Mode of action of insoluble monolayers on mosquito pupal respiration

Abstract

THE emergence of resistant species and an increased awareness of the environmental problems associated with the use of insecticides has resulted in a search for alternative methods of mosquito control¹. One possibility is lecithin (phosphatidylcholine), which is lethal to the pupae of many species of mosquito and to some fourth stage larvae, when spread as a monomolecular layer at high surface pressure on the air–water interface^2,3. Other analogous water-insoluble surfactants, such as cetyl, stearyl and oleyl alcohol ethers of ethylene glycol are equally effective, and successful trials using several of these monolayers have been carried out on rice fields in Kenya⁴. These have not, however, thrown much light on the mechanism of the anoxic process. Observations by high-speed photography suggested that it is not simply a decrease in surface tension, with consequent wetting of the respiratory trumpet of the insect, but also involves a physical barrier to surface penetration (demonstrated in a 35-mm film available from A.I.M.). We report here the importance of the rheological nature of the monolayer, and show that those exhibiting ‘liquid’ properties at high concentrations form efficient barriers while ‘solid’ monolayers do not. We suggest that liquidity at high surface pressure enables (1) rapid repair of the monolayer after penetration or other disturbance; (2) rapid monolayer transfer to the penetrating trumpet during its displacement from the surface followed by ‘wetting’ and the formation of a bilayer-stabilised water ‘plug’, and (3) complete coverage of the water catchment surface in spite of, for example, projecting vegetation.