Introduction

Insecticide resistance is an increasing problem  for mosquito control in different  parts of the world (Canyon and Hii, 1999 ;Katyl et al  2001; Saleh et al.,2003; Nazni et al .,2005;Tawatsin et al.,2007). It is necessary, from time to time, to monitor the susceptibility status of local mosquito  vectors to the insecticides used in the control programmes. Documentation of insecticides resistance will identify insecticides that are no longer effective and is a critical first step towards developing a resistance management programmes (Ponlawat et al., 2005). Given the limited information on susceptibility  levels of mosquito vectors to insecticides in Jeddah, our objective was to determine the current susceptibility status of A.aegypti ,the primary vector of dengue fever, to some insecticides commonly used in mosquito control programmes .

Materials and Methods

Mosquito strain

Tests were performed on a field strain of A.aegypti 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 %  RH., with a 14:10 (L:D) photoperiod.

Insecticides

The insecticides used for larval bioassay were

Two pyrethroids: Snap 230 SC (permethrin 11 % + Tetramethrin 1% + Piperonyl Butoxide 11 % ; Astrachem Co). And Icon 2.5 EC (Lambada cyhalothrin 2.5 %; Astrachem Co).

The bacterial insecticide Bacilod, a wattable powder formulation of Bacillus thuringiensis israelensis (1200 ITU/mg.;LOD, Ltd.)

Two insect growth regulators: Baycilod WP25 (Triflumuron 25 %; Bayer Env. SC. SAS) and Sumilarv (Pyriproxyfen) 5 G(Sumitomo chem.. Co).

 Larval bioassay

The larval susceptibility test was conducted according the method of WHO (1980).Treatments were carried out by exposing early 4^th instar larvae of  A.aegypti  to  various concentrations  of  the tested insecticides for 24 hr, in  groups of glass beakers containing 100 ml 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 these experiments. Larval mortalities were recorded at 24 hr post-treatment for the pyrethroid insecticides Snap and Icon as well as the biocide Bacilod. The dead larvae were identified when they failed to move after being probed by a needle in siphon or cervical region. In the case of the IGR Sumilarv and Baycidal, cumulative mortalities of larvae and pupae were recorded daily. Live pupae were transferred to untreated water in new beakers for further observation, i.e. normal emergence, presence of morphologic abnormalities or death. Partially emerged adults or these found completely emerged but unable to leave the water surface were recorded and scored as dead. Therefore, the biological effect of Sumilarv Baycidal was expressed as the percentage of larvae that do not develop into successfully emerging adults, or inhibition of adult emergence (WHO, 2005). Log concentration – Probability regression lines were drawn for the tested insecticides and statistical parameters were also calculated using the method of Litchfield and Wilcoxon (1949).

Results and Discussion

Table 1 shows the toxicity of pyrethroid insecticides Snap and Icon as well as the bacterial insecticide Bacilod against mosquito larvae of A.aegypti.The effective concentrations of the above insecticides against 4^th instar larvae were 0.02 – 0.015 ppm, 0.005 – 0.03 ppm and 0.2 -0.6 ppm, respectively. The corresponding larval mortalities were in respect 18 – 91, 20 – 93 and 24 – 95 %. Taking LC50  values obtained from toxicity lines into Consideration  (Fig.1), the records showed that  Icon (0.01 ppm) proved to be the most effective compound, followed by Snap (0.048) while  Bacilod (0.3) was the least effective. These results indicate that mosquito larvae of A.aegypti were more susceptible to Icon than Snap and Bacilod by about 4.8 and 30 folds, respectively.

However , it has been suggested that  the variation in susceptibility  status of the present  mosquito larvae to the test insecticides is a dynamic process depending on the frequency of use, type of insecticides and its concentration (Salah and wright , 1989; Paul et al .,2006). The fluctuations in the percentage mortalities obtained for the different concentrations of the test insecticides against the present A.aegypti larvae support this conclusion (Canyon and Hii, Sulaiman et al, 2007).

The results presented in table 2 show the percentage of larval mortality, pupation and the inhibition of adult emergence following larval treatments with different concentrations of the IGR Baycidal and Sumilarv. The effective concentrations of Baycidal and Sumilarv ranged from 0.003 – 0.006 ppm and 0.001 – 0.02 ppm, respectively. In general   4 – 31 % and 6 – 15 % larval mortalities were obtained when the early 4^th instar larvae of A.aegypti were treated with the above compounds, respectively. However, the biological effects of the test compounds were often manifested by the formation of a type of larval–pupal intermediate. These abnormalities in the metamorphosis might be due to inbalance in the hormonal system (Bridges et al., 1977).  Moreover, most pupae that pupated successfully often died either before the adult emerged or as albino pupa. Many adults emerged incompletely or left their tarsi attached in the pupal exuvia (Salah et al., 1981). Generally the corresponding percentages of inhibition of adult emergence were in respect 36 – 89 % and 20 -92 %. Taking IC₅₀ values (concentration which to inhibit the emergence of 50 % of adults) into consideration (fig 2), A.aegypti larvae proved to be more susceptible to Baycidal (0.0007ppp) than Sumilarv (0.003ppm) by about 4.3 times. Similar studies  in this respect were carried out by  several  authors to determine the susceptibility  level of different mosquito  vectors to IGRs (Saleh and Wright  (1990)) using cyromazine  against Culex pipiens and A. epacticus; EL_Shazly and Refaie (2002) using Sumilarv against C. pipiens; Tawatsin et al(2007) using  Novoluron  against C.quinquefasciatus). Generally, the present  work suggested  that continuous  insecticide  susceptibly  monitoring  should be  conducted in different areas  in Jeddah regularly  to identify  the efficacy of  insecticides  used for mosquito  vector control and to facilate  selection of insecticides with  greatest promise  for halting or minimizing  dengue infections .

Table 1.  Susceptibility levels of A. aegypti mosquito larvae to Snap, Icon and Bacilod. 

a .Five replicates, 20 larvae each ; control mortalities ranged from 0.0 – 2%

b .Obtained from toxicity lines (Litchfiled and Wilcoxon,(1949) ).

Table 2: The biological effects of the IGR Baycidal and Sumilarv on the developmental stages of A. aegypti .

a .Five replicates, 20 larvae each

b corrected with Abbott’s formula (Abbott, 1925).

c .Obtained from toxicity lines (Litchfiled and Wilcoxon, (1949) ). 

Acknowledgement

The authors  are indebted to the Deanship of Scientific Research for funding this project under No. 164/28. We are also thankful to all our colleagues and friends for their co-operation  and encouragement during this work.

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