The impact of some environmental factors on the abundance of mosquitoes larvae in certain localities of Sharkia Governorate in Egypt

Mosquitoes play a major role in diseases transmission and distributions, such as filariasis and malaria. The understanding of environmental factors that affect their larval abundance is a key factor for their control. Thus this study aims at investigating mosquitoes' larval abundance in a region known to be endemic for malaria mosquitoes' related diseases in Sharkia Governorate. Measuring environmental factors were: pH, total dissolved solids “TDS”, turbidity, water and air temperatures, relative humidity, wind speed, water hyacinths, sun light, water flow, and width and depth of drains in relation with these environmental factors on larval abundance. Six mosquitoes species were recorded and identified in different locations from study sites of 9 drains: Abu Kbeer, Dyarb Negm and Al-Zakazik. Culex pipiens, Culex perexiguus, Culex antennatus, Culex pusillus, Culex sinaiticus and Ochlerotatus detritus were found to be dominant in these localities. Culex pipiens was the most dominant species (79.79%) in all localities. Results were shown and discussed in detail.


INTRODUCTION
Mosquitoes are considered a very important vector for filariasis and malaria.Filariasis is a widespread disease in Sharkia Governorate (Shawarby et al., 1965 andKaschef et al.,1982) and transmitted mainly by Cx. pipiens.Malaria has an old history as the only outbreak of malignant (falciparum) malaria occurred in the Nile Delta (Farid, 1940).
Understanding of mosquitoes bionomics is of key importance in the epidemiology of mosquito-borne diseases and in planning methods of mosquitoes control.Climatic factors play an important role in species distribution, behavior, survival and vectorial status.Water is an essential component of the mosquitos' environment and whether it is running, standing, clean or polluted, sweet or brackish, shaded or sunlight, frequently determines which species of mosquitoes breeds in it.The environments of the immature stages and adult mosquitoes are interdependent since the adult mosquitoes must have access to water for egg laying.(WHO manual of mosquitoes control, 1988).
Cx. pipiens was the main vector disease of human for filariasis in Egypt (Gad et al., 1987).Wuchereria bancrofti has been endemic in Egypt since pharaonic times (Laurence, 1967), and it was considered as a major health problem throughout the first half of the 20 th century (Baz, 1946).
Cx. pipiens breeds in seewage canals and drains, wells and cesspits, but not in rice fields in which Cx. antennatus prefers as breeding habitat.Cx. antennatus breeds also in the same breeding sites of Cx. pipiens, Cx. univittatus breeding habitats were found to resemble more or less those of Cx. antennatus.On the other hands, Theobaldia longiareolata larvae were recovered from the same breeding sites of Cx. pipiens.Rice fields were found also negative for this species.Wassif (1969) stated"Ae.caspius breeds in seepage, canals and drains".
Anopheles pharoensis was present in summer and autumn, Uranotaenia unguiculata from October to December; Culiseta longiareolata from December to June (with a peak in March), Ae. caspius in late autmn and winter; Cx. pusillus from September to November, Cx. perexiguus from September to November, Cx. pipiens which was so numerous throughout the year that it was difficult to determine its seasonal peak.(Kirkpatrick, 1925).
Sharkia Governorate covers about 4911 km² at the east of Nil Delta with a population of 5,354,041 inhabitants (2010 public census).Sharkia bordered from the north by El Manzlah Lake, from the east by Ismailia, from south by Al Qalyubiah and from west by El Daqahliyha.It represents various topographic areas including agricultural, semi desert, and desert areas.It contains 15 towns and over 4000 villages with various human activities.
The agricultural drainage water reuse is defined as the use of water discharge repeatedly, source "Nevada's Water Dictionary" (Mostafa et al., 2004).The degraded water quality threatens the expansion and even the continuation of the reuse of drainage water from the main drains in the Nile Delta.(Hussein et al., 2008).
The objective of the present study is to evaluate the effect of some environmental factors on the mosquitoes larvae abundance.The former was undertaken in selected sites in one of our Governorates, Sharkia, known to have plenty of mosquitoes habitats and this may have an influence on man's hygiene.

