Mosquitoes and their potential predators in rice agroecosystems of the Mekong Delta, southern Vietnam.

Abstract

Culex tritaeniorhynchus, Cx. gelidus, and Cx. quinquefasciatus, known vectors of Japanese encephalitis (JE), are distributed in rice agroecosystems in Asian countries. Very few integrated studies on the breeding habitats of rice-field mosquitoes, including JE vectors, have been conducted in Vietnam. We investigated the mosquito fauna and potential predators in 8 rice growing areas in the Mekong Delta region of southern Vietnam, during the wet and dry seasons of 2009. Mosquitoes and their predators were collected from a variety of aquatic habitats (rice fields, ponds, wetlands, shrimp ponds, ditches, canals, and rivers). We collected 936 Culex spp. (354 Cx. tritaeniorhynchus, 240 Cx. vishnui s.l., 189 Cx. fuscocephala, and 42 Cx. gelidus), 33 Uranotaenia, 25 Anopheles, and 9 Mimomyia (4 Mi. chamberlaini) in the dry season. During the rainy season, we collected 1,232 Culex spp. (132 Cx. vishnui s.l., 66 Cx. tritaeniorhynchus, 9 Cx. gelidus, 4 Cx. fuscocephala, and 2 Cx. bitaeniorhynchus), 236 Anopheles spp. (40 An. vagus and 1 An. sinensis), and 7 Uranotaenia (3 Ur. lateralis). Heteroptera such as Micronecta, Veliidae, and Pleidae were abundant and widely distributed in both seasons. Based on a stepwise generalized linear model, the abundance of mosquitoes and their predators in rice fields was high when the rice plant length was short and water depth was shallow. Therefore, the use of insecticides during the earlier stages of rice growth should be avoided in order to preserve the predator populations.

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JOURNAL OF THE AMERICAN
MOSQUITO CONTROL ASSOCIATION
Mosquito News
MOSQUITOES AND THEIR POTENTIAL PREDATORS IN RICE
AGROECOSYSTEMS OF THE MEKONG DELTA, SOUTHERN VIETNAM
SHIN-YA OHBA,1,3T. T. TRANG HUYNH,2LOAN LUU LE,2HUU TRAN NGOC,2SAN LE HOANG2
AND MASAHIRO TAKAGI1

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MOSQUITOES AND THEIR POTENTIAL PREDATORS IN RICE
AGROECOSYSTEMS OF THE MEKONG DELTA, SOUTHERN VIETNAM
SHIN-YA OHBA,1,3T. T. TRANG HUYNH,2LOAN LUU LE,2HUU TRAN NGOC,2SAN LE HOANG2
AND MASAHIRO TAKAGI1
ABSTRACT. Culex tritaeniorhynchus, Cx. gelidus, and Cx. quinquefasciatus, known vectors of Japanese
encephalitis (JE), are distributed in rice agroecosystems in Asian countries. Very few integrated studies on the
breeding habitats of rice-field mosquitoes, including JE vectors, have been conducted in Vietnam. We
investigated the mosquito fauna and potential predators in 8 rice growing areas in the Mekong Delta region
of southern Vietnam, during the wet and dry seasons of 2009. Mosquitoes and their predators were collected
from a variety of aquatic habitats (rice fields, ponds, wetlands, shrimp ponds, ditches, canals, and rivers). We
collected 936 Culex spp. (354 Cx. tritaeniorhynchus, 240 Cx. vishnui s.l., 189 Cx. fuscocephala, and 42 Cx.
gelidus), 33 Uranotaenia, 25 Anopheles, and 9 Mimomyia (4 Mi. chamberlaini) in the dry season. During the
rainy season, we collected 1,232 Culex spp. (132 Cx. vishnui s.l., 66 Cx. tritaeniorhynchus, 9 Cx. gelidus, 4 Cx.
fuscocephala, and 2 Cx. bitaeniorhynchus), 236 Anopheles spp. (40 An. vagus and 1 An. sinensis), and 7
Uranotaenia (3 Ur. lateralis). Heteroptera such as Micronecta, Veliidae, and Pleidae were abundant and
widely distributed in both seasons. Based on a stepwise generalized linear model, the abundance of
mosquitoes and their predators in rice fields was high when the rice plant length was short and water depth
was shallow. Therefore, the use of insecticides during the earlier stages of rice growth should be avoided in
order to preserve the predator populations.
