Its larvae have pupated inside the leaves all winter. In May, they emerge as gnat-like adults, ready to fly, mate and lay their eggs in the new leaves. The eggs will hatch in a few weeks and new larvae will feed throughout the summer.
All varieties of boxwood are susceptible, but the slower-growing English varieties are less susceptible than the American cultivars. The leafminer feeds between the upper and lower sides of the leaf. This feeding causes blisters on the underside of the leaf. Leaves infested by this pest become yellow and smaller than a normal leaf. In fact, plants that are severely infested appear completely unhealthy.
Photo: Brian Kunkel, University of Delaware, Bugwood.org