Larvicide tests were conducted on five species of mosquitoes, each of which had one or more DDT-resistant strains. The high potencies of DDT and, to a large degree, of DDD were completely lost by resistance. Other compounds were affected in different degrees according to the resistance mechanisms present, as indicated by resistance spectra and the effects of synergists. DDT-resistant strains of Culex pipiens fatigans Wied., Anopheles quadrimaculatus Say and A. stephensi List, showed highly specific resistance to DDT, probably dependent on a dehydrochlorination mechanism. DDT-resistance in Aedes aegypti (L.) and A. gambiae was also high, but there was definite evidence of cross-resistance to biodegradable DDT-analogues (about ×4 and ×10, respectively). This low-level, but definite, cross-resistance extended to a number of other compounds, notably pyrethroids, insect development inhibitors, amines, etc. The presence of synergistic action by piperonyl butoxide suggested that this depended on a microsomal oxidation system.
Isotopically labelled (14C) DDT and malathion were used to study pick-up and penetration of these insecticides by larvae of normal and resistant Ae. aegypti. Both the actual and the percentage penetration of DDT were greater in the resistant than in the normal strain. Whatever the reason for this, it disposes of the possibility of reduced pick-up and penetration as a factor in DDT-resistance. With malathion, the percentage penetration was always higher in the susceptible strain than in the resistant one, though in some cases the actual amount was less.