Studies on insecticide-resistant anophelines. 2. Chromosome arrangements in laboratory-developed DDT-resistant strains of Anopheles atroparvus.

The development of several DDT-resistant strains of Anopheles atroparvus is described. They were selected by larval treatment only (RL), selection of larval and adult females (RLAF), and selection of adult females and males (RAFM). Their rate of DDT-resistance was measured in the larval and adult states (mainly in the 32nd generation). The results were compared with those of a dieldrin-selected strain (R/dieldrin) and several normal and semi-normal reference strains. In the larval assay DDT-resistance was of the following order: RL>RLAF>RAFM>R/dieldrin. Topical application to adults gave: RLAF>(RL?)>RAFM>R/dieldrin; while adult tarsal contact gave: RAFM>RLAF>RL>R/dieldrin These different orders of resistance are discussed.The strains were subjected to cytological examination by the method of Frizzi. The percentages of heterozygous inversions found were as follows: R/dieldrin, 77.4%; RL, 55.3%; RLAF, 54.0%; RAFM, 24.4%; all normal and semi-normal strains about 12%-21%.The rate of DDT-resistance could not be correlated with the percentages of heterozygous inversions found in the various strains.