Content uploaded by Jérémy Bouyer
Author content
All content in this area was uploaded by Jérémy Bouyer on May 05, 2015
Content may be subject to copyright.
SAMPLING,DISTRIBUTION,DISPERSAL
Stratified Entomological Sampling in Preparation for an Area-Wide
Integrated Pest Management Program: The Example of Glossina
palpalis gambiensis (Diptera: Glossinidae) in the Niayes of Senegal
JE
´RE
´MY BOUYER,
1,2,3
MOMAR TALLA SECK,
2
BABA SALL,
4
ELHADJI YOUSSOU NDIAYE,
4
LAURE GUERRINI,
1,5
AND MARC J. B. VREYSEN
6
J. Med. Entomol. 47(4): 543Ð552 (2010); DOI: 10.1603/ME09149
ABSTRACT The riverine tsetse species Glossina palpalis gambiensis Vanderplank 1949 (Diptera:
Glossinidae) inhabits riparian forests along river systems in West Africa. The government of Senegal has
embarked on a project to eliminate this tsetse species, and African animal trypanosomoses, from the Niayes
area using an area-wide integrated pest management approach. A stratiÞed entomological sampling strategy
was therefore developed using spatial analytical tools and mathematical modeling. A preliminary phyt-
osociological census identiÞed eight types of suitable habitat, which could be discriminated from LandSat
7 ETM
⫹
satellite images and denominated wet areas. At the end of March 2009, 683 unbaited Vavoua traps
had been deployed, and the observed infested area in the Niayes was 525 km
2
. In the remaining area, a
mathematical model was used to assess the risk that ßies were present despite a sequence of zero catches.
The analysis showed that this risk was above 0.05 in 19% of this area that will be considered as infested during
the control operations. The remote sensing analysis that identiÞed the wet areas allowed a restriction of the
area to be surveyed to 4% of the total surface area (7,150 km
2
), whereas the mathematical model provided
an efÞcient method to improve the accuracy and the robustness of the sampling protocol. The Þnal size
of the control area will be decided based on the entomological collection data. This entomological sampling
procedure might be used for other vector or pest control scenarios.
KEY WORDS stratiÞed sampling, vector control, tsetse, remote sensing, African animal trypano-
somosis
In the subhumid savannah of West Africa, riverine
tsetse species such as Glossina palpalis gambiensis
Vanderplank 1949 inhabit riparian forests along river
systems, where they are major vectors of African an-
imal trypanosomosis (AAT) (Bouyer et al. 2006, Guer-
rini and Bouyer 2007) and human African trypanoso-
mosis or sleeping sickness (Camara et al. 2005). In
Senegal, as in other parts of West Africa, AAT is a
major obstacle to the development of more efÞcient
and sustainable livestock production systems (Itard et
al. 2003), and the presence of tsetse is considered a
major root cause of hunger and poverty (Feldmann et
al. 2005).
G. p. gambiensis thrives normally in areas that re-
ceive a minimum of 600 mm of rain annually (Brunhes
et al. 1998), but in western Senegal the ßies survive
under very dry conditions, where annual precipitation
is limited to 400 Ð500 mm. In addition, the ßies seem to
have adapted to peri-urban, densely populated areas
like the Parc de Hann located in the city center of
Dakar and neighboring locations (Se´bikotane, Pout).
The G. p. gambiensis populations in western Senegal
seem to be well adapted to speciÞc habitats called the
Niayes that owe their name to vestiges of Guinean
forest in low-lying interdune depressions that are pe-
riodically or permanently ßooded. Well described in
their original state by Toure´and Morel ⬎30 yr ago
(Morel and Toure´1967, Toure´1971, 1974), the Niayes
are situated northeast from Dakar, but have suffered
from an increase in human encroachment, which
1
Centre de Coope´ration Internationale en Recherche Agronomique
pour le De´veloppement, Unite´Mixte de Recherche Centre de Coope´r-
ation Internationale en Recherche Agronomique pour le De´veloppe-
ment-Institut National de la Recherche Agronomique Controˆle des Mal-
adies Animales, Campus International de Baillarguet, F34398,
Montpellier, France.
2
Institut Se´ne´galais de Recherches Agricoles-Laboratoire National
dÕElevage et de Recherches Ve´te´rinaires, Service de Parasitologie, BP
2057, Dakar-Hann, Se´ne´gal.
3
Corresponding author: Centre de Coope´ration Internationale en
Recherche Agronomique pour le De´veloppement-De´partement
Syste`mes Biologiques Unite´Mixte de Recherche Controˆle des mal-
adies animales exotiques et e´mergentes, Institut Se´ne´galais de Re-
cherches Agricoles-Laboratoire National dÕElevage et de Recherches
Ve´te´rinaires, Service de Parasitologie, BP 2057 Dakar-Hann Se´ne´gal
(e-mail: bouyer@cirad.fr).
4
Direction des Services Ve´te´rinaires, 37, avenue Pasteur, BP 67,
Dakar, Se´ne´gal.
5
Centre de Coope´ration Internationale en Recherche
Agronomique pour le De´veloppement, Unite´Propre de Recherche
Animal et gestion inte´gre´e des risques, Campus International de
Baillarguet, F34398, Montpellier, France.
