Boxwoods infested with this leafminer develop blisters on the lower leaf surface. Infested leaves are usually smaller, off-color and drop sooner than healthy leaves. Heavily infested boxwoods usually have sparse foliage and poor color. Shrubs are generally not killed by leafminers.
To control leafminers, foliar applications of pesticide should be applied during the adult egg-laying period which is during April and May in Western North Carolina. Adult leafminer flies emerge over a two-week period in early spring just after the boxwoods have put out their new growth. These flies are like tiny, orange mosquitoe-like (about 1/8 inch long) insects which swarm around or cling to boxwoods. After laying eggs in the leaf tissue, the flies die. There is one generation per year.
Tiny, whitish maggots hatch and feed inside the leaf. As they grow (up to 1/8 inch long), the maggots become bright yellow. Several maggots may develop in a single leaf. Their feeding induces the formation of blisters on the lower leaf surface inside of which the maggots develop for about a year.
Products approved for home use are:
imidacloprid (Merit) 75% wettable powder:
0.7 to 1.4 level teaspoons per foot of shrub height in no less than 10 gallons of water per 1000 square feet (Use as soil drench.) May also be used as spray at shoot expansion.
Spray during female oviposition period