Where frequent re-entry is a problem, or where disease bearing mosquito species are involved, it becomes necessary to apply residual sprays to the surfaces on which mosquitoes are likely to rest. Residual sprays differ from space sprays principally in possessing a greater concentration of the toxicant material. Only insecticides with long lasting effects are suitable for use in residual sprays. Where rough absorbent surfaces are involved, the use of a suspension made by mixing a water-dispersible powder is more effective than the use of either a solution or emulsion. When resistance to an insecticide is suspected, contact the nearest entomologist for assistance or advice.
Treatment using aerosols or mists is recommended for the outdoor control of adult mosquitoes.
When control of breeding sources is not possible, aerosols are considered to be a desirable method for preventing annoyance by mosquitoes in limited bivouac areas. Aerosols will often effect complete control within a limited region and will bring adequate protection for short periods. However, in any area where reproduction is continuous and dominated by migratory species, the use of aerosols alone is satisfactory only if done on a repetitive basis. When properly applied, aerosols do not leave dangerous or unsightly deposits. Where reinfestation is not a problem, such as in less populated areas, insecticide application by means of a mist blower may provide satisfactory control.