Activyl is a monthly spot-on flea control for dogs. The active ingredient in Activyl is indoxacarb, which works by bioactivation - enzymes inside the flea activate indoxacarb and release its full flea-killing power. There is no known flea resistance to indoxacarb. Activyl stops fleas from feeding, kills all flea stages (egg, larvae and adult), and inhibits flea development in the environment.