KEY WORDS
control
Agriculture, Culex vishnui subgroup, integrated vector management, malaria, mosquito
INTRODUCTION
Mosquito-borne diseases such as malaria,
Japanese encephalitis (JE), and dengue are major
public health threats in Asian countries. Common
measures for mosquito control include bed-nets
and indoor residual sprays using dichloro-diphe-
nyl-trichloroethane (DDT) or pyrethroids (Lind-
blade et al. 2006). However, there is concern
about toxicity to nontarget organisms (Henry and
Kishimba 2006) and vector resistance to chemical
insecticides (Van Bortel et al. 2008). Because
chemical insecticides also kill mosquito predators
(Service 1977), the use of chemical insecticides
therefore requires caution. In the Mekong Delta
region, southern Vietnam, the use of DDT has
been stopped since 1985. Current prevention and
control of malaria rely on the use of pyrethroids
in long-lasting insecticidal mosquito nets for
households and residual spraying. Interest in
formulating ecological approaches has been
growing over the past 3 decades, particularly
because of the limitations of chemical use,
including insecticide resistance of mosquitoes,
the health risks for human and domestic animals,
and ecological disturbances to the natural balance
such as predator–prey relationships (Yasuoka et
al. 2006, Yasuoka and Levins 2007). Therefore,
further understanding of mosquito ecology in rice
agroecosystems is crucial for developing mosqui-
to control strategies in the future.
The spread of JE throughout Asia may have
originated in the Indo-Malay region of Asia
(Solomon et al. 2003), with cases reported in
India, Nepal, Sri Lanka, Bangladesh, Myanmar,
Laos, Cambodia, Thailand, Vietnam, Malaysia,
China, Philippines, Indonesia, Korea, Japan,
Papua New Guinea, and, most recently, in the
southern part of Australia. In Asian countries,
Culex tritaeniorhynchus Giles, Culex vishnui s.l.
Theobald, Culex fuscocephala Theobald, and
Culex gelidus Theobald are known JE vectors
(Van Peenten et al. 1975, Gingrich et al. 1992,
Reuben et al. 1994, Vythilingam et al. 1997,
Stoops et al. 2008). Species in the Culex vishnui
subgroup, such as Cx. tritaeniorhynchus, Cx.
pseudovishnui Colless, and Cx. vishnui, breed in
a wide range of aquatic habitats, i.e., rice fields,
furrow pits, puddles, cisterns, and permanent and
transient ground pools (Stoops et al. 2008). Of
these, the most important breeding habitat is the
rice-field ecosystem (Mogi 1978, Victor and
Reuben 1999), and their population abundance
is closely related to rice agrosystems (Lacey and
Lacey 1990, Takagi et al. 1995b, Takagi et al.
1997, Keiser et al. 2005).
In wetlands, including rice fields, wide varieties
of natural enemies of mosquitoes are have been
reported (Sunahara et al. 2002, Ohba and
Nakasuji 2006, Mogi 2007). The impact of
natural enemies on Cx. tritaeniorhynchus larvae
was determined in several ecological studies in
1Department of Vector Ecology & Environment,
Institute of Tropical Medicine (NEKKEN), Nagasaki
University, Sakamoto, Nagasaki 852-8523, Japan.
2Pasteur Institute, Ho Chi Minh City, Vietnam.
3Present address: Center for Ecological Research,
Kyoto University, Otsu 520-2113, Japan.
Journal of the American Mosquito Control Association, 27(4):384–392, 2011
CopyrightE2011 by The American Mosquito Control Association, Inc.
