Polarity of Insecticides

The polarity of insecticides has been regarded as an important factor for cuticular penetration. As mentioned earlier, the typical insect cuticle should be considered a two-phase system, the outer layer (epicuticle) having hydrophobic properties and the inner layers (procuticle) having hydrophilic properties. Thus, whether the insecticide is lipid soluble or water soluble, its tendency to move through the cuticle as a whole depends on whether it can pass through the hydrophobic or hydrophilic barrier, whichever the case may be. The efficiency of such movement will probably depend on the oil-water partition coefficient of the insecticide, the nature of the surfactant or solvent—which may be a part of the insecticide formulation—and the nature of the cuticle itself (Terriere, 1982). 

Oil-water partition coefficient refers to the tendency of a chemical to distribute itself between lipid and aqueous phases when both are present. This can be measured for comparative purposes by adding the compound to a two-phase system such as olive oil-water or octanol-water, mixing the three components thoroughly, allowing the two phases to separate, and then determining the amount of the compound in each of the phases. Using the olive-water system, DDT has a partition coefficient of 316, indicating that the concentration of DDT in the olive oil is 316 times that in the water phase. 

Insecticide Half-time (min) Partition coefficient (olive oil-water) 

Source From Olson, W.P. and O Brien, R.D., /. Insect Physiol. 9, 777, 1963. With permission. 

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