What is leaf miner: Leafminer is an insect pest in which the larvae feed on the inside of the boxwoods leaves. These insects feed inside the leaf between the upper and lower leaf surface. Larvae will turn into adults and break through the lower leaf surface when fully matured.
What to look for: Blistered leaves, leaves with something resembling a “bubble” protruding out on the leaf surface, leaves may be discolored brown to yellow or they can remain green.
Life cycle of leaf miner: Larvae stage causes the most damage to boxwood plants. Larvae over winter in leaf blisters. Once spring arrives with warmer temperatures the larvae use this as a signal to become active again and start feeding inside the boxwood leaves. Larvae can feed inside the boxwood leaves from spring to summer. Adult leafminers can emerge out of the leaf in May. Adults look like small yellow/orange flies that can swarm around boxwood plantings. Shortly after adults emerge they mate. After mating a fertile female can deposit her eggs into the underside of boxwood leaves directly injecting the leafminer eggs into the boxwood leaves. Adult females die shortly after depositing their eggs. This starts the cycle again as the eggs grow into larvae within the leaf in about two weeks. The larvae can feed and grow to maturity inside the boxwood leaf bringing a lot of damage to the boxwood plant as it feeds and steals nutrients from the plant itself.
How to treat Leaf miner with insecticides & systemics: Use insecticide around the first week of May and a second application around mid June and mid July. Carbaryl or Malathion can control adult populations. Acephate can control larvae developing in the leaves when applied in mid May after adults emerge from leaves. You can also use the systemic insecticide imidacloprid also known as Merit to control larval populations. Apply systemic insecticide at the base of the plant near roots because this insecticide will be taken up through the roots of the boxwood and concentrated inside the boxwood to prevent further infestation.
How to treat Leaf miner naturally: Select more resistant cultivars to begin with, English and Japanese varieties are better than American varieties. Use natural insect predators such as green lacewing to control larval population. You can prune new growth of Boxwoods in May after adults deposit eggs to reduce the larval population and reduce damage on your boxwoods. Keep Boxwoods healthy, with proper watering and air circulation. A healthy plant is less likely to be susceptible to insect damage.