The effect of six pyrethroids, NRDC 104 (resmethrin), NRDC 107 (bioresmethrin), NRDC 119 (cismethrin), NRDC 143 (permethrin), NRDC 156 and allethrin, against larvae and adults of the Egyptian mosquito Culex pipiens L. has been evaluated. Five strains were used, a field strain and DDT-susceptible, DDT-resistant, malathion-susceptible and malathion-resistant strains. Results obtained were also referred to a DDT highly susceptible strain of another mosquito Aedes aegypti (L.)

The toxicity curves showed that NRDC 156 proved to be the most effective compound tested, followed by NRDC 143, NRDC 119, NRDC 107 and NRDC 104, respectively, while allethrin was the least effective. Such a fact was highly pronounced either on the LC50 or on the LC95 basis.

As larvicides, data obtained proved that the DDT-susceptible strain was less susceptible to all the tested pyrethroids than the DDT-resistant strain by about 42.6-, 5.1-, 8.6-, 132-, 290.7- and 4.8-fold in the case of NRDC 104, 107, 199, 143, and 156 and allethrin, respectively. Tests also showed that there is no definite variation in susceptibility of both malathion-susceptible and malathion-resistant strains, except in the case of NRDC 143 and 156. Comparing tests against larvae of A. aegypti confirmed that the susceptibility to these pyrethroids was closely correlated with insect species and its strain, and cross-resistance might be developed among larvae of these species.

As adulticides, the pyrethroids used could be arranged in the following descending order according to their LC50 values: NRDC 156, NRDC 143, NRDC 104, allethrin, NRDC 107 and NRDC 119. The change in the trend of toxicity is possible due to the stage tested and the mode of action of these compounds on adult or larval stage.