Dawn dish soap does kill fleas. It works by reducing the surface tension of water. Fleas sink and drown instead of remaining on the water’s surface. The dish detergent also prevents insects’ waxy cuticles from repelling water. As a result, water can enter their respiratory system to suffocate them.
Adding dish soap to water is a common way to drown fleas. For instance, placing a plate of soapy water beneath a lamp works as a flea trap. Or fleas can be drowned in a bowl of soapy water after being combed off a pet.
Dawn Works as a Surfactant
Forcing Fleas to Sink
Dawn contains surfactants, which are compounds that lower the surface tension of water. Fleas will sink and drown when a couple drops of Dawn are added to water. Without a surfactant, fleas won’t break the water’s surface tension. This is because of their small size and water-repellent layer of wax. They’ll stay on the surface, flailing around and possibly escaping. Fleas can survive up to seven days in water, and up to 24 hours when submerged.
Allowing Water into the Respiratory System
Insects don’t drown easily. Their respiratory systems are resistant to oxygen deprivation. Fleas, and other insects, are covered in a layer of wax that repels water and makes them non-wettable (hydrophobic). The waxy layer extends into the trachea, preventing water from entering their respiratory systems.
Dawn’s surfactant properties make fleas more wettable. The waxy cuticle won’t repel water, and thus it’ll enter into their respiratory systems. The fleas drown as the water displaces air. In addition, the soap creates suds which may physically block the spiracles (respiratory entries), preventing the exchange of air and causing suffocation.
Bathing Pets with Dawn
Dawn is sometimes used as an alternative to flea shampoos. The detergent drowns fleas on pets by the same mode of action as previously mentioned. However, it’s important to keep in mind that dish soap isn’t made for animals. Dawn is much harsher than pet shampoos. It’ll remove an animal’s natural protective oils, and their skin can get extremely dry and irritated. Even with gentle, pH balanced pet shampoos, it’s advised not to bathe pets more than once a month. Avoid using Dawn on animals with skin infections.
Insecticidal Effect of Soap
Soapy water has been used as an insecticide for thousands of years. It’s believed that soaps break down cell membranes and remove the wax from insect cuticles. As a result, insects can’t retain water and quickly desiccate. Soaps may also interfere with the cellular metabolism of insects, working as growth regulators. However, glycerin soaps show little to no effect on fleas.