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Studies on the behavior of peridomestic and endophagic M form Anopheles gambiae from a rice growing area of Ghana

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  • College of Health Sciences Asmara

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The 'paddy paradox', the occurrence of large populations of vectors but low amounts of malaria transmission where irrigated rice is grown, was investigated in a village in Ghana where M form Anopheles gambiae are common. Peridomestic and indoor host-seeking mosquitoes were collected in tent traps and light traps over 21 consecutive nights at the start of the rainy season in June 2009 when the population increased exponentially from less than 100 per night to over 1000. Infection rates in the overall mosquito population were 0.3% and in the estimated parous population were 1.9%. Numbers of An. gambiae in the tent trap peaked between midnight and 02:40 am. The majority of insects were taking their first blood meal, as virgins or shortly after mating. More than expected were collected in the light trap during a rainstorm at the start of the rains but overall numbers were not affected. Fewer than expected were collected after a subsequent storm. Recruitment to the adult population decreased over the following days. It is hypothesised that the 'paddy paradox' is due to young pre-gravid insects dispersing more widely than gravid ones, not necessarily to low survival in the mosquito.
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Studies on the behaviour of peridomestic
and endophagic M form Anopheles
gambiae from a rice growing area of Ghana
J.D. Charlwood
1,2
*, E.V.E. Tomás
2
, P. Salgueiro
3
,
A. Egyir-Yawson
4
, R.J. Pitts
5
and J. Pinto
3
1
DBL Centre for Health Research & Development, 57 Thorvaldensvej,
Fredriksberg 1871, Denmark:
2
MOZDAN, PO Box 8, Morrumbene,
Inhambane Province, Mozambique:
3
Centro de Malária e outra Doenças
Tropicais, Rua da Junqueira 100, Lisbon 1300, Portugal:
4
Biotechnology and
Nuclear Agriculture Research Institute, Ghana Atomic Energy Commission,
Kwabenya, Accra, Ghana:
5
Vanderbilt University, Department of Biological
Sciences, Nashville, Tennessee, USA
Abstract
The paddy paradox, the occurrence of large populations of vectors but low
amounts of malaria transmission where irrigated rice is grown, was investigated in a
village in Ghana where M form Anopheles gambiae are common. Peridomestic and
indoor host-seeking mosquitoes were collected in tent traps and light traps over 21
consecutive nights at the start of the rainy season in June 2009 when the population
increased exponentially from less than 100 per night to over 1000. Infection rates in
the overall mosquito population were 0.3% and in the estimated parous population
were 1.9%. Numbers of An. gambiae in the tent trap peaked between midnight and
02:40 am. The majority of insects were taking their first blood meal, as virgins or
shortly after mating. More than expected were collected in the light trap during a
rainstorm at the start of the rains but overall numbers were not affected. Fewer than
expected were collected after a subsequent storm. Recruitment to the adult
population decreased over the following days. It is hypothesised that the paddy
paradoxis due to young pre-gravid insects dispersing more widely than gravid ones,
not necessarily to low survival in the mosquito.
Keywords: Anopheles gambiae, rainfall, behaviour, paddy-paradox, tent trap
(Accepted 21 February 2011)
Introduction
In order to properly understand the epidemiology of
malaria, information on the ecology and bionomics of the local
vector population is required. Information on factors such as
population density, survival rate, infection rate, blood-feeding
frequency, time and location of biting, as well as host
preference, all affect the ability of a mosquito population to
transmit malaria. In West Africa, the most important vectors
are the M and S forms of Anopheles gambiae. These are
incipientspecies in which hybrids are fertile and can be
readily produced in the laboratory. Where they occur in
sympatry in the wild, however, they often show low rates of
hybridization. (Diabate et al., 2009).
