Remember all those yellow spots on boxwood leaves we had this spring? These were the feeding sites of the overwintering larvae of boxwood leafminer (Monarthropalpus buxi, M. flavus).
Damage to boxwoods by this leafminer can be devastating. The next generation of leafmining larvae are feeding inside 2018 foliage, as I write this actually. Those little midges! Management with insecticides can be especially challenging in the landscape.
Cedar leafminer (Argyresthia thuiella and other species) is a tiny moth whose larval stage feeds on the inside of leaf scales, causing them to turn yellow, then brown and dead. New generation larvae are feeding inside foliage now and populations are expected to build in some areas of southern Ontario.
If you haven’t already pruned them, boxwood and eastern white cedar foliage can be lightly sheared to reduce populations of larvae that would normally persist into 2019. There is no need to collect clippings because the clippings will desiccate and will not be able to support larval development.
Annual summer sheering of these evergreen hosts can, over time, reduce infestations of leafmining pests, and also create a more full look by supporting increased lateral bud growth.