Boxwood Leafminer are small flies that attack several varieties of boxwood, causing the usually dense foliage to appear weak and lackluster. These pests can kill boxwood twigs and cause mature plants to become susceptible to other diseases, including winter-kill in colder climates. Larvae are yellow maggots approximately 1/8-inch long that consume the green, food-producing tissue within the leaf. Adult gnats are about 1/10-inch long, yellow to orange-red in color, and produce one generation each year.
Named for the evergreen they infest, Boxwood Leafminer feeds on the upper and lower sides of boxwood leaves. Some varieties of English boxwood are infestation-resistant, including: Argenteovarigata, Pendula, and Suffruitiosa.
Look for new leaves to become spotted-yellow by late spring and blistered by mid-summer. Leaves also may drop prematurely. It can take several weeks for Boxwood Leafminer infestation to be noticeable. Signs include egg punctures on the underside of leaves and larvae living inside leaf blisters.
Apply liquid fertilizer to combat maggots and egg-laying gnats shortly after new growth in early June. Proper fertilization can help promote new growth, which improves overall vigor and appearance.