The most likely explanation is insecticide resistance.

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We learned long ago that insecticide use represents a selection
factor in a flea population. The resistant individuals survive and
pass their genes on to offspring.  Eventually a resistant population
is produced.  We would like to avoid creating a population of fleas
who laugh at our best insecticides and there are two ways of doing
this:

The first way to avoid resistance:  Change products periodically.

This seems simple and even obvious. If you want to make a resistant
population then keep exposing the population to the same insecticide
and after enough generations your population will be resistant.  If
you switch to another insecticide, the group will be totally
sensitive to the new insecticide.  After a few more generations,
change again.

The second way to avoid resistance:  Use a flea sterilizer.

A group of fleas that survives exposure to Frontline® or Advantage®
cannot pass on their resistance genes if they have been sterilized by
a second product.  Program® interferes with the production of chitin
(the hard material making up the insect exoskeleton).  The adult flea
has already made its chitin but its off-spring need to develop a
chitin egg-tooth to escape their eggs after development into larvae.
A larva whose mother has had a big drink of lufenuron-laden blood
will not be able to hatch.

Another such sterilizer is Methoprene (the "plus"in FrontlinePlus®).
Methoprene was developed as an additive to flea sprays and foggers.
It is totally non-toxic (it does not even kill fleas) and represents
a group of insect control agents called "insect growth regulators" or
"juvenoids". Methoprene mimics a youth hormone of the flea so that
larvae who consume it in flea dirt cannot mature and eggs laid by
female fleas who have been topically treated with it cannot develop.
Twenty years ago, this compound was a miracle in flea control. It
enabled the life cycle to be broken in two places for the first time.
Now methoprene is rather "old hat" but it has been included in
Frontline to prevent Frontline resistance.

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