If you hang around an orchard long enough something unusual will show up, especially when seasons change and there's more rain than usual and it's cold, but not so cold that it freezes and it's prolonged.  So out of San Diego comes a request for an identification of a brown bump on avocado stems.  It's a brown aphid.  Is it something of concern?  Likely not.  Over the years there have been reports of several aphids on avocado.  Check out Walter Ebeling's "Subtropical Fruit Pests" at Avocadosource.com:

http://www.avocadosource.com/papers/research_articles/ebelingwalter1959b.pdf

These things come and go, and they don't do any damage because once the biocontrol bugs get going, they are fresh meat for them.