6
Entomology Unit, Food and Agriculture Organization/Interna-
tional Atomic Energy Agency Agriculture and Biotechnology Labo-
ratory, Joint Food and Agriculture Organization/International
Atomic Energy Agency Programme, Vienna, Austria.
0022-2585/10/0543Ð0552$04.00/0 䉷2010 Entomological Society of America
has dramatically altered this entire ecosystem. La
Petite Coˆte is a similar, albeit drier ecosystem sit-
uated south of Dakar, which expands along the At-
lantic coast toward Joal and the Sine Saloum river.
The tsetse inhabiting the Niayes and La Petite Coˆte
belong to the most northwestern distribution of the
tsetse belt in West Africa (Fig. 1). No updated
information is currently available on the present
status and ecology of the tsetse populations in this
special habitat.
Fig. 1. Location of the study area. The suitability of the vegetation for harboring G. p. gambiensis after a phytosociological
census, and the wet areas as obtained from a supervised classiÞcation are presented. Gray cells at the top and the bottom
of the maps represent areas where no satellite imagery was available.
544 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 47, no. 4
The Niayes area has particular climatic conditions
that allow intensive cropping and cattle breeding even
during the dry season. In addition to the local breeds,
important exotic cattle populations are maintained for
the milk production, but their cost effectiveness is
continuously threatened by exposure to the trypano-
somoses. Horses that are mainly used for the transport
of food crops are also present in high densities, and the
area contains numerous stud farms with horses of high
economic value. Tsetse bites are also a continuous
nuisance for human populations, especially in Sebiko-
tane and Pout.
The seriousness of the tsetse and trypanosomosis
problems in the Niayes and La Petite Coˆte was re-
cently revealed during a parasitological and serolog-
ical survey of resident cattle, where AAT herd prev-
alence rates of 10Ð90% were observed (Seck et al.
2010). The survey showed that Trypanosoma vivax is
the most abundant species, followed by Trypanosoma
congolense. The parasitological prevalence may be
grossly underestimated in reality, because of the poor
sensitivity of the diagnostic Buffy coat technique that
was used (Pinchbeck et al. 2008).
During consecutive 2 yr in the 1970s, an attempt was
made to eliminate the G. p. gambiensis populations
from ⬎150 km of linear habitat in the Niayes area using
selective bush clearing and residual ground spraying
with a 2% formulation of dieldrin. After the campaign,
no tsetse were detected during two consecutive sur-
veys (Toure´1973). However, tsetse reappeared in the
1980s, necessitating a second campaign combining in-
secticide spraying with the deployment of traps and
insecticide-impregnated screens. The tsetse problem
seemed to have disappeared until ßies were detected
again in 1998 (B. Sall, unpublished data). These ob-
servations prompted the Direction de lÕElevage
(DIREL) and the Institut Se´ne´galais de Recherche
Agricole (ISRA) to carry out more extensive surveys
in 2003Ð04, which conÞrmed the presence of G. p.
gambiensis in the Niayes (B. Sall, unpublished data).
At the time of these surveys, it was not clear whether
the resurgence of the ßy population could be attrib-
uted to reinvasion from the main tsetse belt in the Sine
Saloum region, located ⬎100 km south of Dakar, or
from small residual pockets inside the control area.
In 2005, the DIREL initiated a tsetse control cam-
paign called Projet de lutte contre les glossines dans
les Niayes (tsetse control program in the Niayes),
which was funded by Senegal and technically and
Þnancially supported by the International Atomic En-
ergy Agency (IAEA). The project is being imple-
mented in the context of the Pan African Tsetse and
Trypanosomiasis Eradication Campaign (PATTEC), a
political initiative of the African heads of state that
calls for increased efforts to manage the tsetse and
trypanosomosis problem, which is considered a se-
rious impediment to sustainable agricultural rural
development in most sub-Saharan African countries
(http://www.africa-union.org/Structure_of_the_
Commission/depPattec.htm).
The tsetse project in Senegal has adopted an area-
wide integrated pest management (AW-IPM) ap-
proach that aims at integrating various control tactics
(e.g., traps, insecticide-impregnated targets, live baits,
the sterile insect technique) (Dyck et al. 2005) to
target an entire tsetse population within a circum-
scribed area (Klassen 2005). To develop an appropri-
ate AW-IPM strategy, detailed and accurate data are
required of the target population. The DIREL and
ISRA, in collaboration with the Centre de Coope´ra-
tion Internationale en Recherche Agronomique pour
le De´veloppement, the Food and Agriculture Orga-
nization, and the IAEA therefore developed a speciÞc
entomological sampling protocol to deÞne as precisely
as possible the present distribution of tsetse in the
Niayes and La Petite Coˆte. These entomological sur-
veys are part of an entire package of baseline data that
include parasitological and serological disease preva-
lence, tsetse population genetics, and socioeconomic
and environmental impact data that will be published
elsewhere. A comprehensive analysis of these data will
assist the decision-making process of selecting a strat-
egy of sustainable elimination or perpetual suppres-
sion of the tsetse populations (Vreysen et al. 2007).
This study presents the development of a sampling
process that was designed for this particular area, com-
bining modern tools such as remote sensing (RS) to map
suitable habitats, geographic information systems, and
the Global Positioning System (GPS) with mathematical
modeling to maximize efÞciency and accuracy of the
sampling protocol in preparation for an AW-IPM tsetse
campaign in the Niayes. The stratiÞed sampling devel-
oped in this study could serve as a template for other
tsetse control campaigns presently launched within the
PATTEC initiative in West Africa.
