Fig. 2C). The unusual
and universally low mosquito collections on Day 8
(data not shown) could have been related to the
lowest overnight temperatures (48.98F [9.48C])
recorded during the experiment. On Day 16, the
collection of Cx. tarsalis outside the TFL-treated
perimeter was 0 specimens compared with 13
specimens collected outside the control perimeter
that night (100% reduction).
In the COP scenario, Cx. tarsalis percent reductions inside the treated tents showed 61% reduction
on Day 0 but steadily declined to approximately 33%
on Day 1 and nearly 8% on Day 2 (Fig. 2B).
However, following a period of zero effect on Day 8,
the coldest overnight collection period, the efficacy
rebounded to just over 45% reductions in treated
tents on Day 16. Collections outside the TFL-treated
tents on Days 0 and 2 showed up to 75% reduction
compared with controls, but the presence of TFL was
not efficacious outside tents on Days 1 and 16 where
collections were actually 60–67% higher than
untreated control areas. For the Bivouac scenario
we had intended to suspend TFL-treated ULCANS
from each of the tread-in posts around the 2 areas
with the highest biting pressure but mistakenly
selected the 2 areas with the lowest biting pressure
and, except for Day 8 collections, Cx. tarsalis percent
reduction data were uniformly moderate across the
study period (Fig. 2C).
This is the 1st known investigation of the efficacy
of transfluthrin as a residual spatial repellent applied
to US military materials. We found that transfluthrin
could reduce target insect incursion into protected
areas by up to 61% (COP scenario Day 1) and
90.9% (FOB scenario Day 1), and in some cases
reduce collections adjacent to protected areas by up
to 75% (COP scenario Day 2). With regard to
longevity, the presence of TFL-treated material was
associated with reductions of approximately 45%
(COP scenario; inside treated tent) or 100% (FOB
scenario; outside treated HESCO perimeter) at 16
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strip presented approximately 8 g of TFL in the
HESCO perimeter. In Scenario 2 we suspended a 6in. (15-cm) 3 48-in. (1.2-m) strip (doubled over to
make a 24-in. [0.6-m] strip) of TFL-treated ULCANS
in each of the 2 doorways of each tent in 2 of the sites
with the highest biting pressure to form a hypothesized protective barrier to entry into those tents, for a
total of 1.44 g TFL across the 4 tents with strips. In
Scenario 3 we suspended a 6-in. (15-cm) 3 48-in.
(1.2-m) strip (doubled over to make a 24-in. [0.6-m]
strip) of TFL-treated ULCANS from each of the
tread-in posts around the 2 areas with the highest
biting pressure to form a hypothesized protective
barrier reducing mosquito attack on the simulated
human at the center, for a total of 1.8 g TFL across
the 2 bivouac sites.
We continued overnight EVS trap surveillance for
5 nights: October 23–24 (Day 0), October 24–25
(Day 1), October 25–26 (Day 2), October 31–
November 1 (Day 8), and November 8–9 (Day 16).
We also placed Kestrel 5500 (Nielsen-Kellerman
Co., Boothwyn, PA) weather recorders with wind
vane kits at each scenario as shown in Fig. 1 to
monitor temperature, relative humidity, and wind
speed and direction throughout the experiment. We
identified all collections to species and tabulated
results by scenario and trap position. The low number
of replicate sites in each scenario were not suitable
for statistical analysis; instead we visualized efficacy
of the TFL-treated materials by calculating the
percent reduction in collections in replicates with
TFL compared with replicates lacking TFL in each
scenario, using the formula:
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Fig. 2. Percent reduction data for Culex tarsalis in the (A) forward operating base (FOB) scenario; (B) combat outpost
(COP) scenario; and (C) individual on open terrain (Bivouac) scenario. Bars above the x-axis indicate reduced numbers of
mosquitoes in treated areas compared with untreated areas.
MARCH 2020
SCIENTIFIC NOTE
among layers. This layering concept could of course
be applied to civilian settings, for instance by setting
up perimeters of TFL-treated vegetation or synthetic
screens around areas where people congregate
outdoors.
Future investigations with TFL-treated materials
should also be conducted in seasons with higher
biting pressures across a range of habitats such as in
temperate, tropical, and hot-arid ecological regions
against natural populations of a range of nuisance and
disease vector mosquitoes, filth-breeding flies, biting
midges, and sand flies (Britch et al. 2014). Also, the
thresholds of repellency and toxicity of TFL on US
military materials need to be examined more closely.
