Question
Any idea why the aphids have not yet harmed the other plants I have nearby?
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Followup To

Question -
I have 2 holly bushes and they both are suffering from insects or fungus of some sort.  Some of the branches were dead by the time I noticed it this spring.  Most of the leaves on the living branches are turning black, patch by patch.  The undersides of the leaves have elongated white fuzz spots, about a third of an inch long.  Any ideas what it is or if I can successfully treat it?
Thanks!

Answer -
Sounds like aphids--I would check with your local garden/nursery type store and buy an insecticide for aphids and spray the bush. Spray both sides of the leaves. The black is a fungus called black sooty mold that grows on the sugar excrement from the aphids--contol the aphids and the black will go away.

Answer
In general--Specific Insects feed on specific plants. Exp.: tomato horn worm feeds on tomatoes--elm leaf beetle feeds on elm. and aphinds that feed on holly for example do not feed on some other plants. There are insects that will feed across a wide variety of plants and some plant are resistant to aphids problems. You did not say what other plants are there so I will assume that the aphids on the hollies are not going to feed on them.