The need for effective biological control organisms for mosquito larvae
is great, since insecticide resistance increases in areas having received
extensive insecticide treatment. Impact of pesticides in nature and the
nature of mosquito resistance should be spelled out. Corixids represent
one of the most important predators; they can feed on algal cells, filamentous
blue-greens, small invertebrates, diatoms, fish eggs and microscopic protozoa
(Bachmann, 1981; Schaefer and Panizzi, 2000). Earlier studies in Saudi
Arabia have shown that Culex quinquefasciatus Say (Diptera: Culicidae)
is the most widespread haematophagous insect and it is considered as an
important source of nuisance in Riyadh Region (Biittiker, 1981; Alahmed
and Kheir, 2005). The water bug Sigara hoggarica Poisson
(Hemiptera: Corixidae) is known to be one of the efficient predators of
mosquito larvae and has considerable mosquito control potential in permanent
and semi permanent water habitats (Tawfik et al., 1990).
The predatory efficacy and seasonal abundance of Sigara hoggarica
in Saudi Arabia has not been investigated. In this study, an attempt was
made to assess the predatory efficacy of Sigara hoggarica
against Culex quinquefasciatus larvae in the laboratory and in
the field during summer and winter seasons. The seasonal activity of the
predator in Riyadh City was also investigated.
MATERIALS AND METHODS
Collection, Identification and Rearing of the Predator
This study was conducted during the period Sept. 2003 to 2004 in Al
Haeir area, 50 km South of Riyadh City (24°27` N, 46°47` E). Sigara
hoggarica was collected from four sites at Wadi Hanifa (Riyadh
City), using a D-shape hand-net and then taken to the laboratory. The
predator was kept in glass aquarium (50x40x25 cm) and supplied with oxygen.
The predator was fed ad libitum on the first or second mosquito
larval instars. Representative sample of the predator was sent to The
Natural History Museum in London for identification. The mean monthly
temperature, humidity and rainfall in El Haeir area were obtained from
the Meteorology Department, Riyadh City.
Collection, Identification and Rearing of the Prey
Larvae of Culex quinquefasciatus were collected from Riyadh
City and identified according to the available identification keys (Hopkins,
1936; Mattingly and Knight, 1956; Harbach, 1988). The larvae were reared
in mosquito rearing cages (61x61x61 cm, Bioquip Company, C.A., USA) in
the laboratory and the emerged adult were fed on concentrated sugar solutions.
For egg laying, the adults were fed on a pigeon as a source of blood.
When mosquito larvae are required, some egg masses were taken and put
in metal trays (40x30x12 cm) with some water. Some animal food concentrates
were added as food for larvae and the trays were covered with iron mesh.
When the mosquito larvae reached the 4th larval instar, they were transferred
to a special mosquito rearing cages and the emerged adults were taken
by an aspirator to a big metal cage where they were fed on sugar solutions
and blood until they are required.
Predatory Efficacy of Sigara hoggarica in the Laboratory
Six treatments of mosquito immature stages, each consisted of 4 replicates,
were prepared in plastic containers partially full of water. The first
5 experimental treatments consisted of 400 either L1, L2, L3, L4 or pupae
of Culex quinquefasciatus. The sixth treatment was left as control
treatment. A starved predator for 24 h was added to each experimental
replicate and each container was tightly closed and put at room temperature
in the laboratory. After 24 h, the mean numbers of larvae and pupae consumed
were determined and the mean predatory efficacy of the predator against
each larval instar and pupa were calculated.
Assessment of Predatory Efficacy of Sigara hoggarica in the
A shade made of plastic materials and iron frame (6x4x2 m) was made
in the Agricultural Research Station at College of Food and Agricultural
Sciences, King Saud University, Riyadh. Inside the shade, a table (80x60x100
cm) was placed. Six treatments, each consisted of four replicates were
made and a starved predator was added to each replicate (except the control
group) as described earlier for determination of predatory efficacy of
Sigara hoggarica in the laboratory. The six treatments were
put on the table inside the shade (to simulate the field conditions where
the temperature, humidity, light and wind are not under control) and covered
with iron mesh, to protect them from other insects. After 24 h, the mean
number of each larval instar and pupae consumed by the predator were determined
and the mean predatory efficacy against each larval instar and pupae were
Study of the Seasonal Activity of Sigara hoggarica
Four sites (different ponds and water collections at Wadi Hanifa)
were selected for collection of Sigara hoggarica. Each site
was visited every two weeks and the predators were collected by an aquatic
D-shaped hand net and the collected Sigara hoggarica were
counted and recorded. The water temperature was recorded and the mean
monthly temperature, humidity and rainfall of the study area were obtained
from Meteorology Department, Riyadh City.