The study sites
Work was carried out from April to June 2012 in different mosquitoes breeding sites in Sharkia Governorate.Nine drains in three different cities were selected to conduct this study, Figs.1&2.The impact of some environmental factors on the abundance of mosquitoes larvae

Habitat characterization
Classification of aquatic habitats based on their size and appearance.Emergent vegetation cover was defined as the proportion of the water surface area that was covered by emergent vegetation.Floating vegetation cover was estimated as the proportion of the water surface area that was covered by floating vegetation.Any plant below the water surface was classified as submerged vegetation, and the proportion of water surface area covered by this vegetation was estimated (Ephantus et al., 2007)

Larval surveys
Larval surveys were carried out for three monthes (April, May and June 2012).Larvae were collected by aquatic net having half circular net with 45 cm diameter fixed at the end of a long stick.After collection, larvae were kept in 70% ethyl alcohol in glass vials, labeled and transferred to the laboratory for classification.Keys given by Kirkpatrick (1925) and Harbach (1988) were used for mosquitoes identification.

Environmental factors measurements
Parameters that measured for each sample; pH, Total Dissolved Solids (TDS) and turbidity.Other water parameters were taken in site of collection: water & air temperatures, percent of relative humidity (RH), wind speed, water hyacinths capacity, sun light, water flow speed, width and depth of drains.

Geographical information system mapping
Digital map of Egypt was obtained from Geographic information system unit (GIS) in Egyptian ministry of environmental affairs agency (EEAA), appearing various divisions of Egypt with different feature classes for drains, rivers and roads.Coordinate system used in digital maps was GCS_WGS_1984 and Datum D_WGS_1984.Computer program Arc view 9.1 was used for producing maps showing distribution of mosquitoes larvae in the Governorate.

Data analysis
The SSP (Smiths Statistical Package, version 18) computerized program was used for statistical analysis.Means, standard deviations and standard errors, were calculated for larval densities of mosquitoes species.Means were compared by the one-way ANOVA and if significantly different, they were exposed to pairwise comparison by Tukey test.Multiple Regression analysis was used to examine the relation of larval density to some environmental variables of the breeding water.

a-Relative abundance of mosquitoes species
Mosquitoes larvae were collected from nine sites in three towns in Sharkia Governorate Table 1.A total of 3572 larvae were recorded of which the lowest number of larvae was 27 larvae in site 9 (0.8%) and the highest number of larvae was 764 in site 3 (21.4%).Mosquitoes larvae were higher in Abu Kbeer (1710 Larvae) than in the other towns.
Sunlight has strong positive effect on larval abundance (0.625, p>0.05) and relative humidity has weak positive effect on larval abundance (0.079, p>0.05).While pH, TDS, turbidity, water temperature, air temperature, wind speed and water flow have moderate effects on larval abundance.
Water hyacinths has strong negative effect on larval abundance (-0.830, p<0.01) and drain wide has the weak negative effect on larval abundance (-0.352, p<0.01).While drain depth has moderate negative effect.
Data in