384

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rice agroecosystems (Mogi and Miyagi 1990,
Mogi 1993, Takagi et al. 1996b). Fish, aquatic
insects, and spiders are predacious on Cx.
tritaeniorhynchus (Watanabe et al. 1968, Mogi
2007). In addition, abiotic factors such as sunlight
and vegetation cover alter mosquito abundance
in aquatic habitats. Previous studies revealed that
dense vegetation covering water surface lowers
the larval density (Mogi and Miyagi 1990, Mogi
et al. 1995, Takagi et al. 1995a, Takagi et al.
1996a, Sunish and Reuben 2001).
In southern Vietnam, Cx. tritaeniorhynchus,
Cx. gelidus, and Cx. quinquefasciatus Say are
distributed in rice agroecosystems (Nguyen et al.
1974). Hasegawa et al. (2008) showed that the
proximity of hosts to breeding sites in a rice-
cultivating village in northern Vietnam affected
the abundance of Cx. gelidus but not of Cx.
vishnui s.l. However, very few studies concerning
the breeding habitats of mosquitoes in rice
agrosystems have been conducted in Vietnam.
We conducted an ecological study on mosquito
fauna and its potential predators in 8 rice
production areas in the Mekong Delta region,
southern Vietnam, in the rainy and dry seasons of
2009. We also evaluated other factors, such as
plant length and water depth, affecting mosquito
abundance in rice fields.
MATERIALS AND METHODS
Study site
We conducted this study from February 11 to
19, 2009 (dry season) and from July 6 to 14, 2009
(rainy season) in 8 districts in the Mekong Delta
region in southern Vietnam (Table 1 and Fig. 1).
Farmers cultivate rice 3 times a year in these
areas. Farmers in Ben Tre and Long An
provinces alternate between rice cropping and
shrimp production. Rice fields and adjoining
aquatic habitats such as ponds (water depth .
1 m), wetlands (water depth , 1 m), shrimp
ponds, ditches (width , 1 m), canals (width .
1 m), and rivers were selected as potential
mosquito habitats. Although it is important to
investigate the mosquito fauna during the breed-
ing periods of shrimp, we did not address this in
the present study. The aquatic habitats for
sampling in each district were selected based on
the relative area of each aquatic habitat. We
measured the length of 10 rice plants and water
depth at 10 locations made at 10 points, each
.1 m, in each rice field, and used these values in
statistical analysis mentioned below.
Sampling methods
We monitored the abundance of mosquito
larvae and other insects by dipping. The diameter
and depth of the dipper used were 12 cm and
5 cm, respectively. We collected 1 sample from
each study plot, and each sample consisted of 30
dips made at 30 points, each .1 m. In case 30
dips could not be made owing to the small size of
some aquatic habitats, we made 10 or 20 dips and
then converted these to 30 dips. Using this
method alone, a number of predatory insects
and fish resting on the foliage might have been
missed. We brought all samples to the laboratory
of the Pasteur Institute in Ho Chi Minh City.
Insects, excluding mosquito larvae, were identi-
fied to the order, family, or genus level as far as
possible, by using a binocular microscope. Based
on the literature (Mogi 2007, Shaalan and
Canyon 2009), potential predators were classified
into invertebrate predators (Coleoptera, Hemip-
tera, and Odonata) and vertebrate predators (fish
and anuran larvae).
We preserved the collected mosquito larvae in
70% ethanol until identification. We identified all
larvae, excluding damaged and/or 1st to 3rd
instars, based on taxonomic keys (Rattanarithi-
kul et al. 2005a, Rattanarithikul et al. 2005b,
Rattanarithikul et al. 2006a, Rattanarithikul et
al. 2006b). Pupae were reared until emergence for
species identification. Larvae smaller than the 4th
instar were identified to the genus level and
counted. Moreover, we asked 1–5 farmers
whether they applied insecticides to the rice
fields.
Data analysis
Similar to the previous studies (Yasuoka et al.
