This tree doesn't rate as high as the one planted next to the front door, and you're willing to let it take some hits, particularly from an insect that only feeds on leaves. So the best strategy is to do nothing, knowing that your ladybug buddies will finish off most of the aphids in about a month. Control by exclusion means simply keeping pests at bay. Fences used to keep rabbits out of vegetable gardens are one example. Netting placed over plants when pests show up can safeguard plants from birds and insects.
Landscape fabric laid out under mulch in border gardens is an effective weed-buster.
Floating row covers made from lightweight fabrics can be placed over plants to provide a barrier to insect pests, particularly while the plants are young. They won't protect plants from soil insects, and they have to be in place before the insects show up. You'll have to remove the covers when plants are pollinating, at least for a few hours in early morning. They also provide some protection from frost, but can hurt plants in hot weather. Chemical control includes pesticides, some of which are more environmentally friendly than others, according to B. Rosie Lerner, Purdue Extension horti-culture specialist.