Studies on aqueous Suspensions of Insecticides

Mosquitos were exposed for short contact periods to deposits on plaster blocks from aqueous suspensions of insecticides. There is an inverse relation between the size of ground DDT crystals fractionated into size ranges by sedimentation and their effectiveness. Smaller particles are more effective than larger ones even when equivalent numbers are compared. Differences in effectiveness are attributed to differences in the amounts of insecticide picked up and retained by the insect. There is no significant increase in kills of mosquitos exposed for the same contact period to dosages of 10–20 micron DDT crystals above 3 mg. per sq. ft. “Persistence” of deposits of these crystals, however, increases as the dosage increases. Crystal length alone is not critical, for fine 60 micron DDT needles are slightly more effective than ground crystals in the 10–20 micron range. The shape and mass of crystals together influence pick-up and determine their effectiveness. There is also an inverse relation between crystal size and effectiveness of methoxychlor and DDD. The effectiveness of Compound 497 and of the gamma isomer of benzene hexachloride is influenced much less by particle size than is that of DDT. The inert ingredient of a wettable powder has a masking effect on the insecticide, and this is intensified as the proportion of diluent to insecticide increases. The availability of the deposit from an aqueous suspension of DDT crystals depends largely on the nature of the surface to which it is applied.