2006, Yasuoka and Levins 2007), niche width
was calculated for each species, using the
formula: niche width 5 number of habitat types
where a species was collected divided by 7, which
is the total number of habitat types included in
this study. Knowledge of the environmental
factors affecting mosquito abundance in rice
fields is indispensable for future mosquito man-
agement because rice fields are the most common
and widespread wetlands in Vietnamese agroeco-
systems. We used a generalized linear model
(GLM) with negative binomial (‘‘glm.nb’’ pack-
age) in R statistical package, version 2.12.1 (R
Development Core Team 2010) to determine
mosquito abundance in rice fields. Season (dry or
rainy), predator abundance, rice plant length,
and water depth were the factors used in the
analysis (Zuur et al. 2009). The forward–back-
ward stepwise method using the step Akaike’s
information criteria package was used for model
selection. Furthermore, forward–backward step-
wise GLM with negative binomial was used to
reveal the relationship between predator abun-
dance and other factors such as season, mosquito
abundance, rice plant length, and water depth.
Because each sampling site could not be sampled
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Table 1.
Mosquito sampling sites in the dry and rainy seasons of 2009 in southern Vietnam.
Season
Province
District/town
Commune
Habitat type
Total
Rice fields (cm)
Rice
fields
Ditch
Pond
Wetland
Shrimp
pond
Canal River
Rice length
Water depth
Dry
season
Ben Tre
Binh Dai
Thanh Phuoc
5
3
2
1
11
None recorded
None collected
Tien Giang
Cai Lay
Tan Binh
2
1
3
83.1
3.1
Long An
Can Duoc
Tan Chanh
1
1
7
9
None recorded
None collected
Ben Tre
Chau Thanh
An hiep
5
1
1
1
3
11
48.7
5.6
Tien Giang
Go Cong
Binh Dong
4
2
1
1
1
9
84.6
6.6
Tien Giang
Go Cong Dong
Tan Thanh
2
3
3
1
9
98.6
7.7
Tay Ninh
Tay Ninh
Ninh Thanh
3
1
2
1
7
74.7
4.8
Vinh Long
Vinh Long
Tan Ngai
2
3
2
3
10
None recorded
None collected
Total
24
1
15
12
3
12
2
69
Rainy
season
Ben Tre
Binh Dai
Thanh Phuoc
4
3
4
1
12
None recorded
None collected
Tien Giang
Cai Lay
Tan Binh
2
1
1
2
6
26.8
3.3
Long An
Can Duoc
Tan Chanh
2
6
3
1
12
None recorded
None collected
Ben Tre
Chau Thanh
An hiep
5
2
4
11
58.3
4.8
Tien Giang
Go Cong
Binh Dong
4
2
1
1
8
60.3
11.6
Tien Giang
Go Cong Dong
Tan Thanh
5
1
3
1
1
11
63.7
11.7
Tay Ninh
Tay Ninh
Ninh Thanh
6
2
1
9
28.3
7.0
Vinh Long
Vinh Long
Tan Ngai
2
4
2
4
12
47.8
8.8
Total
30
2
23
6
5
13
2
81
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twice (dry and rainy season) necessarily, each
sampling site was not incorporated as random
effect in the statistical model.
RESULTS
Mosquito abundance in each aquatic habitat
In the dry season, 936 Culex spp. (including
354 Cx. tritaeniorhynchus, 240 Cx. vishnui s.l., 189
Cx. fuscocephala, and 42 Cx. gelidus), 33 Urano-
taenia, 25 Anopheles, and 9 Mimomyia (including
4 Mi. chamberlaini Ludlow) were collected
(Table 2). Culex tritaeniorhynchus was present in
all the aquatic habitats. In contrast, Cx. fuscoce-
phala was present in the ditch habitat but not in
the rice fields. Although we investigated only 1
ditch, exclusively higher densities of Culex and
Anopheles were observed in this habitat in the dry
season. The ditch habitat did not contain running
water, and therefore, it was not used for irrigation
of the rice fields in the dry season.
In the rainy season, we collected 1,232 Culex
spp. (including 132 Cx. vishnui s.l., 66 Cx.
tritaeniorhynchus, 9 Cx. gelidus, 4 Cx. fuscoce-
phala, and 2 Cx. bitaeniorhynchus Giles), 236
Fig. 1.Map showing the study sites (district or town name) in southern Vietnam.
Table 2.List of mosquitoes collected in February 2009 (dry season).