To paraphrase Voltaire, If Anopheles gambiae did not exist,
man would have created him. Humans created the larval
ecological niche, puddles in exposed forest, exploited by
An. gambiae. We have also provided shelter and food for the
adults since we first arrived on the planet but especially since
the beginning of agriculture when the species went through a
*Author for correspondence
E-mail: jdcharlwood@gmail.com
Bulletin of Entomological Research (2011) 101, 533539 doi:10.1017/S0007485311000125
©Cambridge University Press 2011
First published online 15 March 2011
population expansion (Donnelly et al.,2001). The creation of
other larval habitats with expanding agriculture may be an
evolutionary driving force in the An. gambiae complex in
general and in An. gambiae s.s. in particular. The M and S forms
differ in their most commonly used breeding sites. The pan-
African S form predominates in temporary pools, the
archetypal An. gambiae breeding site, whilst high population
densities of M form An. gambiae are associated with rice culti-
vation. In many such irrigated sites, despite the large numbers
of mosquitoes, transmission is low, leading to the so called
paddy paradox(Ijumba & Lindsay, 2001).
Examination of the behaviour of young mosquitoes has
rarely been undertaken (Charlwood et al., 2003a), and the
reason for the relatively low survival and infection rates in
mosquitoes from rice growing areas remains unknown. It may
have something to do with their behaviour when newly em-
erged. From their emergence as adults to the time of their first
oviposition, female An. gambiae undergo a number of physio-
logical changes such as a rise in juvenile hormone (Noriega,
2004) and perform a number of one-offbehaviours, including
mating and the taking of a pre-gravid blood meal, that are not
repeated in subsequent oviposition cycles.
Should, as a result of these activities, they enhance the
likelihood of their acquiring an infection, it may affect eventual
transmission since, assuming age specific mortality rates,
these are the mosquitoes that are most likely to survive to be-
come vectors. In previous studies, parous/nulliparous rates,
obtained by dissection at dispersed intervals, have been used
to assess mosquito survival. More detailed dissections, under-
taken on a nightly basis, provide more detailed information
(Charlwood et al., 1985; Holmes & Birley, 1987; Mutero &
Birley, 1989).
Mosquitoes taking their first blood meal can be distin-
guished from other age groups by the early state of
development of their ovaries in which follicles have little or
no yolk and which are surrounded by tightly coiled trachea.
They can, by examination of the reproductive tract, be further
separated into virgins, insects that have recently mated (with a
mating plug) and those that are mated but in which a mating
plug is not seen. Whilst mating per se is unlikely to affect biting
behaviour in anophelines (Klowden, 2001; Charlwood et al.,
2003b), information on the proportion of first-feeding insects
that feed as virgins or that have recently mated can provide
insights into what a female does following emergence.
Differences in behaviour between M and S form An.
gambiae when newly emerged may affect their eventual vec-
torial capacity. Factors that might influence the likelihood of
acquiring an infection are time and place that the mosquitoes
feed, late biting endophagic insects being more likely to bite
humans than exophagic early biting ones. The development of
risk-free tent traps, such as the Furvela trap (Govella et al.,
2009), for the collection of biting mosquitoes before they enter
houses enables a comparison between indoor and outdoor
biting mosquitoes to be easily performed. The trap, which
collected similar numbers of S form An. gambiae to the CDC
light trap when tested in Tanzania (Govella et al.,2009), has yet
to be tested in West Africa where M form An. gambiae
predominate.
Both M and S form An. gambiae at high densities have been
described from Okyereko, a village in an irrigation scheme in
Ghana (Yawson et al., 2004,2007; Okoye et al., 2005). Tem-
perature is one factor that appears to determine the distri-
bution of M and S forms of An. gambiae in Ghana, the M form
being apparently better adapted to higher temperatures than
the S form (De Souza et al., 2010). We therefore examined the
population dynamics of both endophagic and exophagic
mosquitoes at the start of the rains from Okyereko and applied
a more detailed dissection than has previously been applied to
these mosquitoes (Dzodzomenyo & Simonsen, 1999). A
possible explanation for the paddy paradox is provided.
Methods
Okyereko (5° 24.87N, 0° 36.25W), some 70km to the west
of Accra, consists of 80 relatively run-down cement houses,
5km from the coast, and is bordered on two sides by extensive
irrigated rice fields. According to Yawson et al.(2007), the
village had 35% S to 65% M form An. gambiae whilst, according
to Dzodzomenyo & Simonsen (1999), the proportion was 99%
M form. During the study, some of the fields, each 100 × 50 m
in size, were being harvested whilst others were recently
planted. At the start of the study, one field in particular, 350 m
to the south of the collection sites, with rice at an early stage of
cultivation (fig. 1), contained large numbers of small pools,
Fig. 1. Map of Okyereko showing the location of sample sites and
a selection of fields. The other fields to the south, east and west of
the village were not mapped. The rice paddies observed to contain
large numbers of Anopheles gambiae larvae at the start and end of
the study are indicated by a star ( , canal; , road;
, footpath; ., tent-trap; , light-trap; , larvae).