Materials and Methods
The Study Area. The study area of 7,150 km
2
is located
along the Atlantic coast at 13.5Ð15.5⬚N and 16.5Ð17.5⬚W.
The area is 180 km long and 30Ð35 km wide and can be
divided into three main sections: the Niayes in the North,
the Sine Saloum in the South, and La Petite Coˆte in
between (Fig. 1). Mean daily temperatures vary be-
tween 25⬚C and 30⬚C and relative humidity between 60
and 80%. Annual precipitation is 200Ð500 mm with a
rainy season from July to September.
Preliminary Surveys. A5⫻5-km grid overlaying the
entire target area (286 cells) was developed with let-
ters and numbers allocated to rows and columns, re-
spectively, to facilitate the Þeld sampling procedures
(Leak et al. 2008). During preliminary surveys be-
tween September and December 2007, a phytosocio-
logical census was carried out in 277 sites, of which 87
were selected as learning sites, to achieve the super-
vised classiÞcation (see below), and 190 as validating
sites, to test the speciÞcity and sensitivity of this clas-
siÞcation (Fig. 2). All woody species were identiÞed
together with their stratum and cover class (Þve
classes from 0 to 100%). Simultaneously, 50 unbaited
Vavoua traps, constituted of a cone of mosquito net-
ting placed over three panels of black and blue fabric
with an angle of 120⬚between them (Laveissie`re and
Gre´baut 1990), were deployed in the various habitats
July 2010 BOUYER ET AL.: STRATIFIED SAMPLING IN PEST MANAGEMENT 545
encountered to update our knowledge of tsetse hab-
itat preferences. The phytosociological census was
used to group the learning and validation sites in three
classes of habitat suitability, based on the presence and
abundance of forest plant indicators described previ-
ously (Bouyer et al. 2005), as follows: 1) not suitable
when forest species or equivalent (tree crops) were
absent; 2) suitable (S) when forest species (or culti-
vated trees) cover was above 50%; and 3) suitable, but
degraded (SD), when forest species were present, but
with a cover of ⬍50%.
Remote Sensing Analyses. A riverine tsetse species
such as G. p. gambiensis cannot survive without suit-
able habitat containing forest tree species (or equiv-
alent), so all vegetation with important photosyn-
thetic activity was identiÞed during a supervised
classiÞcation from a LandSat 7 ETM
⫹
image of April
2001, using the learning data set (see methodological
details in Guerrini et al. 2008). The learning sites were
used to deÞne the regions of interest using the ENVI
4.3 software. These areas corresponded to the pres-
ence of ground water at the end of the dry season and
are henceforth denoted wet areas. It was demon-
strated before that photosynthetic activity at a smaller
spatial scale is positively correlated with density and
survival of G. p. gambiensis (Rogers and Randolph
1991). In view of the importance of obtaining an ac-
curate picture of the distribution of G. p. gambiensis in
the area, it was considered important to sample all S
and SD sites. The sensitivity of this classiÞcation was
calculated as the number of S and SD validation sites
located within the obtained wet areas divided by the
total number of S and SD sites. The speciÞcity was
calculated as the ratio of S and SD sites within wet
areas divided by the total number of sites within wet
areas.
Entomological Sampling Strategy. Within each grid
cell of the study area, one to 43 unbaited Vavoua traps
(Vestergaard Frandsen ApS, Kolding, Denmark) were
deployed in the available wet areas for tsetse ßy sam-
pling (Fig. 2). Males and females were counted sep-
arately, but their densities were merged in the present
distribution analysis because they harbor similar hab-
itat preferences. The Þeld teams were equipped with
GPS (Map 75 Garmin) handheld instruments that had
the grid and polygons of the wet areas uploaded as
polylines converted into track objects using the soft-
ware DNR Garmin 5.2.25. Traps were deployed for 3 d
and removed when at least one tsetse was captured
during this period. When no tsetse was caught, traps
remained deployed for up to 30 d and were checked
every 1Ð5 d. In each trap site, ecological data were
collected, including the vegetation type and tree
cover.
Each grid cell was assigned to one of three possible
categories, as follows: 1) no wet area in dry season:
considered as tsetse-free, and no trapping was at-
tempted; (2) no tsetse caught: if no tsetse ßies were
caught after sampling of the wet areas; (3) infested: if
one tsetse ßy was caught in at least one of the wet
areas.
Probability Maps. Zero catch of tsetse ßies does not
mean absence of tsetse in the sampled area. Therefore,
a recently published model (Barclay and Hargrove
2005) was used to evaluate the probability (or risk)
that tsetse are actually present in a grid cell when not
Fig. 2. Selection of trapping sites in the identiÞed wet areas. The example of grid cells G11 to H13 is presented.
546 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 47, no. 4
sampled through a given sampling effort (number of
traps ⫻days), i.e., for all the cells in the second
category “no tsetse caught.”In this model, the prob-
ability of observing a sequence of zero catches if in fact
there are insects in the sampled area is given by the
following: P⫽exp (⫺St
), where Sis the number of
traps deployed in the total area, tthe number of days
for which each trap is operated,
the trap efÞciency,
and
the population density (number of insects/area
of suitable habitat). This probability was calculated for
each grid cell using the speciÞc number of traps, duration
of trapping, and the total wet area surface in each grid
cell. In the absence of any control effort, the minimum
number of ßies in the sample area was set at 10, consid-
ering that this is an underestimation for any resident
tsetse population in the absence of control effort. The
goal of this exercise was to detect resident tsetse popu-
lations and was not related to assessing dispersal. This
issue will be addressed later when mark-release-recap-
ture experiments and population genetics (Bouyer et al.