Mosquito species
Habitat type
Total
Rice fieldsDitch PondWetland
Shrimp
pond Canal River
Culex tritaeniorhynchus
Cx. vishnui s.l.
Cx. fuscocephala
Cx. gelidus
Mimomyia chamberlaini
Culex spp.
Uranotaenia spp.
Anopheles spp.
Mimomyia spp.
Aedeomyia spp.
Not identified
Total
No./site
12
156
204
81
189
24
17 97 174
3
3 354
240
189
42 162
31
4
4
1
4
36 307 2212111
33
25
29
6 153
55
11
249
222
19.6
7,431
531
7,962.0
96
27
8.2
1,396
156
129.3
10
41
17.0
17
24
3.4
16
3
9.5
9,215
10,219
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Anopheles (including 40 An. vagus Doenitz and 1
An. sinensis Wiedemann), and 7 Uranotaenia
(including 3 Ur. lateralis Ludlow) (Table 3).
Culex vishnui and Cx. tritaeniorhynchus were
present in the rice fields, wetlands, and ponds
and in the rice fields and ponds, respectively.
Anopheles vagus and An. sinensis were present in
the rice fields. In contrast, Cx. fuscocephala was
present in the wetlands, but not in the rice fields.
The number of mosquito larvae was higher in the
rice fields and ponds, but a few mosquito larvae
were present in the other aquatic habitats. In the
ditch habitat, the number of mosquitoes was
extremely low because this ditch contained
running water, which irrigated the rice fields.
Potential mosquito predators
In the dry season, we collected 2,613 Hemiptera,
97 Coleoptera, and 132 Odonata nymphs from all
thehabitats(Table 4).Poolingthesamplescollected
from 8 study sites, Micronecta (total abundance 5
2,052, niche width 5 0.86) and Veliidae (217, 0.86)
were widely distributed, followed by Anisops (122,
0.71) and Pleidae (140, 0.57). Coleoptera (62
Dytiscidae and 35 Hydrophilidae) were not highly
abundant relative to Hemiptera but were widely
distributed (niche width 5 0.86). Similarly, Odo-
nata and fish were less abundant than Hemiptera
but occupied a wider niche (0.71–0.86). The
number of natural predators per site was the
highest in the wetland (99.7), followed by the ditch
Table 3.List of mosquitoes collected in July 2009 (rainy season).
Mosquito speciesRice fields DitchPond Wetland
Shrimp
pond Canal RiverTotal
Culex vishnui s.l.
Cx. tritaeniorhynchus
Anopheles vagus
Cx. gelidus
Cx. fuscocephala
Uranotaenia lateralis
Cx. bitaeniorhynchus
Anopheles sinensis
Culex spp.
Anopheles spp.
Uranotaenia spp.
Not identified
Total
No./site
129
25
40
12 132
66
40
41
639
4
3
2
1
4
3
2
1
488
189
6 51465
4
1,019
1952
44
1,062
878
142
571
31.0
1,204
2,6797
3.5
10
1.7
0
0.0
9
0.7
0
0.064.7
Table 4.List of potential mosquito predators collected in February 2009 (dry season).
OrderFamily or genus
Habitat type
Total
Niche
width1
Rice
fieldsDitch Pond Wetland
Shrimp
pondCanalRiver
HemipteraGeriidae
Veliidae
Hydrometridae
Mesoveliidae
Anisops spp.
Notonecta spp.
Sigara spp.
Pleidae
Micronecta spp.
Belostomatidae
Nepidae
Hydrophilidae
larva
Dytiscidae adult
Zygoptera
Anisoptera
Fish
Tadpole
415 0.29
0.86
0.57
0.57
0.71
0.29
—
0.57
0.86
0.43
0.57
0.86
1106 43
1
11
108
31
2
8
6
14
4
2
4
13 217
18
18
1
2
39
122
11
3
9
0
4 95
165
401140
7001,077 26786
1
2,052
2
1
3
12
1
2
15
5
35
2
2
1
Coleoptera2422
3
6
1818
6
20
61
96
640
42.7
8
6
3
1
6
1
6
9 62
30
102
80
113
0.86
0.86
0.71
0.71
0.29
Odonata110
5
2
69
6
17
944
39.3
Vertebrate 10
Total
No./site
51
51.0
1,197 29 127
10.6
48
24.099.79.7
1Niche width 5 the number of habitat types where a species was collected by 7, which is the total number of habitat types.