J.D. Charlwood et al.534
each with large numbers of larvae of An. gambiae s.l. By the end
of the study, this was no longer suitable for mosquito larvae
(the rice having grown and the water having largely dried up),
but pools in another set of recently cultivated fields, 200 m
to the west of the collection sites, contained mosquito larvae
(fig. 1).
Peridomestic and indoor biting mosquitoes were collected
with a Furvela tent trap and a CDC light trap, respectively. The
traps sample mosquitoes at slightly different phases of the
hunting cycle. Tent traps catch mosquitoes that are attracted to
odour (i.e. in the initial approach to a bait), rather like the
Odour Baited Entry Trap (OBET) of Costantini et al. (1993),
with the difference that no entry behaviour is required of the
insect, whilst light traps catch hungry mosquitoes in the act of
leaving a bedroom after an unsuccessful attempt to feed on a
host protected by a mosquito bednet. Some species may also
be attracted to light per se, which might bias the light trap
results whilst the way the mosquitoes respond to the tent trap
may reflect a similar approach to houses rather than a
completely exposed host in the open. Nevertheless, previous
results indicate that the Furvela tent trap and light traps have a
very similar sampling efficiency (Govella et al., 2009).
During the study, a CDC light trap was run on a daily basis
inside the bedroom of a house close to the edge of the village.
The trap was hung *1.5 m from the floor, close to the end of
the bed in which the householder slept under a mosquito net.
The trap was run from 18:00h to 06:30h of the next day.
A slightly modified version of the Furvela tent trap
described by Govella et al. (2009) for the collection of outdoor
biting mosquitoes was also run on a daily basis. The trap dif-
fered in that wire, instead of string, was used to attach the trap
to the tent. A three-person bell tent, with two adult hosts (JDC
& EVET) inside it was used. On five nights, the collection bag
on the tent trap was changed at three-hourly intervals,
enabling an assessment of biting activity by time to be made.
Following identification and separation into species or
species group, females of the An. gambiae complex were sep-
arated according to abdominal condition and dissected under
a stereo-microscope using transmitted light to determine
their gonotrophic age. Based on the appearance of their
ovaries, female mosquitoes were separated into the following
classes:
(i) First blood-feeding
Virgin: spermatheca empty, ovarioles Stage I.
Plug: sperm in the spermatheca, a mating plug in the
common oviduct, ovarioles Stage I.
Nulliparous I: sperm in the spermatheca but without a
mating plug ovarioles at Stage I.
(ii) Second blood-feeding
Plug-blood: mating plug present, old dark blood from
previous feed in stomach.
Nulliparous II: spermatheca with sperm, mating plug not
present, ovarioles Stage II, i.e. yolk present in terminal
ovariole.
(ii) Subsequent blood-feeding
Parous with sacs: a sac with some distension still present,
indicating that the mosquito had returned to feed shortly
after oviposition.
Parous without sacs: the sac from the previous oviposition
had contracted, indicating that there had been a delay
between oviposition and re-feeding.
The presence of retained Stage V eggs in parous females,
according to the sac stage, was also noted.
Estimated numbers of each age group collected were
determined by multiplication of the number caught in a trap
by the proportion in that category.
Samples of the An. gambiae complex were stored over silica
gel for later species and form determination and for an
assessment of infection with Plasmodium falciparum.
DNA extraction was performed individually following
Collins et al. (1988). DNA samples were used to determine
the molecular form by polymerase chain reaction (PCR)-
restriction fragment length polymorphism (RFLP) (Fanello
et al., 2002).
The presence of circumsporozoite (CS) antigens of
P. falciparum was determined using the sandwich enzyme-
linked immunosorbent assay (ELISA) using the protocols of
Wirtz et al. (1987).
Rainfall data was obtained from the National Meteo-
rological Service at Winneba, 5 km to the west of Okyereko,
and temperature data was obtained from Apam, 30 km to
the east of Okyereko (no thermometer being available in
Winneba).
