The Science of Citrus, Botanical Oils, & Surfactants
Fleas have a waxy outer skeleton that protects them from the effects of many insect-killing products. The University of Ohio lists these Botanicals as effective skeletal-penetrators and flea killers: "Linalool (Demize House Spray), a citrus peel extract, is a natural, fast-acting flea killer with short residual control. Other botanicals include d-Limonene, citronella oil (Citrus Flea Defense), eucalyptus oil, pennyroyal oil, balsam, lavender oil, calendula, comfrey, rosemary, tea tree oil and yucca." They all help to penetrate the flea's "armor" to expose the internal organs to killing conditions.
Surfactants or wetting agents (Citrus Flea Defense) also break down the armor-coating to allow the insect to die of dehydration or worse.
Advantages of all of these ingredients is their safety to humans and animals as well as the environment. They are extremely quick kill - often within 20 seconds of contact. AND development of immunity by insects is unlikely.