Upon completion of development, the mature larva produces a silk-like cocoon in which it pupates. The cocoon is ovoid, whitish, and loosely spun. Because the cocoon is sticky, it quickly becomes coated with debris from the environment which helps camouflage it. In most homes, the pupation can be completed within 5 to 14 days in ideal conditions. Once the pupa has fully developed, the adult flea (pre-emerged adult) will remain inside the cocoon for several days to several weeks until stimulated to emerge from the cocoon. Physical pressure (being stepped on), carbon dioxide (that mammals exhale), vibration (from walking, or vacuuming) and heat (generated by potential hosts or heated homes) can all stimulate emergence. It is interesting to note that flea pupae do not hatch all at once regardless of the stimuli. It is believed there is some type of communication involved (Pheromonal?, Genetic?) that causes flea pupae to stagger there hatching over a varied period of time. This effect is sometimes mistakenly perceived as a flea control product failure when it is in fact a normal mechanism of the flea to increase the likelihood of survival.

The fully formed adult flea residing in the cocoon (pre-emerged adult) is the stage that can extend the longevity of the flea, and makes it the most troublesome from a control standpoint. If the pre-emerged adult does not receive the proper stimulus to emerge, it can remain dormant in the cocoon for several weeks and possibly as long as one year in rare conditions, until a suitable host arrives. Added to this is the fact that there are no chemical sprays available on the market today that can penetrate the pupael cocoon. House sprays will kill exposed eggs and larvae, but pupae will remain unaffected and will hatch at some point long after the residual effect of most sprays has expired. Successful treatment of pupae must usually wait until it has hatched into an adult where it may then be killed with an adulticide. Cocoon's are typically spun around the base of carpet fibers or bedding material which make them impossible to remove with regular vacuuming. Vacuuming is recommended though as the vibration from the vacuum may stimulate the flea to the point of emergence.

Pupae commonly make up 10% of the infestation in a home. Depending upon temperature and humidity, the entire life cycle of the cat flea can be completed in as little as 12 to 14 days under ideal conditions, or be prolonged to 6 months and possibly as long as a year in rare instances. However, under average household conditions, cat fleas will complete their life cycle within 3 to 4 weeks.

Pupal Window Effect

Once adult fleas begin to emerge from cocoons, they are vulnerable to insecticides. However, after treatment with an adulticide pupae will continue to emerge for at least 2 to 4 weeks. In fact, small numbers of fleas may continue to emerge for periods up to one year; a phenomenon called "delayed emergence." Or "Delayed Pupal Hatching" This continued emergence of fleas following treatment is called the Pupal Window.

The pitfalls of the Pupal Window Effect are:

• Re-treatment with adulticides on the pet is needed to control the emerging fleas; and
• Pet owners unaware of the effect may believe the control measures used aren't working and either discontinue them or switch to other, perhaps less effective methods. 

A common story owners relate, is they visit a cottage for the first time since last season. Within minutes of entering they notice that their pet is covered in fleas or that their legs are being bitten by small fleas. These smaller fleas are newly emerged adults that have remained dormant since the last visit and the presence of the family has stimulated the emergence.