A SECONDARY EFFECT OF DDT UPON THE AQUATIC MICROFLORA AND MICROFAUNA

Observations were made on the microflora and microfauna of infusions in which larvae of Anopheles maculipennis atroparvus were being subjected to DDT selection pressure (2 μg/ml). Comparisons with controls indicated that the insecticide influenced neither the species representation nor the succession of the organisms concerned. However, heavy initial larval mortality took place. Dead larvae remaining in the containers overnight became centers of very rapid bacterial multiplication, and the maturation of the experimental infusions was accelerated. The larval cultures became polysaprobic after about two weeks, by which time the surviving larvae were in the fourth instar. These survivors were large and healthy, having spent their early developmental period in the presence of a rich supply of infusion organisms answering their food requirements, and successfully withstood the deteriorating environmental conditions and consequent heavy infestation with epibionts (Chlamydobacteriales and peritrichous ciliates). On the other hand, fourth instar larvae from similar infusions in the laboratory colony of A, m. atroparvus were often undersized and sickly because of early overcrowding linked with the initially longer time required for the development of an adequate yield of food organisms. Such larvae, also pupae, frequently died of exhaustion due to undernourishment and complicated by the hampering of their activity by a vestiture of bacteria and ciliates. This explained the paradox that adults derived from the laboratory colony tended to be less robust than those hatching from developmental stages subjected to DDT selection pressure. Besides pointing to the importance of paying due regard to larval environmental factors in resistance studies, these observations suggest the need for field investigations of the significance of similar chains of events in nature.