The village boundaries, selected rice fields, and locations
of tent and light traps were marked with a hand-held GPS
unit (Garmin).
Results
A total of 235 individual An. gambiae s.l. were used for
species and molecular form identification, half of them
were collected with the tent trap and the other half with a
light trap. Of these, 234 were An. gambiae s.s., (230 (98%) being
M molecular form, three S form, one an M/S hybrid and one
was An. melas). All four S form specimens, including the
hybrid, were collected indoors. Given their overwhelming
frequency, in all further discussion, it is assumed that we were
dealing only with M form An. gambiae.
The total number of mosquitoes collected in tent and light
trap is shown in table 1. In both traps, An. gambiae was by far
the most common mosquito collected.
The great majority of the mosquitoes collected were unfed.
Only 3.4% of the mosquitoes in the tent trap (including an
unspecified number of those seen to enter the tent when
collection bags were being changed) and 2.8% in the light trap
were either blood fed or part fed. A total of 2110 An. gambiae
s.l. were dissected, 795 from the light trap and 1315 from the
Table 1. Total number of mosquitoes collected by species in tent and light traps, Okyereko, Ghana, June 2009.
Anopheles Culex Mansonia
gambiae funestus pharoensis zeimanni quinquefasciatus tritaeniorhynchus sp. africana
Tent 5532 69 130 3 557 54 5 24
Light 7761 91 48 4 725 83 0 185
Anopheles gambiae in Ghana 535
tent trap. Parous rates from both collections were very low
throughout the experiment (table 2).
Mean numbers of unfed, part-fed, engorged and gravid
females collected in the tent trap by night time from five
collections in which collection bags were changed are shown
in fig. 2. Most unfed and gravid females were collected in the
middle two periods of the night, whereas numbers of part-fed
and engorged insects, although small, increased during the
night. The proportion of the collection that was parous de-
creased during the night (fig. 3) although the proportion of
these that had sacs remained more or less constant, as did the
proportion of first-feeding insects with mating plugs.
On one night of collection, the battery used in the light trap
was not properly charged and, although numbers collected
were sufficient for the purposes of dissection, they were a
fraction of the number expected. Hence, the collection data
from the light trap on this day was not included in the analysis.
Overall, approximately two thirds (0.62) of recently em-
erged mosquitoes had mating plugs in both traps (192 of 311
dissected from the light trap and 354 of 574 dissected from the
tent trap). Similarly, there were no differences between overall
proportions of the other age groups collected in the two traps.
This again implies that both traps are sampling the same
population of mosquitoes.
The wet season started on the 11th of June (148 mm being
recorded in Winneba, the town 5 km from Okyereko). More
An. gambiae than expected were caught in the light trap and
less in the tent trap on this night. The sum of the numbers
collected in both traps was, however, similar to the expected
number, as was the proportion of newly emerged insects
that had a mating plug (0.64). Thus, the rain had not
apparently affected mating behaviour nor overall host seeking
activity.
After the 11th, a total of 251.2 mm of rain was recorded on
11 of the remaining 13 days of the study. On four of these days,
two of them at night, more than 20mm of rain fell. On those
two nights, the proportion of the total collection was lower
than usual in the tent trap (fig. 4). The number of mosquitoes
in both traps was also less than expected on the 19th of June
when 144.2 mm of rain fell from the evening until 02:40 am on
the 20th, although the drop was greatest in the tent trap. The
proportion of newly emerged insects with a mating plug was
higher on this day than on all other days (0.9 having plugs
compared to a mean of 0.63 on the other days). The correlation
coefficient between the numbers collected in the light trap and
tent trap for all nights (including those when more than 5 mm
of rain fell) was 0.506. This improved to 0.761 when the nights
when more than 5 mm of rain fell were excluded from the
calculations. Variation in numbers collected in both traps was
also greater in days subsequent to the rainstorm on the 19th
than in the period leading up to it. After the rainstorm, num-
bers of An. gambiae in the tent trap declined, whilst numbers in
the light trap continued to increase (fig. 5).