2009) will be used to delineate the Þnal target area. The
trap efÞciency, deÞned as the probability that a trap
catches a ßy in an area of 1 km
2
/d, was deÞned as 0.01.
This Þgure was calculated from a reanalysis of raw cap-
ture data of ⬎400,000 released ßies during the elimina-
tion campaign against the same subspecies in Side´rado-
ugou, Burkina Faso (data not shown) (Cuisance et al.
1984, Politzar and Cuisance 1984).
Results
Suitable Habitats. Eight types of habitat were iden-
tiÞed as being suitable for G. p. gambiensis, which was
conÞrmed by tsetse catches during the survey: 1)
natural Guinean forest galleries close to permanent
springs with riparian tree vegetation such as Nauclea
latifolia, Phyllantus mullerianus, and Lonchocarpus sp.
as predominant plant species; 2) semidisturbed Guin-
ean riverine thickets with bushy vegetation replacing
the original trees and where Alchornea cordifolia, Phyl-
lantus mullerianus, and Phoenix reclinata predominate;
3) Euphorbia spp. fences located in ßooded areas and
generally mixed with the riverine thickets; 4) swampy
forests, more or less natural, around permanent ponds
and generally protected by a wall, as in city parks (Parc
de Hann, Pasteur-Mbao); 5) riverine forests located
inland adjacent to the mangroves, which do not ßood
during high tide and where the presence of fresh water
springs allows the occurrence of non- or semihalo-
phyte forest tree species such as Elais guineensis, P.
reclinata, and A. cordifolia; 6) palm tree crops with E.
guineensis as relics from gallery forests where people
have kept only the useful plant species (often this
habitat is reduced to food crops); 7) other tree crops
(citrus fruit, mango), some of which are not well
trimmed, and are watered the entire year or are in the
vicinity of an underground hydrological network; and
8) lakes or swamps with reeds, which can be up to 3 m
tall.
All of these habitats rely on the presence of fresh
water during the dry season, which can be provided by
underground river networks, springs, or human wa-
tering activities. In these sites, transpiring plants allow
the maintenance of a certain level of humidity critical
for the survival of riverine tsetse, and for this reason
they cannot thrive in open savannah habitats (Challier
1973).
Mapping of Wet Areas. The sensitivity of the clas-
siÞcation was 0.96 (1 S and 3 SD not detected on 110
validation sites), whereas its speciÞcity was only 0.43,
which means that ⬍50% of the wet areas actually
harbored suitable habitats for tsetse.
From the 683 trap locations, only 61% were frequented
by cattle, and insecticides for crop protection were ap-
plied in 48% of the sites. Permanent and temporary sur-
face water was observed in 27 and 7% of the sites, re-
spectively. The depth of ground water was shallower
than5min58%ofthesites, between 5 and 10 m in 12%,
and deeper than 10 m in the remaining sites.
Table 1 shows the distribution and rates of infesta-
tion in the various suitable habitats described above
among the wet areas (some sites had a mixed habitat).
The differences in infestation rates between habitats
were signiÞcant (Pearson
2
,df⫽10, P⬍0.0001).
Much of the original vegetation (gallery forests, riv-
erine thickets, palm tree forests) has disappeared, and,
if present, was less infested with G. p. gambiensis than
some artiÞcial habitats like citrus tree plantations or
Euphorbia hedges (P⬍0.05), indicating that this spe-
cies of tsetse is well adapted to artiÞcial habitats in the
Niayes, given suitable humidity.
Entomological Results. Of the 286 grid cells in the
study area, wet areas were absent in 87 cells, and 105 of
the remaining 199 cells were surveyed. Traps were set
from December 2007 to March 2009 in 683 sites, corre-
sponding to 3,564 trapping events. G. p. gambiensis was
the only tsetse species sampled, and ßies were captured
in 27% of the trapping sites distributed over 23 grid cells,
of which 21 were located in the Niayes area and two in
the Sine Saloum. Although the mean apparent density of
G. p. gambiensis was 0.41 ßies/trap/d (SD 1.70), ßy
catches ßuctuated between 0 and 18 ßies/trap/d, indi-
cating a very heterogeneous ßy distribution (Fig. 3). The
Table 1. Description of tsetse habitats in the Niayes area
Habitats % trapping sites
within habitat
% infested sites
within this habitat
Citrus tree plantations 16 (109) 31 (34)
Euphorbia hedges 17 (116) 22 (26)
Swampy forests 7 (48) 16 (8)
Mango tree plantations 35 (239) 15 (36)
Riverine thickets 16 (109) 12 (13)
Lake or swamp with reeds 4 (27) 12 (3)
Palm tree plantations 15 (102) 6 (6)
Food crops 20 (137) 6 (8)
Other tree plantations 6 (41) 5 (2)
Others 8 (55) 2 (1)
Forest galleries 1 (7) 0 (0)
The frequency distribution of the 683 trap locations in relation to
the various preferred habitats encountered in the target area and
percentage of sites infested by G. p. gambiensis are presented. The
percentages in the Þrst column are not cumulative to 100% because
some of the trapping sites harbored mixed habitats. The habitats are
classed from most to least infested. The number of trapping sites for
each class is given in parentheses.
