Introduction

With increasing problems of environmental pollution by chemical insecticides and development of resistance in many species  of mosquito vectors, intensive work was currently paid to the use of non-conventional insecticides  as insect growth regulator (IGR) and plant extracts for mosquito control in different parts of the world (Saleh and wright,1989; Chowdhury et al.,2007; Seng et al.,2008).

The present work was planned in part to evaluate the biological effects of two non-conventional insecticides, (IGR) , Dudim and  the plant extract Neem oil, against  Cx. pipiens, the dominant mosquito species in Jeddah,Saudi  Arabia. Additional test were also conducted to study the side effects of the test compounds on some nontarget organisms associated with mosquito habitats.

Material and methods

Mosquito strain

Tests were performed on a field strain of Cx. pipiens raised from wild larvae, collected from Jeddah governorate, Saudi  Arabia, and had been maintained in the laboratory under controlled conditions of 27± 1 ^oC  and 70 ± 5% R.H, with a 14 : 10 (L :D) photoperiod.

Compounds tested

Two non-conventional insecticides were used:

The insect growth regulator Dudim (4G%); 1-(4-chlorophenyl)-3-(2,6-difluorbenzoyl)-urea, kindly supplied by Dr. KH. Al. Ghamdi, Fac. of science, King Abdulaziz Univ.

The plant extract Neem oil (Azadirachtain indica) kindly supplied by Dr. M. A. Khan, Dept. of Zoology, Saifia Science College Bhopal, India. The stock solution of the plant extract was prepared by  adding 1 ml of it to 99 ml of distilled water containing 0.5% triton X-100 as an emulsifier to ensure complete solubility of the extract in water. Series of concentrations were prepared in distilled water.

Test experiments

 The Larval susceptibility test was conducted according to the method of WHO (1981). Treatments were carried out by exposing early  4^th instar larvae of Cx. pipiens to various concentrations of the tested compounds in groups of glass beakers containing 100ml of tap water. Five replicates of 20 Larvae each per concentration , and so for control trials were set up. The Larvae were given the usual larval food during the experiments. Mortalities of larvae and pupae were recorded daily. The percentage of inhibition of adult formation was recorded and compared with those of the controls under the same conditions of tests  ( Abbott,1925). Log concentration – probability regression lines were drawn for the tested compounds and statistical parameters were also calculated using the method of Litchfield and Wilcoxon (1949).

Additional tests were also conducted to study the side  effects of the IGR Dudim and  the plant extract Neem oil on some aquatic nontarget organisms associated with mosquito breeding habitats. The test organisms were collected form a pond water known to be mosquito breeding sites. The nontarget organisms were Odonata naiads, aquatic bugs ( Order : Hemiptera) and aquatic beetles (Order: Coleoptera).

The concentrations corresponding to IC₉₀ values (concentration which to inhibit the emergence of 90% of mosquito adults emergence) were drived form the toxicity lines of Dudim and  Neem oil. These concentrations were prepared and chosen for treating the aquatic organisms. Treatments were carried out by exposing the individuals to the test concentrations in 500 ml glass beakers with 100 ml of tap water. Three replicates of 4 individuals each per concentration, and so for control were set up. Mortalities were recorded after 24hr post-treatment.

Results and Discussion

 Percentage mortalities of Cx. pipiens larvae and inhibition of adult emergence following treatments with the IGR Dudim and  the plant extract Neem oil are shown in table 1. In general, 2-30% and 11-65% larval mortalities were obtained when 4^th instar larvae of Cx. pipiens were treated with the effective concentrations of the above compounds, respectively. This means that the test compounds did not appear to give high  percentages of  mortality against larval stages. In other words, the results thus may confirm the unsuitability of larval mortality records as a measurement for evaluating the efficacy of such compounds (saleh,1985).

The biological effects of the present compounds were often manifested by the formation of a type of larval – pupal intermediate. Moreover, most pupae were died before the adult emerged. Some adults emerged incompletely or left their tarsi attached in the pupal exuvia (Al-sharook et al.,1991). Therefore, the biological effects of Dudim and  Neem oil were expressed as the percentage of larvae that  do not develop into successfully emerging adults or the inhibition of adult emergence (WHO,2005).

Generally, the effective concentrations of Dudim and  Neem oil in respect were 0.0001-0.005 ppm and 40-150 ppm. The corresponding percentages of inhibition of adult emergence were 22.5 – 94.6% and 23.4 -92.5%, respectively. According to IC₅₀ values (concentration which to inhibit the emergence of 50% of mosquito adults emergence), the IGR Dudim (0.0003 ppm) proved to be more effective against Cx. pipiens mosquitoes than  the plant extract Neem oil (70 ppm). It can be concluded that the response of  Cx. pipiens to the test compounds depends entirely on the differential mode of action of these compounds and its effective concentrations. Studies in this respect were carried out by other investigators using the IGR cyromazine against  Cx. pipiens and Aedes epacticus (Saleh and wright,1989);  the IGR methoprene against Ae. nigromaculis (Cornel et al., 2000) and the plant extract Solanum villosum against Cx. quinquefasciatus (Chowdhury et al.,2007). However, consideration must be taken to study the effect of environmental conditions on the effectiveness of such compounds when applied for field control measures.

On the other hand, the side effect of the IGR Dudim and  the plant extract Neem oil on three nontarget organisms viz. Odonata naiads, aquatic bugs (Order: Hemiptera) and aquatic beetles (Order: Coleoptera) was evaluated in the laboratory. In general, the records indicated that treatment with the concentrations corresponding to IC₉₀ values of  Dudim (0.0032ppm , Fig.1)  and  Neem oil (70ppm , Fig.2) did not exhibit any marked effect on the test nontarget organisms. Similar findings have been obtained by Miura and Takahashi (1975) who found that treatment with the IGR TH6040, at rates used for mosquito control, did not have any adverse effects against aquatic beetles and spiders associated with mosquito larvae in the pond water. Gunasekaran et al.(2004) reported that treatment with the sub-lethal doses of the bioinsecticide Teknar HP-D did not affect on Gambusia fish and water bugs of  Notonecta sp.  and Diplonychus indicus. Moreover, treatments with the IGR pyriproxyfen (sumilarv) against mosquito larvae did not have any impact on aquatic hemipterans  ( Vythilingam et al., 2005).

Figure 1: The relation between concentrations of  Dudim  and the percentage of inhibition of adult emergence after treatment of Cx. pipiens larvae.   Click here to View figure

Figure 2 : The relation between concentrations of  Neem oil and the percentage of inhibition of adult emergence after treatment of Cx. pipiens larvae.   Click here to View figure

Generally, it can be concluded that the present IGR Dudim and the plant extract Neem oil seem to have a good margin of safty to aquatic natural enemies prevailing in mosquito breeding habitats. Therefore, the use of such non-conventional compounds  in mosquito control programs offers certain advantages over some currently used chemical insecticides.

Table 1: The percentage inhibition of adult emergence following treatment of Cx. pipiens mosquito larvae with the IGR Dudim and the plant extract Neem oil.

a: Five replicates, 20 larvae each

b: Corrected by Abbotts formula (Abbott, 1925)

c: Litchfield and Wilcoxon (1949)

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