Q: I have boxwoods that are about four feet tall and I have never pruned them extensively. Last year’s growth is mottled looking.
A: Your boxwoods are most probably infested with boxwood leaf miner. August is the month when homeowners notice the splotchy brown and yellow leaves caused by the pests.
A fly-like (some say mosquito-like) adult insect laid eggs on the boxwood leaves in spring. When the eggs hatched, the larvae chewed into the center of the leaf and happily munched away, protected from predators and the elements. Now you easily notice the mottled appearance of the damaged leaves.
Leaf miners are very difficult to control once they are inside the leaves. The best way to control their numbers is to spray when the adults are present in April and May. Insecticidal soap (click for sources) or horticultural oil (click for sources) will kill them if the shrubs are sprayed thoroughly. Make three applications at weekly intervals beginning three weeks after new spring foliage appears. Systemic insecticides (click for sources) are absorbed by plant tissue, making it deadly to chewing insects in the leaves.
You can prune the foliage now without harming your boxwoods but plant to use an insecticide on the miners next year.