July 2010 BOUYER ET AL.: STRATIFIED SAMPLING IN PEST MANAGEMENT 547
total observed infested area was 575 km
2
, of which 525
km
2
was situated in the Niayes area. The trapping results
seemed to indicate that the infested area was surrounded
in the north, east, and south by grid cells where no tsetse
ßies were caught.
Probability Maps. Of the 105 grid cells where traps
were deployed, the trap surveys had already demon-
strated the presence of ßies in 21 cells. The probability
model described above was applied to the remaining
84 grid cells where no ßies were trapped. The analysis
indicated a probability of tsetse presence below 0.05
(the level of risk accepted) in 68 grid cells and above
0.05 in the remaining 16 grid cells. Fig. 4 presents
graphically the probability of tsetse presence in those
cells around the conÞrmed tsetse-infested area.
Discussion
The collection of entomological baseline data from
an area that is the target of an AW-IPM program is a
prerequisite for the development of an appropriate
control strategy (Vreysen 2005, Leak et al. 2008). Data
Fig. 3. The apparent densities of G. p. gambiensis in the Niayes area of Senegal. Trap catches (number of ßies/trap/d)
are given for the dry season of 2008.
548 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 47, no. 4
on tsetse species present, and the relative abundance,
distribution, and temporal and spatial dynamics of the
population will enable program managers to select
appropriate control tactics and develop a plan of how
and where to deploy these as adapted to the charac-
teristics of the target zone/population. The baseline
data will also allow accurate monitoring of the control
operation and to continuously assess progress made
(Vreysen 2005). For large targeted areas (e.g., the
tsetse project in Ethiopia) (Alemu et al. 2007), logis-
tics, time, and economics will limit the amount of
trapping that can be carried out. Therefore, a repre-
sentative grid-based sampling approach (Leak et al.
2008) has been proposed that aims to collect data from
carefully selected habitats within a given grid cell that
are considered to be representative for other similar
habitat areas in that grid cell. The sampling process
described in this work is based upon this approach, but
was Þne-tuned and improved using modern tools of
spatial analysis (geographic information systems/RS/
GPS) (Cox and Vreysen 2005), mathematical model-
ing, and accounting for the ecological afÞnities of
a riverine tsetse species such as G. p. gambiensis
(Challier 1973). Although the operational phase of the
elimination campaign has not been initiated yet in
Senegal, and most other countries under the PATTEC
initiative are in the phase of data collection, we believe
that the methodology described in this study would
assist in making the data collection more cost effec-
tive.
The Niayes of Senegal receives ⬍500 mm of rain
annually, and human development has signiÞcantly
modiÞed the habitats suitable for G. p. gambiensis,
which illustrates the tremendous adaptability of the
Fig. 4. Probability of presence of G. p. gambiensis. This probability was implemented in the grid cells where no ßies were
trapped and which are situated around the conÞrmed infested area. The black rectangle at the top represents the cells G11
to H13 shown in Fig. 2.
July 2010 BOUYER ET AL.: STRATIFIED SAMPLING IN PEST MANAGEMENT 549
species. A G. p. gambiensis population is even found in
the Parc de Hann located in the densely populated city
center of Dakar. The park is a 0.4-km
2
forest swamp
protected by a stone wall, and contains a zoo with
caged animals, a pony club, and a high number of daily
visitors, i.e., all the requirements for the species to
proliferate.
In the Niayes, high rates of tsetse infestation were
encountered in perennial crop areas, especially citrus
and in Euphorbia hedges, swamps with reeds, and
riparian thickets. Tsetse presence was highly corre-
lated with wet areas, where particular conditions al-
low fresh water to be available during the dry season.
Mangrove and inland riverine forests adjacent to man-
groves were absent in the Niayes area, but were
present in the southern part of the study area, i.e., the
Sine Saloum (Fig. 1). The inland riverine forests ad-
jacent to mangroves should not be confounded with
the mangroves themselves, where Rizophora spp. and
Avicenia spp. are the predominant species. Mangrove
vegetation is not suitable for G. p. gambiensis breeding
(i.e., larviposition) because it is ßooded every day
(Toure´1971, 1974), but this habitat is often fre-
quented by ßies seeking hosts for a blood meal, re-
sulting in high population densities. In Senegal, G. p.
gambiensis is absent from the northern part of the Sine
Saloum, where a decrease in annual rainfall has led to
a disappearance of the inland riverine forests adjacent
to mangroves. In addition, the area has experienced a
reduction of the Rizophora because of an increase in
salinity, resulting in a habitat that cannot offer enough
shelter to tsetse anymore. Even in the infested area of
the southern part of the Sine Saloum, the distribution
of G. p. gambiensis is very patchy, and mostly conÞned
to islands or riverbanks covered with inland riverine
forests adjacent to mangroves.
The remote sensing analysis allowed us to restrict
the sampling area to only 4% of the original target area.
