Variation in Mortality with Differences in Humidity among Mosquitoes exposed to BHC, Dieldrin and DDT

Abstract

SINCE 1949 a long series of trials of residual insecticides has been carried out by this Unit at Taveta, Kenya, using the trap-hut method originated by Thompson¹. In a number of the trials carried out since 1952, using BHC and dieldrin, a pronounced seasonal effect has been observed. During the dry season, the percentage mortality among female anophelines entering the houses has frequently declined, increasing comparatively rapidly at the onset of the next rainy season. This has been noted in huts lined with an impervious surface (banana leaf) treated with dieldrin and in others with reed roofs and walls plastered with mud, and treated with either dieldrin or BHC. The phenomenon has not been noted with DDT in this series of trials; but it possibly occurred in the earlier set conducted in the same locality by Davidson (ref. 2, Table 3) using DDT oil-bound suspension on a mud-and-gravel wall. Hocking³ observed an increase in kills in a tent treated with DDT dissolved in kerosene when heavy rain fell, but his observation seems to have been overlooked.