Leafminers are the most problematic insect pest for boxwoods in the New York area. These gall midges overwinter in the leaf tissue and become apparent in the spring when adult flies emerge from the leaf underside, leaving the evidence of holes and/ or tiny casings behind. Females then deposit eggs in the newest leaves, typically in protected positions low and inside the plant. Yellow - orange swelling, sometimes blisters, appear on the leaves in late summer as the insects grow. If you remove the lower layer of a leaf, larval leafminers are exposed.
If you have problematic leafminer damage to your boxwood, control techniques should be applied at the time the adult insects are exposed and most vulnerable. Ruffle the leaves of the shrub and look for orange, winged, adult midges, beginning in late April, when they emerge from the undersides of leaves and, for a period of three weeks, preparing to deposit their eggs. There is one life-cycle per year.
These very common boxwood pests can cause conspicuous damage to a plant but it is usually merely cosmetic. Characteristic cupping and bleaching of leaves takes place in the spring when the nymphs feed inside the leaf, along with waxy deposits secreted on the new leaves in spring. Growth may be stunted but further damage to the health of the plant is unlikely. Populations do not often reach levels requiring control.
Mites can be a significant problem, particularly on Buxus sempervirens (common or American boxwood) cultivars. Shade reduces mite damage, as does summer irrigation because hot, dry conditions speed the velocity of this insect's life cycle and promote population growth.
Mites feed on the leaf surfaces and make tiny punctures as they go, removing chlorophyll with the fluids of the leaf. The result is stippling that can be hard to see, but over time the leaves become pale and silvery. You should not treat mites unless they have reached a problematic level as there are natural, predatory insects that will feed on them if allowed to do so.