Parous mosquitoes classified as having sacs were more
likely to have retained Stage V (i.e. unlaid) eggs in their ovaries
than mosquitoes classified as not having sacs (two-tailed
probability from a Fishers exact test P=0.005) (table 3). This
implies that such eggs are voided with time after oviposition
as the sacs contract. Mean air temperatures in Apam decreased
during the study (from a mean of 28.4°C on the 9th of June to
24.4°C on the 16th), as did the proportion of mosquitoes dis-
sected without sacs (r
2
between the proportion of parous
insects with sacs and mean temperature=0.299), giving rise to
a lowered estimation of the mean oviposition cycle length over
time (fig. 6).
Table 2. Age structure of unfed M form An. gambiae collected from tent and light traps, Okyereko, Ghana, June 2009.
Virgin Plug Nulliparous
Stage I
Nulliparous
Stage II
Parous
with sacs
Parous
without sacs
Parous
rate
Tent trap 203 334 216 153 136 269 0.33
Light
trap
110 200 119 99 80 183 0.30
Fishers exact test (two-tail) virgin/plug ratios tent/light trap P= 0.507; sac/no-sac tent/light trap P=0.399; NI/NII tent/light trap P= 0.38.
0.6
0.5
0.4
0.3
0.2
Proportion parous
0.1
0
12 34
Period of the ni
g
ht
Fig. 3. Parous rates by collection period of the night of M form
An. gambiae collected using the Furvela tent trap, Okyereko,
Ghana, June 2009.
10 200
175
150
125
100
75
50
25
0
5
Number fed collected
Number unfed collected
0
1234
Period of the night
Fig. 2. Outdoor biting activity of M form An. gambiae determined
using the Furvela tent trap, Okyereko, Ghana, June 2009 (&, part-
fed; , engorged; , gravid; , unfed).
J.D. Charlwood et al.536
As expected from the low parous rates, infection in the
mosquito was low. No oocysts were seen on the stomachs of
parous insects dissected and only four (0.3%) of the 1296 mos-
quitoes analyzed in the ELISA tested positive for P. falciparum
circumsporozoite protein. All of the positive mosquitoes came
from the 753 tested from the light trap, each of the three days
of collection yielding at least one positive mosquito, compared
to none of the three days tested for the tent trap. Among the
expected parous population from these days, overall sporo-
zoite rates increased to 1.9%, whilst the rate from the light-trap
samples increased to 5.2%.
Discussion
Our results indicate that, in both East and West Africa, on
calm nights, the Furvela tent trap and CDC light trap sample
similar fractions of the local An. gambiae populations. The tent
trap has a number of advantages over the light trap, par-
ticularly for spatial studies. It collects host-seeking mosquitoes
that are attracted to kairomones rather than relying on light.
Light traps collect mosquitoes in the act of departing from a
house and can only be hung where there are houses. The kind
of house in which a trap is hung influences the number of
mosquitoes entering, hence the number caught. Tent traps, on
the other hand, provide a uniform sample, and can, within
reasonable limits, be located anywhere. They also use con-
siderably less power than a light trap but do require the
payment of a sleeper that increases their running costs.
The population of female An. gambiae, an archetypal
invasivespecies, went through a log-scale change in density
during the three weeks of the study. Given this increase, it
is not surprising that parous rates and malaria transmission
were low and similar to the rates observed by Okoye et al.
(2005) who found four of 2411 (0.2%) An. gambiae from
Okyereko positive for sporozoites. Such low sporozoite rates
have been described from other rice-growing areas of West
Africa (Robert et al., 1985,1987; Ijumba & Lindsay, 2001; Diuk-
Wasser et al., 2007). Although in the study presented here we
could not distinguish between catastrophic mortality in young
insects vs. exodous from the study area, our data on parous
rates and sporozoite rates provides insight into what might be
happening in the population. Assuming age-specific mortality
(Clements & Paterson, 1981), young insects should be the age
group whose survival is at a premium, and even small
mosquitoes that manage to take a blood meal should survive
as well as any (Takken et al., 1998a). Although sporozoite rates
among the whole population were low, when only the ex-
pected parous population was taken into consideration, rates
were typical for a population of African vector. Similarly,
when first-feeding insects were removed from survival rate
calculations, an estimate similar to other studies was obtained
1000 150
125
100
75
50
25
0
800
600
400
Number collected
Rain (mm)
200
0
6-Jun 11-Jun 16-Jun 21-Jun 26-Jun
Date
Fig. 5. Numbers of M form An. gambiae collected in CDC light
traps indoors and Furvela tent traps outdoors and daily rainfall,
Okyereko, Ghana, June 2009 (——, Light trap; ----, Tent trap).
