It is suggested that if the vapour of an insecticide is rapidly adsorbed by a substrate the rate of volatilisation of the molecules of insecticide particles lying on the substrate is greatly increased. This explains the rapid loss of effectiveness of wettable powders on some soils. After the insecticide has been sorbed it is concentrated at and near the treated surface but afterwards slowly diffuses inwards. This diffusion reduces recoveries of insecticides from surface scrapings.

DDT can be recovered quantitatively from Uganda mud blocks 12 months after sorption is complete. Therefore, on this mud at least, catalytic decomposition does not occur.

A test has been devised for measuring the sorptive capacities of soil samples for non-polar vapours. It indicates if a soil is highly active but those with lower or negligible activities do not give a good correlation.

A survey of 18 soils from different parts of the world suggests that rapid inactivation of an insecticide by sorption should be expected on any red lateritic soil. Generally, others are inert or show only low activity.

Attempts to reduce or abolish the sorptive powers of Uganda soil have not been successful and the only effective method known at present is to limewash the walls before spraying. Pretreatment with a size solution also stops sorption but, unlike limewashing, at the same time reduces the efficiency of a subsequently applied wettable powder.

Finally, work by various authors on the inactivation of insecticides on mud surfaces is reviewed.