Entry of Insecticides into Animal Systems

Abstract

The body surface of insects consists of a hard skin known as the cuticle. At first glance it appears that this barrier should be very effective against the entry of insecticides, but there are two main reasons why it is not: (1) the insect exposes a far greater surface area relative to its volume than the mammal, and (2) the insect cuticle is hydrophobic (i.e., lipophilic) so that it can resist desiccation and drowning. Most modern insecticides are apolar and therefore easily penetrate the insect cuticle. Mammalian skin, by contrast, is relatively resistant to the entry of insecticides. As a result, it is common that the acute oral toxicity of an insecticide is much higher in mammals than the contact toxicity, whereas the contact and oral toxicities are almost equal in insects. Though in insects cuticular penetration is usually the major pathway, in some instances insecticides do enter by way of the mouth, repiratory system, and other vulnerable places such as the antennae, eyes, and tarsi.