Hydrophilic properties cuticle

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). 

It seems reasonable that molecules of a surfactant may diffuse from the spray droplet into the cuticle of leaves perhaps via imperfections and cracks and then align themselves in monolayers with their nonpolar ends oriented in the cutin and wax. The polar ends will thus also form a layer whose size depends on the length of the hydrophilic chain of the surfactant molecule. These layers or hydrophilic channels will presumably attract water, causing swelling of the cuticle, and thus channels or pores are formed along which herbicide molecules can diffuse according to their various chemical properties (solubility, residual chemical charge, polar properties, etc.). 

In contrast, no insecticide has been developed to interfere specifically with the process associated with the steroidal insect molting hormone, 20-hydroxyecdysone. Although active ecdysteroids can be obtained from plant and animal sources (S), the main reason for the failure to develop them as insecticides has been that (a) their structures are too complex to produce economically, (b) their hydrophilic nature prohibits their penetration into insect cuticle, and (c) insects have powerful mechanisms to eliminate ecdysones between molts. One approach to solve these problems is to synthesize a simple molecule which can mimic ecdysone, but with appropriate chemical and transport properties, and acceptable metabolic stability. 

---