For statistical analysis, means were compared using Least Significant
Difference (LSD), (SAS, 1990). Correlation coefficients (r) between the
numbers of Sigara hoggarica and temperature and relative humidity
were also calculated (SAS, 1990). All statistical analyses results were
compared at 5% level for significance.
The Predatory Efficacy of Sigara hoggarica in the Laboratory
The highest predatory efficacy was shown against the first larval
instars (Fig. 1) and it decreased as the size and age
of the larval instars increased (means followed by similar letters are
not significantly different).
The Predatory Efficacy of Sigara hoggarica in the Field
The results showed that the predatory efficacy of Sigara hoggarica
was highest against the first larval instar and it decreased as the size
and age of the prey increased (Fig. 2). The results
also showed that the predatory efficacy of Sigara hoggarica
during summer is significantly higher than that during winter against
each respective larval instar (means followed by similar letters are not
The Seasonal Activity of Sigara hoggarica
The water bug, Sigara hoggarica was collected throughout
the year, but the numbers obtained started to increase gradually in November
and a peak was reached in next April, where 218 insects were caught. During
this peak, the mean ambient temperature was 25.5°C (Fig.
3), mean water temperature was 28°C (Fig. 4)
and relative humidity was 39.6% (Fig. 5). The activity
of the bug started to decline in June and it reached the lowest density
during August to October, where less than 3 insects were collected. During
this period, the ambient temperature ranged between 31 to 35.9°C (Fig.
3), the water temperature varied between 28 to 30°C (Fig.
4) and the relative humidity varied between 15.5% to 23.1% (Fig.
|| Mean predatory efficacy of Sigara hoggarica
against Culex quinquefasciatus immature stages in the laboratory.
L1: First larval instars, L2: 2nd larval instars, L3: 3rd larval instars,
L4: 4th larval instars, P: Pupal stage of Culex quinquefasciatus.
Means followed by similar letters are not significantly different
||Predatory efficacy of Sigara hoggarica
against Culex quinquefasciatus immature stages during summer
and winter in the field. L1: First larval instars, L2: 2nd larval
instars, L3: 3rd larval instars, L4: 4th larval instars, P: Pupal
stage of Culex quinquefasciatus. Means followed by similar
letters are not significantly different (p>0.05)
||The effect of ambient temperature on the seasonal activity
of Sigara hoggarica at El Haier area, Riyadh City
||The effect of water temperature on the seasonal activity
of Sigara hoggarica at El Haier area, Riyadh City
||The effect of relative humidity on the seasonal activity
of Sigara hoggarica at El Haier area, Riyadh City
Statistical analysis have shown that there is a non significant negative
correlation between ambient and water temperature and the numbers of Sigara
hoggarica (r = -0.175 and r = -0.145, respectively). The results also
have shown that there is a positive significant correlation between the
relative humidity and the numbers of Sigara hoggarica (r = +0.481).
The predatory efficacy of Sigara hoggarica, both in the laboratory
and in the field, was highest against the first larval instar and it decreased
as the age and size of the prey increased. It seems that the size of the
prey in relation to the size of the predator is very crucial in determining
the predatory potential of the predator. Similar results were reported
by Streams (1992), who found that the age of the prey has great effect
on the predatory potential of the predator. Another factor which may influence
the predatory potential of a predator, is the prey density. Predators
may eat more prey at higher prey densities than at lower prey densities.
When they eat more, predators may grow faster and the predation rate is
thus a function of growth of predator size (Murdoch, 1973).
The mean predatory efficacy of the water bug in summer was significantly
higher than that during winter and this might be due to the relatively
high temperature during summer which makes the bug more active and predacious.
Similar results were reported by Morton et al. (1988), who found
that the predatory efficacy of mosquito fish Gambusia affinis during
summer was significantly higher than that in winter.
These results have shown that Sigara hoggarica was available
in its breeding sites throughout the year, but a peak of seasonal activity
was attained during the period April to June. During this period, the
ambient temperature varied between 25 to 30°C, the mean water temperature
was 30°C and the mean relative humidity was 60% suggesting that these
are the optimum environmental conditions for the bug activity.
The results of this study have shown that the water bug, Sigara
hoggarica, is an efficient predator of Culex quinquefasciatus
larvae and it has great potential to reduce immature stages of mosquitoes
in permanent and semi permanent water collections, but further studies
on prey-predator relationship are required.