I have leafminers in my boxwoods. I opened a leaf the other day and there was the ugly, orange maggot inside. I live in central Virginia, zone 7, and wonder when to expect the miners will emerge and when to spray. I found this info online, but it was written in 1976. I wondered if it's still accurate in terms of stuff to kill the miners and when to put it on. Read on:
"The boxwood leaf miner is the larva of a small gnatlike fly. In spring, the flies inject their eggs into the young boxwood leaves. Larvae from the eggs develop slowly during the summer, hollowing out areas inside the leaves as they feed. They winter inside the leaves. The larval, or feeding, stage of the life cycle is completed late in April or early in May. The pupal stage follows; it lasts about 10 days. During this stage, the larvae turn to pupae. The pupae break through the surfaces of the leaves and work themselves partway out. The adult flies then emerge from the pupae. Adults of this insect are easily controlled with properly-timed applications of carbaryl. To determine the right time to apply carbaryl, watch the development of the pupae. Every 2 or 3 days during the pupal period, break open a leaf and examine the pupae. A pupa's head and wing pads turn dark brown near the end of the pupal period-just before the adult fly emerges from the leaf. This is the time to apply carbaryl. To control the leaf miner when in the young larvae stage, spray with carbaryl about June 15 just after they hatch. Spraying later in the summer or autumn also controls the larvae, but their mines will remain as yellow spots in the leaves. Dimethoate sprays will control mature larvae if applied in early spring as plants resume growth."