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(51.0), pond (42.7), rice field (39.3), river (24.0),
canal (10.6), and shrimp pond (9.7).
In the rainy season, 1,990 Hemiptera, 142
Coleoptera, and 94 Odonata were collected from
all of the habitats (Table 5). Micronecta (total
abundance 5 1,142, niche width 5 1.00), Veliidae
(386, 0.86), and Pleidae (305, 0.71) were abundant
as well. Coleoptera consisted of individuals in the
Dytiscidae (71, 0.71) and Hydrophilidae (71,
0.71). Odonata consisted of individuals in the
Zygoptera (24, 0.71) and Anisoptera (70, 0.63).
Fish (291, 0.71) were more abundant in the rainy
season than in the dry season (80, 0.63). The
number of natural predators per site was the
highest in the wetland (39.8), followed by the
pond (38.1), rice field (34.7), canal (23.6), ditch
(20.5), shrimp pond (9.2), and river (9.0).
Mean length of rice plants and water depth in
rice fields are shown in Table 1. Insecticides were
used in all rice fields.
Mosquitoes in the rice fields
Stepwise GLMs (with negative binomial) re-
vealed that rice plant length and water depth
negatively affected mosquito abundance in the
rice fields, but season and predator abundance
were excluded from the model (Table 6). In
addition, the stepwise GLMs (with negative
binomial) revealed that rice plant length and
water depth affected the predator abundance in
rice fields. However, season and mosquito
abundance were excluded from the model. These
results suggest that mosquito and predator
Table 5.List of potential mosquito predators collected in July 2009 (rainy season).
Order Family or genus
Habitat type
Total
Niche
width1
Rice
fieldsDitch Pond Wetland
Shrimp
pondCanal River
Hemiptera Veliidae
Hydrometridae
Mesoveliidae
Anisops spp.
Notonecta spp.
Sigara spp.
Pleidae
Micronecta spp.
Belostomatidae
Nepidae
Hydrophilidae larva
Dytiscidae adult
Zygoptera
Anisoptera
Fish
Tadpole
1434 175 60
3
7
22 386
10
24
69
0.86
0.29
0.57
0.29
0.14
0.29
0.71
1.00
0.29
0.43
0.71
0.71
0.71
0.71
0.86
0.71
7
87
9
2
60
22
32
6
1
6
7
210
280
28
82
1
15
1
4
305
7693645 1,142
43
6
71
71
24
70
291
53
1
9
41
1Coleoptera33
23
5
20
21
1
877
38.1
25
2
1
6
2
35
239
39.8
3
35
14
37
7
3
110
2
5
258
12
307
23.6
Odonata2
2
2
2
Vertebrate1
Total
No./site
1,042 41
20.5
46 18
34.7 9.29.0
1Niche width 5 the number of habitat types where a species was collected by 7, which is the total number of habitat types.
Table 6.Generalized linear model results for the mosquito and predator abundance in rice fields. Factors in
boldface type are significant (P , 0.05).
Response variableSource1
Parameter estimateSE
ZP
Mosquito
abundanceIntercept
RL
WD
(Season)
(Predator abundance)
8.58
20.06
20.24
—
—
1.005
0.013
0.079
—
—
8.54
24.47
23.08
—
—
,0.0001
, ,0.0001
, ,0.0001
—
—
Predator
abundanceIntercept
RL
WD
(Season)
(Mosquito abundance)
6.05
20.03
20.10
—
—
0.637
0.009
0.05
—
—
9.5
,0.0001
, ,0.0001
0.0042
—
—
24.01
22.03
—
—
1RL, rice length; WD, water depth.
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abundance were high when rice plant length was
short and water depth was shallow.
