Boxwood leafminer: A serious pest of a favorite landscape plant
Spring time reveals damage from a boxwood pest that was active late last summer.
Boxwood leafminer can be a serious pest of a favorite landscape plant, boxwood. In some areas of Michigan, yearly feeding of this insect can seriously disfigure the shrubs, as damaged leaves are shed mid-summer. The insect is actually a fly, but its larval form, which looks like a tiny yellow worm (maggot), does the damage to the leaves as it feeds between the upper and lower layer of the leaf. The damage we see now actually began last summer, but damage is seldom noticed until spring.
As spring temperatures warm up, the larvae resume feeding, making very obvious “blisters” on the leaf, which when ripped open reveal the tiny yellow maggots. Heavily infested plants actually sound like they are crackling. When the adult flies emerge, you will see tiny pinprick-sized emergence holes on the underside of the leaf.
Boxwood leafminer may only be an occasional pest in some areas; however, west Michigan has seen this pest for quite a few years in a row. Some cultivars of boxwood are more resistant than others and it is always best to select the most resistant cultivars.
Chemical control with an insecticide spray is difficult because the application must be timed with the emergence of the adult flies. An application of an insecticide spray when the adult flies emerge (this corresponds to when weigela is in bloom) can reduce populations. Insecticide sprays containing bifenthrin (Ortho Bug-B-Gon), carbaryl (Sevin), cyfluthrin (Bayer Advanced Yard and Garden Spray) or malathion are among the recommend materials that can be applied to control the adult flies.
Imidacloprid, a systemic insecticide (Merit or Bayer Tree and Shrub Insect Control), will control the larvae inside the leaves and does not require precise timing. This product should be applied in mid- to late April. Imidacloprid has very low vertebrate toxicity; however, like other neonicotinoid insecticides, imidacloprid is implicated as a possible cause of honey bee decline.
Confirm the presence of larvae before applying a pesticide, either by dissecting a leaf or holding it up to the light. Otherwise, symptoms could be confused with disease or abiotic causes. Be sure to read and follow all instructions and safety precautions found on the label before using any pesticide.