Larvicidal Activity and Joint Action Toxicity of Certain Combating Agents on Culex pipiens L. Mosquitoes

Aims: The main objective of the present study was to investigate the larvicidal effect of some biological control agents like Beauveria bassiana and Bacillus thuringiensis var. israelensis (B.t.i.) and some natural control agents as Diflubenzuron, Azadirachtin and Emamectin benzoate on Culex pipiens mosquito. The toxicity of binary mixtures of these control agents was also assessed.
Methodology: The larval susceptibility test of C. pipiens was estimated when the third instar were treated with Azadirachtin, B. bassiana, B.t.i., Diflubenzuron, Emamectin benzoate and Deltamethrin (reference compound). Series of concentrations for each compound in addition to control were replicated four times. Mortality counts were carried out after 24, 48 and 72hr of treatment. To determine the joint toxic action of the tested compounds, the calculated LC12.5, LC25 and LC50 (after 72hr) were used alone (to calculate the expected mortalities) and in bi-mixtures. For each treatment, four replicates of 30 larvae/replicate were used. Percent mortalities of larvae were recorded after 72hr post-treatment. The joint action of different mixtures in terms of co-toxicity factor (C.F.) was estimated. Data of bioassay were analyzed using Probit program.
Results: Data showed that the LC50 of B.t.i., Emamectin benzoate, Azadirachtin, Diflubenzuron and B. bassiana were 0.044ppm, 1.24ppm, 3.02ppm, 10.32ppm and 4.122ml/L, respectively on the third instar of C. pipiens after 24hrs. Azadirachtin showed time related larvicidal activity. Diflubenzuron induced delayed effect on C. pipiens larvae. B. bassiana, had the lowest activity against this mosquitoes (LC50= 1.85ml/L) after 72hr of exposure. Data of joint toxic action of some mixtures such as (B. bassiana + Diflubenzuron) or (Diflubenzuron+Azadirachtin) revealed antagonistic effect while almost other binary mixtures showed potentiating effects. The mixture of LC12.5 B.t.i. + LC12.5 Deltamethrin recorded the highest potentiating activity.
Conclusion: The study suggests that, the most effective tools for C. pipiens larvae eradication included B.t.i. followed by Emamectin benzoate, Azadirachtin, Diflubenzuron then B. bassiana. The use of some binary mixtures of these tested control measures can get better control, save the amount and reduce control cost.