Joint action of six herbicides with malathon against mosquito larvae of Culex pipiens L

Abstract

Results had proved that the 6 herbicides tested (Benthiocard, Cremart, Drepamon, Oxadiazon, Propamil and Trifluralin) manifest extremely low toxicity to larvae of Egyptian Culex pipiens L. The median lethal concentrations of which far exceeded those being recorded for malathion (0.068 and 0.012 ppm for the field and malathion resistant strains, respectively) as mosquito larvicide. These LC 50-values varied from 7, 16, 62, 23, 42 and 66 ppm (in case of the field strain) and 7.6, 9, 60, 20, 44 and 68 ppm (in case of the malathion resistant strain), respectively. Mixtures of malathion and the tested herbicides produced fitted toxicity lines except in case of Cremart (againts both field and malathion resistant larvae), Drepamon (againts malathion resistant larvae) and Trifluralin (againts field larve). In these latter cases, herbicides at maximum sub-lethal concentrations, when combined with different concentrations of malathion, caused a definite increase in the toxicity of malathion. These compounds were the strongest synergists, producing about 100% increase in toxicity. The degree of synergism demonstrated by the other herbicides does not appear to be correlated with their relative toxicity to the mosquito larvae. However, synergism was dose- and strain-dependent. Continuous treatments with maximum sub-lethal concentrations of the tested herbicides caused latent mortalities as larvae (8–100%) or pupae (0–61%) and inhibited the adult formation by 36–100%. The corresponding records were from 2% to 76%, from 3% to 22% and from 16% to 84%, respectively, when larvae were discontinuously treated with maximum sublethal concentrations. Some of these treatments also caused insignificant prolongation in the duration of both larvae and pupae.