Changes in relative humidity had a very marked influence upon the biological activities of γ BHC, dieldrin and DDT sorbed on Uganda mud blocks. In agreement with field observations, kills of mosquitos increased as the humidity increased and this effect was rapidly reversible.

Disappearance of insecticide particles from the mud surface was inversely related to humidity, the rate increasing as humidity decreased. On the other hand, the rates of diffusion of sorbed insecticide away from the surface were greater at higher humidities.

It is suggested, therefore, that when the biological activity of a mud surface is increased by increasing the humidity, the insects acquire a greater dose of insecticide not because the concentration of insecticide in the surface layers is greater but because the insecticide that is already present acquires a greater mobility. This greater mobility results in a more rapid diffusion in the mud itself away from the treated surface and a greater potential for diffusion into insects resting on the mud surface.

Support for these conclusions was obtained by showing that on transferring dieldrin-treated mud blocks from low to high humidity the change in biological activity occurred at about the same rate as the increase in water content of the blocks.

The influence of humidity upon the sorption of DDT and dieldrin on a selection of soil samples of widely differing origin was observed. Both insecticides disappeared from the surface more rapidly at low than at high humidity on all the soils.