Question
Each year I have an aphid infestation on my River Birch.  There are a lot of predatory insects, such as lady bugs, which live in the same tree.  It seems to be quite the little ecosystem.  Should I allow the aphids, as there is only minor damage done to the small tender leaves for about two months and then the tree seems to do just fine throughout the rest of the year?

Answer
You are correct.  They do little damage and are just a nuisance.  If you want to control them it is pretty easy.  You can treat the plant with Bayer Advanced Tree and Shrub Insecticide.  This product is mixed with water and poured on the soil around the base of the plant.  It is taken up systemically and will kill the bugs within a week or so.  It also provides protection from reinfestation for up to a year.  I would wait and do this next spring just as the new growth is coming out and before the aphids show up.  That should cure the problem.  Good Luck.