Field collections demonstrate efficacy of TFL but do
not provide data on the capability of TFL to
nonlethally repel, which is key to the implementation
of TFL as a tool to manage resistance. We are
adapting existing spatial repellent laboratory bioassays (Jiang et al. 2019) for TFL-treated US military
materials that measure repellency without mortality
in colony-reared nonsusceptible and susceptible
strains of Ae. aegypti L. Transfluthrin should also
be tested against other spatial repellents on US
military materials to determine if TFL is the most
effective in field scenarios.
Emerging availability of diverse spatial repellents
and toxicants such as TFL leveraged as residuals on
US military materials offers an important alternative
to standard residual formulations. Spatial repellents
are less likely to induce resistance because target
insects have the opportunity to escape yet would
suffer lethal effects, for example after becoming
trapped in a tent, or if the attractant effect of a human
overwhelmed the repellent effect of the formulation.
Also, both repellent and lethal effects of spatial
repellents like TFL do not require target insects to
touch treated surfaces, which could further reduce
risk of insect–human contact in protected areas
compared with standard residuals that require insects
to land on or bump against the treated surface.
We thank the expert staff at the Coachella Valley
Mosquito and Vector Control District (CVMVCD)
and W. L. Helmey (US Navy Entomology Center of
Excellence [NECE]) for providing key support, and
comments from 2 anonymous reviewers that improved the manuscript. This research was supported
by the USDA–ARS and the US Department of
Defense (DOD) Deployed War-Fighter Protection
Program (DWFP). Mention of trade names or
commercial products is solely for the purpose of
providing specific information and does not imply
recommendation or endorsement by USDA, DOD,
CVMVCD, NECE, the University of Florida, or the
DWFP. Data in this study have been added to the
Mobile Pesticide App operational entomology decision support system database (https://ars.usda.gov/
saa/cmave/PesticideApp; Britch et al. 2014). The
USDA is an equal opportunity provider and employer.
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days posttreatment. It is important to note that Cx.
tarsalis percent reduction data from Day 1 (i.e., the
pretreatment collections) are included in Fig. 2 to
show how pretreatment collections in locations
destined for placement of TFL-treated materials
were generally higher (i.e., bars below the x-axis)
compared with collections from locations destined
to be untreated controls. We specifically chose
locations with the highest biting pressure to provide
the greatest challenge for TFL-treated materials, and
the reversal of the position of the majority of the
bars (i.e., bars above the x-axis) for the Days 0, 1, 2,
and 16 collections highlights the apparent efficacy
of the treatment once the TFL-treated strips were
put into place. The one exception was the Bivouac
scenario where the Day 1 bar was above the x-axis
(Fig. 2C) because we mistakenly chose locations
with the lowest biting pressure. Unfortunately for
this scenario the bars above the x-axis for Days 0, 1,
2, and 16 showing the apparent persistent reduction
of Cx. tarsalis may be attributed to the fact that
biting pressure was naturally lowest in these
replicates.
We elected to estimate the spatial distribution of
biting pressure from 1 pretreatment collection and
assume it to be stable over the 16-day collection
period. We understood that natural population biting
pressure could vary at the chosen locations over the
course of the study, but due to resource constraints
we were not able to increase replicates or randomize
placement of TFL-treated materials to mitigate
positional effects. On the other hand, preliminary
trials with TFL-treated materials at other sites
indicated that TFL effects could persist even when
treated materials had been rotated to other replicates
(Britch and Linthicum, unpublished data) suggesting transference of the spatial repellent to the
surroundings and requiring movement of the entire
experimental setup to mitigate potential positional
effects.
With the exception of a likely weather-related
anomaly midway through the experiment, we observed 45–100% reductions for up to 16 days in and
adjacent to protected areas, showing that this
compound could be an effective element in the US
Department of Defense IVM system. The cool
overnight conditions during the Day 8 collections
could have reduced the evaporative action of TFL
from the treated surfaces (Pettebone 2014), bringing
both treated and nontreated areas to equivalency. A
timed pesticide misting system (Aldridge et al. 2018)
could apply botanical-based repellent during cooler
temperatures to supplement residual TFL applications. A future IVM system could layer TFL-treated
materials for additional protection. For example, a
FOB could be set up with a protective TFL perimeter
surrounding the entire base, along with TFL-treated
materials outside of shelters and near outdoor troop
activity. Future trials should investigate the relative
push–pull efficacy of each layer separated at a variety
of distances to derive maximum effective distance
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