Seresto is a flea collar that actively protects pets from fleas and ticks for 8 months when worn correctly. It works by continuously releasing imidacloprid (an insecticide) and flumethrin (a pyrethroid) at low concentrations through its waterproof collar.
These two substances combine to provide your pet with up to 8 months of continuous protection against fleas, ticks, and other parasites. The collar works quickly, as killing adult fleas typically takes within 24 hours of contact with the active ingredient in Seresto collars. In addition, Seresto collars can kill larvae in just 20 minutes. As such, using Seresto collars helps protect your pet from infestations of fleas and ticks immediately after the application of the collar
All in all
Seresto typically kills adult fleas within 24 hours of being applied and offers total protection against reinfestation for up to 8 months when kept on continuously.