It showed a very high sensitivity, but a low speciÞcity,
which is reßected in the sampling results, i.e., tsetse
ßies were trapped in ⬍30% of the sampling sites. High
sensitivity and low speciÞcity increase the probability
that most of the sites inhabited by tsetse are detected,
which is crucial in an AW-IPM campaign, even if it will
require a more intense sampling effort. In the four sites
(one S and three SD) not detected by the classiÞca-
tion, no ßies were trapped even after deployment of
the traps for 1 mo. Moreover, the wet area classiÞca-
tion permitted the detection of tsetse ßies in unex-
pected sites such as a military camp, where sampling
would never have been implemented without this wet
area classiÞcation. The remoteness of some of the wet
areas (e.g., suitable trapping sites were sometimes
located at a distance of 2 km from a track) and the
small size of the suitable vegetation patches necessi-
tated the use of GPS guidance to locate these sites in
the grids. The entire sampling process was therefore
driven by the remote sensing analyses and could be
implemented by incorporating the RS data in the GPS.
It is still probable that not all potential habitats have
been detected in view of the low resolution (30 m) of
the LandSat image, and the pixels corresponding to
suitable habitat could be easily diluted in surrounding
pixels that might be unsuitable (de La Rocque et al.
2005, Bouyer et al. 2006).
The calculated probability of presence using the
Barclay and Hargrove model (Barclay and Hargrove
2005) provides a method to improve the cost effec-
tiveness of the sampling process. Despite the zero
catches in some of the grid cells, the model likely
overestimated the probability of presence as: 1) ⬍50%
of the wet areas are actually suitable for G. p. gambi-
ensis, and 2) the density of the source tsetse popula-
tions is probably higher than the assumed 10 in 25 km
2
in the absence of control efforts. In Burkina Faso,
densities of ⬎3,500 ßies were observed in riparian
forest along a 10-km river section (Bouyer et al.
2007b), and in some localities of Senegal (e.g., Pout),
⬎500 ßies were captured in 3 d with 10 Vavoua traps
during the present sampling campaign. The applica-
tion of the model proved to be a convenient tool to
prioritize additional sampling efforts in grid cells
where the sampling was considered insufÞcient. The
quantitative methods, such as the remote sensing anal-
ysis that permitted the classiÞcation of the ecological
important areas (i.e., wet areas) for G. p. gambiensis
and the Barclay and Hargrove model enabled the
optimization of the entomological sampling process
before initiation of the operational phase of the actual
control campaign in Senegal, which started in mid-
2009. Taking into consideration the ecological char-
acteristics of other vectors or insect pests, the de-
scribed approach could easily be expanded to improve
sampling procedures in preparation of AW-IPM cam-
paigns.
Although not all grid cells were sampled in the
current study, the survey methodology was adapted
and adjusted as a result of this analysis during the
whole sampling process, and hence, the sampling was
directed toward those cells where the level of conÞ-
dence was the lowest (i.e., where the former proba-
bility of presence was the highest). Some of the grid
cells where no tsetse were captured during the surveys
still have a high probability of being infested with
tsetse; they will so be considered infested and treated
as well, when in contact with infested cells. Additional
sampling in these cells will be carried out during the
control campaign. This information will thus be taken
into consideration when the total target control area
will be determined.
Despite the assumed high accuracy of the described
sampling process, it will be impossible to identify all
the sites where tsetse are present at a given time for
the following reasons: 1) some wet areas might remain
undetected in the classiÞcation process; 2) the prob-
ability of ßy presence will remain high in some areas,
even with zero catches because of the necessity of a
trade-off between accuracy and cost; and 3) the entire
sampling process is based upon dry season conditions,
but it is known that G. p. gambiensis tsetse are able to
move through and breed in other habitats during the
wet season (Cuisance et al. 1985, Bouyer et al. 2007a),
even if restricted again during the following dry sea-
son. Their distribution, therefore, must be seen as
550 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 47, no. 4
dynamic and will be inßuenced by climatic factors
such as rainfall, which can vary from year to year.
The Þrst elimination effort in the 1970s most likely
failed (Toure´1973), because not all ecologically im-
portant areas were identiÞed and treated; e.g., the G.
p. gambiensis population in Pout, which was observed
to have a high population density, was unknown and,
hence, not targeted at the time (S. Toure´, personal
communication). A population genetics study re-
cently conÞrmed the isolation of the tsetse popula-
tions of the Niayes from those of the Sine Saloum, i.e.,
the nearest population of the main tsetse belt in south-
ern Senegal (Solano et al. 2010). The use of old spec-
imens from the Niayes, kept in ISRA collections, will
also assist with testing this hypothesis.
The persistence of small pockets is particularly
problematic when stationary control techniques such
as traps or screens impregnated with insecticides are
used in a fragmented landscape such as the Niayes and
La Petite Coˆte. In such ecosystems, tsetse dispersal is
likely to be low, and the tsetse populations likely to be
highly structured (Bouyer et al. 2007b, 2009), which
corresponds to a metapopulation rather than to an
homogeneous panmictic population (Hanski and Gag-
giotti 2004).
The results that will be derived from a complete
analysis of the collected data sets (entomological, par-
asitological, serological, etc.) will allow the project
managers to select and combine control tactics that
are best suited to create a sustainable zone free of G.
p. gambiensis in the Niayes.
Acknowledgments
We are very thankful to the veterinary staff involved in the
Þeld studies, particularly Abdou Gaye Mbaye, Babacar Ca-
mara, Babel Sow, Aõ¨da Gaye, Mamadou Demba, Alphonse
Manga, and Mansour Fall. We are indebted to the Direction
des Services Ve´te´rinaires and ISRA-LNERV for providing
excellent working conditions.
