THE MONOFACTORIAL INHERITANCE OF RESISTANCE TO DIELDRIN IN LARVAL AND ADULT CULEX QUINQUEFASCIATUS SAY

A susceptible and a resistant strain were isolated from an originally heterogeneous laboratory colony of Culex quinquefasciatus Say (= Culex pipiens fatigans Wiedemann) by use of discriminating concentrations of dieldrin on fourth-instar larvae. By cross-breeding, hybrids of intermediate susceptibility were obtained. By repeated cross-breeding and elimination of susceptibles the authors have shown that resistance to dieldrin is controlled by a single inheritable factor which is neither fully recessive nor dominant in the hybrid genotype, since the ratios of the LC(50) values were 1:19:196. Cross-resistance was shown to lindane but not to malathion or to any of three carbamates.Similar tests were made with adult females exposed to papers impregnated with n-dioctyl phthalate as solvent to secure high concentrations of dieldrin. Resistance in this stage also was neither fully recessive nor dominant, but it can be calculated quantitatively only for the hydrid (approximately 15-fold) since longer exposure was required with the resistant genotype. Determination of dieldrin pick-up showed that this cannot account for the differences in susceptibility of the genotypes. Analysis of resistant females surviving exposure to dieldrin papers showed slow loss of dieldrin and thus added confirmation to the hypothesis that metabolism is not the controlling process in dieldrin-resistance.