G
The impact of some environmental factors on the abundance of mosquitoes larvae
Results reported for pH, ranged from 6.4 to 7.8, are in agreement with previous ones cited by El Badry ( 2010) who shows that pH, ranged from 6.4 to 8. Abdel-Hamid et al. (2009) recorded in Sharkia pH range (7.4-8.4).Salit et al. (1996) reported that larvae of all mosquitoes species in Kuwait State are mostly alkalinophile.Abdel Hamid et al. (2011) in El Ismailia reported that larval density increases as pH increased.Bates (1949) referred to the influence of acidity on the micro biota and its often correlation with landscape type of the breeding place.
Results recoreded for TDS, ranged from 607 to 1640 ppm, were comparable to (134 to 5504 ppm) reported by Al Ahmed et al. (2005).The present study indicates that concentrations of TDS appear to have no intensive connection to the total abundance of all species.This disagrees with previous studies indicating that individual species may be hindered by variations in TDS (TRCA , 2007& TRCA, 2008).
Results reported for turbidity, ranged from 21 to 100 NTU, were comparable to (3.2 to 400.7 NTU) reported by Ammar (2012) & Kenaway and El-said (1990) reported that the turbidity significantly affects larval occurrence.In this study, turbidity has a positive effect on larval abundance and this is in agreement with that of Muturi et al. (2007) who reported that turbidity by organic matter was positively associated with larval occurrence.
Water temperature measurements in the studied drains ranged (23 to 28.8 o C) of the breeding water and this near to with water temperature (21 to 29 o C) reported by Kenawy et al. (1998).Optimum temperature is said to be important for microbial growth upon preferred by mosquitoes larvae (Ramachandra , 1984).Reduced temperatures cause a decline in microbial growth which increases the larval development time exposing them to greater risks of contact with potential predators and competitors (Ramachandra, 1984).
Results indicated that relative humidity has a weak positive effect on larval abundance.This goes in agreement with Rozilawati (2007) who stated that high relative humidity has little impact on eggs development.These results disagreed with Horsfall (1955) who reported that high relative humidity can give high hatching rates and low relative humidity can provide negative impact on egg hatching .
As for results reported for wind speed, ranged from 4.2 to 4.7 m/s, they are in agreement with Service (2000) who stated that slow velocity winds are important as they allow female mosquitoes to place their eggs calmly without disturbance in water containers.
Drains with higher floating vegetation (water hyacinths) have lower larval abundance.This disagreed with Walton et al. (1990) who found that biotic factors like vegetation are being better predictors of larval abundance.Greenway et al. (2003) mentioned that the presence of vegetation and floating plants provide optimal breeding conditions by acting as food sources as well as shelter from predators.Vegetation also creates stagnant conditions by decreasing water movement.Present results indicated that all mosquitoes prefer stagnant water that gives secure area.This is in agreement with that of Abdel-Hamid et al. (2011) and Kenawy et al. (1996) who observed that stagnant water was generally preferable for larval breeding.Abd-El-Magid (1987) recorded that particularly culicine mosquitoes larvae are existed in drainage channels with stagnant water.
Sunlight has strong positive effect on larval abundance likewise Hopkins (1936) that stated "sunlight has favorable effect on mosquitoes larval population for the requirement of algae for larval food".
Results reported for drain width, ranged from 2 to 6 m, with lower drain width, abundance of larvae increase in agreement with Abd-El-Magid (1987) recorded the presence of particularly culicine mosquitoes larvae in drainage channels width between 0.3-5 metre.Salit et al. (1996) observed that Cx. pipiens breeds in a wide range of breeding habitats with various width (0.5-100 m).
Results cited for drain depth, ranged from 0,5 to 2 m, with lower drain depth abundance of larvae increase are in agreement with Horsfall (1955).The later author observed that larval feeding position may be at the water surface or on the bottom and at all levels in between.Horsfall found that in comparatively deep water (56 cm), very few larvae went to the bottom to feed and Abd-El-Magid (1987) recorded the presence of particularly culicine mosquitoes larvae in drainage channels with shallow depth ranging between (5-20 cm).
Water pollutants can be used as mosquito control variables and as a bioindicator.It is proposed that more studies on the effect of physical and chemical properties of water in larval breeding sites in Sharkia Governorate are required.

Fig. 1 :
Fig. 1: Map of Sharkia Egypt Fig. 2: Map of the study area

Fig. 6 (
Fig.6 (A-F): Regression coefficients of some environmental variables that affect different mosquitoes species collected during the present study.

Table 1 :
Mosquitoes larvae species abundance that found in sites of collection in Sharkia Governorate

Table 2 :
Number and averages (SE±) of mosquitoes larvae collected during the study period

Table 3
illustrated that site 3 was the higher one according to number of larvae collected with the higher abundance rate (109.14 ± 89.41larva/site) and site 9 had the lowest larval abundance (3.86 ± 2.26 larva/site).

Table 4 :
Multiple regression analysis for the effect of environmental variables on larval abundance of mosquitoes

m The impact of some environmental factors on the abundance of mosquitoes larvae 55
Figs. 6&7 indicates that all species recorded positively correlated with pH, TDS, turbidity, wind speed and sun light.Except Cx. sinaiticus and Oc.detritus all species positively correlated with water flow and negatively corelated with drain width and depth.All species negatively correlated with water temperature and air temperature except Cx. pipiens and Cx.perexiguus.All species negatively correlated with RH except Cx. pipiens.Cx.pipiens, Cx. perexiguus and Cx.antennatus negatively correlated with water hyacinths and positevly correlated with Cx. pusillus,Cx.sinaiticus and Oc.detritus.