Chemical shelf life

I understand that most garden chemicals have a shelf life and if not
specifically stated, you should replace them after about  3 years.  But
what about Insecticidal Soap?  Do its properties diminish after opening? I
received some Echinops from a very reputible nursery that had aphids on
them.  I didn't notice at first because I'm not particularly
knowledgeable of the plant.  The leaves appeared to be a bit puckered,
but I assumed that's just how they are.  They began to look a bit worse a
few days later, I found the aphids, checked all other plants in the
vicinity, sprayed w/ IS, but days later they were still there and now on
a few other plants.  Sprayed IS  again.  A week later, still a problem. 
So I checked my cabinet for something else to use and came up with an
unopened bottle of Pyola (Canola Oil & Pyrethrins)  One spray and they
seem to be controlled so far.  I know to spray all surfaces, top and
bottom, crotches, to check all plants near them, etc.    Is it time to
buy a new bottle of IS? Kitty

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