Its virtually impossible to give you any advice based on a decription of "brownish things" or "brown creatures" attacking your tree. A picture would be worth more than a 1000 words.

Harry

Avocados don't have a lot of pests; and fewer that do any serious damage. You might just try hosing them off good once a week; it goes a long way to getting rid of simple pests; and the tree likes it. You may have spider mites, but hard to tell without fotos. BTW I live in Guatemala, the original home of most avocados; on my Meyer lemon farm I am growing Queen avocados, only a cupla trees and they are not yet bearing. Of the 100 or so varieties we have here, I think the Queen is the best of all.