Cuticular penetration

Species differences in the rate of penetration of insecticides is shown in Figure 6.2. It is seen that cuticular penetration in the housefly and stable fly cuticle by carbaryl in the first 4 h is seven times that of the rice weevil and five times that of the boll weevil. This could affect the efficacy of this insecticide in these insects. 

Factors Affecting Cuticular Penetration Rates 6.2.4.1 Effect of Solvent... 

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). 

In some cases, increased toxicity of insecticides can be attributed to materials that enhance cuticular penetration of the insecticides without involving inhibition of metabolic detoxification. For example, Sun and Johnson (1972) found that carbaryl was more toxic to houseflies when topically applied in kerosene than in acetone. These authors classified this type of synergism as quasi-synergism. 

Resistance to py rethroids can also be in part attributed to reduced penetration in insects. Ahmad et al. (2006) found that delayed cuticular penetration played an important role in deltamethrin resistance in Chinese and Parkinson strains of the cotton bollworm, Helicov-erpa armigera. The half-time for applied deltamethrin was 1 hr for the susceptible strain and 6 hr for both of the resistant strains. 

Ahmad, M., Denholm, I., and Bromilow, R.H., Delayed cuticular penetration and enhanced metabolism of deltamethrin in pyrethrin-resistant strains of Helicoverpa armigera from China and Pakistan, Pest Manag. Sci., 62, 805, 2006. 

Stevens PJG, Gaskin RE, Hong SO and Zabkiewicz JA Contributions of stomatal infiltration and cuticular penetration to enhancements of foliar uptake by surfactants. Pesticide... 

They lack a cuticle. The cuticular penetration barrier prevents ascertaining whether many new chemical groups are toxic. This barrier can later be overcome by synthesis of analogs of active structures or by better formulation. The knowledge that a chemical is toxic if it reaches the plant cell is probably the most important lead available from cell cultures that whole plant screens lack. The absence of cuticle and short diffusion distances in cell cultures, provide a possibility for performing rapid kinetic. 

Charnel A. Foliar absorption of herbicides study of the cuticular penetration using isolated cuticles. Physiol Veg 1986 24 491-508. 

There now exists an extensive literature on the uptake and translocation of herbicides in plants. In this brief review, it is intended to elucidate the general principles that govern the transport of pesticides in plants. The transport behavior of the different classes of herbicides will be considered, together with ways of using physicochemical properties to predict the likely behavior of new chemicals. It should be noted that the processes of sorption in soil, of cuticular penetration in the case of foliar-applied compounds, and of metabolism in the plant all influence the availability of herbicides for uptake and redistribution in plants, and these factors are the subjects of separate chapters in this book. Interpretation of the literature on systemic transport usually needs substantial consideration of these additional factors. 

The same general principles determine xylem translocation of herbicides whether they are foliage applied or soil applied. Provided that the barrier of cuticular penetration can be largely overcome by appropriate choice of formulation, application to foliage is a more reliable way of getting herbicides into the plant. This route avoids the losses caused by sorption to soil, and it bypasses the endodermal barrier surrounding the vascular tissues in the root. Once through the cuticle and into the leaf tissue. 

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