National Institute of Communicable Diseases, New Delhi
**Present address: C/o. National Institute of Communicable Diseases, Southern India Branch, Coonoor, Madras State.
*The study was carried out under the enquiry on the “Behaviour of vector species and their resistance to insecticides” of the Indian Council of Medical Research.
Laboratory studies on selection for resistance to DDT were carried out with A. stephensi type form. The results showed that:
1. Prolonged selection of A. stephensi (Madras strain) at adult, larval and both adult and larval stages by DDT had resulted in occurrence of DDT-resistance in larval stage and higher tolerance in the adult stage. Selection for DDT resistance also conferred higher tolerance to gamma BHC and dieldrin to both adults and larvae of the three selected strains. The degree of non-specific tolerance was found to be higher in larvae than in adults. This pattern of resistance was in conformity with that observed in the field. The delayed occurrence of resistance is suggested to be due. to slow concentration and suitable organization of the determinants into a close cluster (system).
2. The withdrawal of selection pressure for eight generations did not show material change in the susceptibility status of the three selected strains. Failure of the selected strains to revert to initial susceptibility level was considered to be due to (a) sib-mating in a closed population and/or (b) the shortness of the period for materialization of the phenomenon. Inbreeding may result in integration and disintegration of the minor determinants not closely linked. Integration and disintegration of the determinants was considered as the cause for the observed fluctuation of mean and variance of the tolerance levels of intermediary generations.
3. Continuation of DDT selection in the laboratory for 29 generations failed to increase the larval resistance in Erode strain, the larvae of which were found to be already highly resistant in the field. Non-availability of genetic determinants for further integration into a system for progressive selection was suggested as an explanation of this phenomenon. Laboratory selection, however, resulted in resistance in the adults. Slower concentration and organization of specific determinants having expression in the adult environment alone was suggested as a probable explanation of the phenomenon.