800
700
600
500
400
300
200
Number collected Tent-trap
Number collected Light-trap
100
0
0 200 400 600 800 1000
Fig. 4. Relationship between the numbers of M form An. gambiae
collected in light trap and tent trap on nights with and without
rain, Okyereko, Ghana, June 2009. (The regression equation for the
trend line between the two traps is 0.785x+1.5078.) (, No-rain; &,
Rain).
Table 3. Number of parous M form An. gambiae with and without
sacs and numbers with retained Stage V eggs dissected, Okyereko,
Ghana, June 2009.
With sac Ret V Without sac Ret V
Tent-trap 136 12 269 15
Light-trap 80 15 183 10
Fishers exact test (two-tail) P= 0.005.
29.0 1
0.8
0.6
0.4
0.2
0
28.0
27.0
26.0
Mean tempetature
Proportion without sacs
25.0
24.0
3-Jun 8-Jun 13-Jun 18-Jun 23-Jun 28-Jun
Date
Fig. 6. Proportion of parous M form An. gambiae from both light
and tent traps in Okyereko, Ghana, returning to feed without large
ovariolar sacs and mean daily temperature measured in Apam,
30km to the west, June 2009 ( , mean temp).
Anopheles gambiae in Ghana 537
(Gillies & Wilkes, 1965; Charlwood et al., 1995,1997; Takken
et al., 1998b). This implies that, in older age groups, survival
was unexceptional, which itself implies that rather than dying,
an excess of young insects were leaving the area. Dispersal of
young An. arabiensis was also observed in Tanzania, a pulse
of insects being collected along a transect of traps into the
village of Namawala following a mass emergence at the peri-
phery (Takken et al., 1998b). Greater dispersion of young
An. culicifaces, compared to old ones, was also observed in
Sri Lanka by Rawlings et al.(1981). Such a phenomenon may
be occurring, but with overlapping generations, in Okyereko.
Dispersion of recently mated first-fed (pre-gravid) mos-
quitoes would explain the dearth of plug-bloodmosquitoes
in the collection. Such a phenomenon does not necessarily
require radically different behaviours among insects of differ-
ent ages. Gravid mosquitoes might fly toward oviposition
sites at a variable distance from the village by responding to,
for example, oviposition-site odours or humidity gradients,
whereas first-blood feeders might be expected to fly in less
specific directions, responding to mating cues in the case
of virgins with a first blood meal, or to host cues in the case of
mated females that have taken just the first blood meal. This
might lead to more of one group than the other leaving the
study area, depending on the distribution of resources
and the associated cues that stimulate directed movement.
There is, however, no information available on the pattern
of movement of gravid females to support or challenge this
hypothesis.
Whether the increase in numbers collected during the
study was due to eclosion from several breeding sites or to
the maturation of a single site is also unknown. Bearing in
mind its limitations, the data indicates that the former took
place. Certainly, the very productive field observed at the start
of the study was no longer producing adult mosquitoes in any
number by the end of the study. Not only had it dried up sub-
stantially but also the rice had grown from short to medium
height. Changes observed in a number of population par-
ameters indicate that at least two separate processes were
occurring during the period of observation. The first is the
drop in the estimated duration of the oviposition cycle (fig. 6)
and the second is the rise in the proportion of first feeding
insects (with follicles at Stage I) that were collected mated but
without a mating plug (data not shown).
Temperature appears to determine the distribution of M
and S forms of An. gambiae in Ghana (De Souza et al., 2010),
and it may have influenced the behaviour of the mosquitoes in
the present study. Thus, the change in the observedproportion
of parous mosquitoes collected with sacs could be associated
with one or more of the following factors: a drop in tempera-
tures as a result of the rain, slowing of the contraction of sacs,
or that the population of mosquitoes was generally oviposit-
ing closer to the collection site during the latter period of
observation. Further studies determining the rates of sac
contraction at different temperatures and mapping of breed-
ing sites would help elucidate these points. We do not know
why, among first-feeding mosquitoes collected in the first part
of the study, a higher proportion had apparently mated earlier
than those that were collected in the latter part of the study.
