In addition to being an effective preventative, Flea Destroyer can also be used to battle ongoing/existing flea infestations. Since Flea Destroyer fights fleas from the ground up, it will kill the next generation of fleas that are maturing in the soil. Flea Destroyer starts working right away (killing larvae within 24-48 hours), but you will notice the impact on your yard after about 6-8 days. Applying Flea Destroyer at the end of flea season will also help stop fleas from laying eggs and over wintering.