DISCUSSION
This study found diverse mosquito species in
the rice ecosystem. Most mosquitoes collected
were Culex species in both the dry and rainy
seasons. The presence of Cx. tritaeniorhynchus,
Cx. vishnui s.l., Cx. fuscocephala, and Cx. gelidus
observed in this study was similar to reports from
other Asian countries (Van Peenten et al. 1975,
Gingrich et al. 1992, Reuben et al. 1994, Takagi et
al. 1997, Vythilingam et al. 1997, Stoops et al.
2008). Culex tritaeniorhynchus was present in all
aquatic habitats, showing the largest niche width
in the dry season. Culex vishnui s.l. and Cx.
tritaeniorhynchus were present in the rice fields,
wetlands, and ponds and in the rice fields and
ponds, respectively, in the rainy season. In view
of the abundance of mosquito vectors in rice
ecosystems, control efforts are necessary to
reduce the risk of mosquito-borne diseases in
the region.
In shrimp ponds, a characteristic part of the
agroecosystem in this region, Cx. tritaenior-
hynchus and Cx. gelidus were present in the dry
season (Table 2). This is the first report of JE
vector in shrimp ponds in Vietnam. According to
farmers, insecticides (unknown type, amount, and
frequency) were used in study sites to kill
potential shrimp predators (aquatic insects and
fish) in shrimp ponds, and this might have
impacted mosquito predators as well. Thus,
shrimp ponds may be predator-free habitats for
mosquitoes during the breeding periods of
shrimp.
Culex fuscocephala was present in the ditch in
the dry season and in the wetlands in the rainy
season, but not in the rice fields in both the dry
and rainy seasons. This species was found in rice
fields in Sri Lanka (Yasuoka et al. 2006, Yasuoka
and Levins 2007), Indonesia (Stoops et al. 2008),
and the Philippines (Mogi and Miyagi 1990). This
difference in breeding habitats between the
previous reports and the present study was not
clear. Because serial samples over the whole
season were not collected in the present study,
the distribution of this species might have been
underestimated.
Hemiptera, including Micronecta, Veliidae, and
Pleidae, were abundant in both the rainy and dry
seasons (Tables 4 and 5). More than 60% of
Hemiptera confirmed by polymerase chain reac-
tion showed a positive detection rate for mosqui-
to DNA, indicating that they fed on mosquito
larvae in wetlands in western Kenya (Ohba et al.
2010). We did not confirm the feeding of
Hemiptera on mosquito larvae in rice ecosystems
in Vietnam; they may eat mosquito larvae.
Predatory ability should be determined for each
predator species in future studies.
Stepwise GLMs in this study suggest that
mosquito abundance was high when rice plant
length was short and water depth was shallow.
This finding is similar to that of previous studies
concerning the relationships between vegetation
cover and mosquito abundance (Mogi and
Miyagi 1990, Mogi et al. 1995, Takagi et al.
1995a, Takagi et al. 1996a, Sunish and Reuben
2001). Stoops et al. (2008) suggested that more
emphasis should be placed on killing mosquito
larvae in rice fields in the earlier stages of growth
in Indonesia. However, they did not consider
potential mosquito predators and their relation-
ship to rice plants. In this study, potential
mosquito predators were also abundant when
rice plant length was short and water depth was
shallow, as observed in Japanese rice fields
(Takagi et al. 1996a). Therefore, insecticides that
have a negative impact on mosquito predators
should not be applied to rice fields in the earlier
stages of rice plant growth to preserve the variety
of mosquito predators for integrated vector
management (van den Berg et al. 2007). These
results add to our understanding of mosquito
ecology to applied in mosquito control strategies
in rice ecosystems in Vietnam.
ACKNOWLEDGMENTS
We thank the entomological staff of the Ho
Chi Minh Pasteur Institute. The KAKENHI
19256002, the Japan Society for the Promotion
of Science, the joint research fund between
Nagasaki University and Sumitomo Chemical
(Osaka, Japan), and the Ministry of Education,
Culture, Sports, Science and Technology
(MEXT) to M.T. who provided partial funding
for this study.
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