Given the high densities of larvae observed in Okyereko,
independent estimates of eclosion could be made using
emergence-traps.
Heavy rainstorms tend to have a negative effect on mos-
quito populations, since they drown or flush out the larvae
(Paaijmans et al., 2007); but this, to a certain extent, depends on
the terrain and the level of the water table. In the present study,
the two rainstorms, on the 11th and 19th of June, had different
effects on the mosquito population. On the 11th of June, the
behaviour of the An. gambiae was altered such that they were
collected in greater numbers than expected in the light trap,
whilst fewer than expected were collected in the tent trap.
Overall, though, the rain did not affect the total number
collected on this and subsequent days. Such an outcome in
response to rain may enhance subsequent malaria trans-
mission since the mosquitoes are more likely to take an infec-
ted bloodmeal inside than they are outside the house, where
alternative hosts are to be found.
The rainstorm on the 19th had more profound effects. It
severely reduced the numbers collected on that night and was
associated with a greater subsequent variability in numbers in
both traps. The water from the first rainstorm was largely ab-
sorbed within hours. Subsequent rain left numerous puddles
in the village. The higher water table on the 19th June may
have resulted in some flushing of late larvae in the rice fields,
reducing numbers emerging and, hence, the numbers in the
collections. The very high proportion of mated mosquitoes
collected on that night was probably due to a deficit of virgin
females in the collection rather than an exceptional amount of
mating occurring.
A short but heavy rainstorm (71.5 mm of rain during three
hours during the night) was shown to inhibit host-seeking
behaviour, without affecting mortality, among An. farauti
from Papua New Guinea (Charlwood et al., 1988). In that case,
however, mosquitoes survived and merely delayed coming to
feed until the following day.
Despite the eventual presence of numerous puddles within
the confines of the village, none were observed with anophe-
line larvae. These were sites typically preferred by the S form
of An. gambiae. Less than 2% S form An. gambiae were collected
during the study. These may have started to colonise the
puddles, but numbers may have still been too low for them to
be detected during the larval surveys.
Acknowledgements
We are grateful to the villagers of Okyereko for allowing us
to work in their village. This study was financed in part by the
IAEA (contract 14738) and DBL Centre for Health Research
and Development, Denmark. PS was supported by CMDT and
by a post-doctoral grant (SFRH/BPD/34395/2006) from
Fundação para a Ciência e Tecnologia, Portugal. RJP thanks
Dr Larry Zwiebel and the Department of Biological Sciences,
Mosig Fund, for travel support.
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... It is the only trap that is made from commercially available 'off the shelf' components, including the tent itself. Collections in the Furvela tent-trap are closely correlated to CDC light-trap collections used to monitor indoor biting mosquitoes (Govella et al., 2009;Charlwood et al., 2011;Charlwood et al., 2012), this makes it especially useful for the measurement of changes in indoor/outdoor ratios following the application of methods to control indoor biting mosquitoes. Since changing the collection bag is easy with the Furvela tent-trap it is also possible to examine the biting profile of the mosquitoes throughout the night as was done previously in Ghana (Charlwood et al., 2011). ...
... Collections in the Furvela tent-trap are closely correlated to CDC light-trap collections used to monitor indoor biting mosquitoes (Govella et al., 2009;Charlwood et al., 2011;Charlwood et al., 2012), this makes it especially useful for the measurement of changes in indoor/outdoor ratios following the application of methods to control indoor biting mosquitoes. Since changing the collection bag is easy with the Furvela tent-trap it is also possible to examine the biting profile of the mosquitoes throughout the night as was done previously in Ghana (Charlwood et al., 2011). ...
... This may be due to environmental factors, such as rainfall, that may affect the proportion of the population biting indoors (and so available for capture by the light-trap). For example, in Ghana a greater proportion of the night's catch of An. coluzzii were collected indoors on rainy nights (Charlwood et al., 2011). The total collected in paired indoor light-trap and outdoor tent-trap collections was, however, not different from the number expected. ...
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