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plant resistance to herbivory

Hello to all...

A general school of thought amongst "sustainable" growers is that
plants that are under significant stress are more often the recipient of
herbivory than less stressed plants. I have observed that the
scientific literature substantiates this theory to some extent.

My question for the group is... what is the ecological logic behind
pests consuming weak plants preferentially to strong plants ? and what are
the specific mechanisms by which herbivores discriminate
stressed and less stressed plants ?

In animal ecology it seems very logical that predators have an
easier time catching and killing weak/young/injured prey...

How well does this analogy hold true for plants ? Do strong/less stressed  
plants tend to have more active defense mechanisms than weaker/stressed
plants ? 

...but some stressed plants have elevated levels of alkaloids and
other anti-feedant compounds... 

Any insights or recommended reading on this subject ?

On another subject:
I have read about the insecticidal compounds in Assimina triloba (Pawpaw)
foliage... What is the current status of research on these bioactive
compounds and can anyone provide any recommendations for how one would
prepare pawpaw leaf extract for pest control use ?

Joel Gruver
U of MD, Agronomy