References Cited
Alemu, T., B. Kapitano, S. Mekonnen, G. Aboset, M. Kiflom,
B. Bancha, G. Woldeyes, K. Bekele, and U. Feldmann.
2007. Area-wide control of tsetse and trypanosomosis:
Ethiopian experience in the Southern Rift Valley, pp.
325Ð335. In M. Vreysen, A. S. Robinson, and J. Hendrichs
(eds.), Area-wide control of insect pests: from research to
Þeld implementation. Springer, Dordrecht, The Nether-
lands.
Barclay, H. J., and J. W. Hargrove. 2005. Probability models
to facilitate a declaration of pest-free status, with special
reference to tsetse (Diptera: Glossinidae). Bull. Entomol.
Res. 95: 1Ð11.
Bouyer, J., L. Guerrini, J. Ce´sar, S. de la Rocque, and D.
Cuisance. 2005. A phyto-sociological analysis of the dis-
tribution of riverine tsetse ßies in Burkina Faso. Med. Vet.
Entomol. 19: 372Ð378.
Bouyer, J., L. Guerrini, M. Desquesnes, S. de la Rocque, and
D. Cuisance. 2006. Mapping African animal trypanoso-
mosis risk from the sky. Vet. Res. 37: 633Ð645.
Bouyer, J., A. Sibert, M. Desquesnes, D. Cuisance, and S. de
La Rocque. 2007a. A model of diffusion of Glossina pal-
palis gambiensis (Diptera: Glossinidae) in Burkina Faso,
pp. 221Ð228. In M.J.B. Vreysen, A. S. Robinson, and J. Hen-
drichs (eds.), Area-wide control of insect pests: from
research to Þeld implementation. Springer, Dordrecht,
The Netherlands.
Bouyer, J., S. Ravel, L. Vial, S. The´venon, J.-P. Dujardin, T.
de Meeus, L. Guerrini, I. Sidibe´, and P. Solano. 2007b.
Population structuring of Glossina palpalis gambiensis
(Diptera: Glossinidae) according to landscape fragmen-
tation in the Mouhoun river, Burkina Faso. J. Med. En-
tomol. 44: 788Ð795.
Bouyer, J., T. Balenghien, S. Ravel, L. Vial, I. Sidibe´,S.
The´venon, P. Solano, and T. De Meeuˆs. 2009. Population
sizes and dispersal pattern of tsetse ßies: rolling on the
river? Mol. Ecol. 18: 2787Ð2797.
Brunhes, J., D. Cuisance, B. Geoffroy, and J.-P. Hervy. 1998.
Les glossines ou mouches tse´-tse´. CIRAD/ORSTOM,
Montpellier, France.
Camara, M., D. Kaba, M. Kagbadouno, J. R. Sanon, P. Ouen-
deno, and P. Solano. 2005. La Trypanosomose Humaine
Africaine en zone de mangrove en Guine´e: caracte´ris-
tiques e´pide´miologiques et cliniques de deux foyers
voisins. Med. Trop. 65: 155Ð161.
Challier, A. 1973. Ecologie de Glossina palpalis gambiensis
Vanderplank, 1949 (Diptera-Muscidae) en savane
dÕAfrique occidentale. ORSTOM, Paris, France.
Cox, J.S.H., and M.J.B. Vreysen. 2005. Use of geographic
information systems and spatial analysis in area-wide in-
tegrated pest management programmes that integrate the
sterile insect technique, pp. 453Ð477. In V. A. Dyck,
J. Hendrichs, and A. S. Robinson (eds.), Sterile insect
technique: principles and practice in area-wide inte-
grated pest management. Springer, Dordrecht, The Neth-
erlands.
Cuisance, D., H. Politzar, P. Merot, and I. Tamboura. 1984.
Les laˆchers de maˆles irradie´s dans la campagne de lutte
inte´gre´e contre les glossines dans la zone pastorale de
Side´radougou, Burkina Faso. Rev. Elev. Me´d. Ve´t. Pays
Trop. 37: 449Ð468.
Cuisance, D., J. Fe´vrier, J. Dejardin, and J. Filledier. 1985.
Dispersion line´aire de Glossina palpalis gambiensis et G.
tachinoides dans une galerie forestie`re en zone soudano-
guine´enne (Burkina Faso). Rev. Elev. Me´d. Ve´t. Pays
Trop. 38: 153Ð172.
de La Rocque, S., J. F. Michel, J. Bouyer, G. De Wispelaere,
and D. Cuisance. 2005. Geographical Information Sys-
tems in parasitology: a review of potential applications
using the example of animal trypanosomosis in West Af-
rica. Parassitologia 47: 97Ð104.
Dyck, V. A., G. Hendrickx, and A. S. Robinson. 2005. Sterile
insect technique. Springer, International Atomic Energy
Agency, Vienna, Austria.
Feldmann, U., V. A. Dyck, R. C. Mattioli, and J. J. Jannin.
2005. Potential impact of tsetse ßy control involving the
sterile insect technique, pp. 701Ð723. In V. A. Dyck,
J. Hendrichs, and A. S. Robinson (eds.), Sterile insect
technique: principles and practice in area-wide inte-
grated pest management. Springer, Dordrecht, The Neth-
erlands.
