Adult fleas are easy to kill. But why do they show up again two to three weeks later? The answer lies within the life cycle of the flea.
Eggs—flea eggs (25-40 per day, per flea) are laid on the pet and soon fall onto the pet’s bedding, on carpets, on furniture, the stairs, etc… In two to five days they hatch into larvae.
Larvae—flea larvae have hooks that help them cling to strands of fabrics and carpets along with cat/dog hair, which prevents them from being vacuumed or washed away. Flea adults, larvae, and eggs are easily eliminated with pesticide, but the reason fleas are so hard to get rid of is due to the pupa stage—the stage between the larvae and adult.
Pupa—in the pupa stage the flea is in a cocoon, which protects them from pesticides. The pupa typically emerges from its cocoon in one to two weeks, but if the pupa cannot detect any movement or vibration in its vicinity, it will often delay its emergence for months because the adult flea will need a blood host to feed on. The common result is that all the adults, larvae and fleas are gone for about two weeks or so, but then the pupas emerge from their protective cocoon, and it may take a few days before they succumb to the pesticide residual of the first treatment.
Conclusion: Always use an adulticide and a growth regulator to eliminate the adults, larvae, and eggs along with halting the growth cycle of any survivors to the next stage of the life cycle. It is also important to vacuum thoroughly before treatment, and every day for two weeks after treatment, in order to encourage the pupa to emerge into adults where they will come into contact with the treated surfaces from the original treatment. It takes time and patience, but a proper flea treatment should take care of the infestation in two to three weeks, sometimes a little longer.