Guerrini, L., and J. Bouyer. 2007. Mapping African animal
trypanosomosis risk: the landscape approach. Vet. Ital. 43:
643Ð654.
Guerrini, L., J. P. Bord, E. Ducheyne, and J. Bouyer. 2008.
Fragmentation analysis for prediction of suitable habitat
for vectors: the example of riverine tsetse ßies in Burkina
faso. J. Med. Entomol. 45: 1180Ð1186.
July 2010 BOUYER ET AL.: STRATIFIED SAMPLING IN PEST MANAGEMENT 551
Hanski, I., and O. E. Gaggiotti. 2004. Ecology, genetics and
evolution of metapopulations. Elsevier Academic, Am-
sterdam, The Netherlands.
Itard, J., D. Cuisance, and G. Tacher. 2003. Trypanosomo-
ses: historique: re´partition ge´ographique, pp. 1607Ð1615.
In P.-C. Lefe`vre, J. Blancou, and R. Chermette (eds.),
Principales maladies infectieuses et parasitaires du be´tail:
Europe et Re´gions chaudes. Lavoisier, Paris, France.
Klassen, W. 2005. Area-wide integrated pest management
and the sterile insect technique, pp. 39 Ð68. In V. A. Dyck,
J. Hendrichs, and A. S. Robinson (eds.), Sterile insect
technique: principles and practice in area-wide inte-
grated pest management. Springer, Dordrecht, The Neth-
erlands.
Laveissie`re, C., and P. Gre´baut. 1990. Recherches sur les
pie`ges a`glossines (Diptera, Glossinidae): mise au point
dÕun mode`le e´conomique: le pie`ge “Vavoua.”Trop. Med.
Parasitol. 41: 185Ð192.
Leak, S.G.A., D. Ejigu, and M.J.B. Vreysen. 2008. Collection
of entomological baseline data for tsetse area-wide inte-
grated pest management programmes: FAO animal pro-
duction and health guidelines. Food and Agriculture Or-
ganization of the United Nations, Rome, Italy.
Morel, P. C., and S. Toure´. 1967. Glossina palpalis gambi-
ensis Vanderplank 1949 (Diptera) dans la re´gion des
Niayes et sur la Petite Coˆte (Re´publique du Se´ne´gal).
Rev. Elev. Me´d. Ve´t. Pays Trop. 20: 571Ð578.
Pinchbeck, G. L., L. J. Morrison, A. Tait, J. Langford, L.
Meehan, S. Jallow, J. Jallow, A. Jallow, and R. M.
Christley. 2008. Trypanosomosis in Gambia: prevalence
in working horses and donkeys detected by whole ge-
nome ampliÞcation and PCR, and evidence for interac-
tions between trypanosome species. BMC Vet. Res. 4: 7.
Politzar, H., and D. Cuisance. 1984. An integrated campaign
against riverine tsetse ßies Glossina palpalis gambiensis
and Glossina tachinoides by trapping and the release of
sterile males. Insect Sci. Appl. 5: 439Ð442.
Rogers, D. J., and S. E. Randolph. 1991. Mortality rate and
population density of tsetse ßies correlated with satellite
imagery. Nature 351: 739Ð741.
Seck, M. T., J. Bouyer, B. Sall, Z. Bengaly, and M.J.B. Vreysen.
2010. The Prevalence of African animal trypanosomoses
and tsetse presence in Western Senegal. Parasite (in
press).
Solano, P., D. Kaba, S. Ravel, N. Dyer, B. Sall, M.J.B. Vreysen,
M. T. Seck, H. Darbyshir, L. Gardes, M. J. Donnelly, T.
de Meeuˆs, and J. Bouyer. 2010. Tsetse population genet-
ics as a tool to choose between suppression and elimina-
tion: the case of the Niayes area in Senegal. Plos Tropical
Neglected Diseases 4: e692.
Toure´, S. 1971. Les glossines (Diptera, glossinidae) du
Se´ne´gal: ecologie, re´partition ge´ographique et incidence
sur les trypanosomoses. Rev. Elev. Me´d. Ve´t. Pays Trop.
24: 551Ð563.
Toure´, S. 1973. Lutte contre Glossina palpalis gambiensis
dans la re´gion des niayes du Se´ne´gal. Rev. Elev. Me´d. Ve´t.
Pays Trop. 26: 339Ð347.
Toure´, S. 1974. Note sur quelques particularite´s dans
lÕhabitat de Glossina palpalis gambiensis Vanderplank,
1949 (Diptera, Glossinidae) observe´es au Se´ne´gal. Rev.
Elev. Me´d. Ve´t. Pays Trop. 27: 81Ð94.
Vreysen, M.J.B. 2005. Monitoring sterile and wild insects in
area-wide integrated pest management programmes, pp.
325Ð361. In V. A. Dyck, J. Hendrichs, and A. S. Robinson
(eds.), Sterile insect technique: principles and practice in
area-wide integrated pest management. Springer, Dor-
drecht, The Netherlands.
Vreysen, M., A. S. Robinson, and J. Hendrichs. 2007. Area-
wide control of insect pests, from research to Þeld im-
plementation. Springer, Dordrecht, The Netherlands.
Received 10 June 2009; accepted 24 March 2010.
552 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 47, no. 4
