Q: I have a low hedge of boxwoods that are looking somewhat orange-tinted and less dense than usual. What might be causing this?
A: Boxwood Leafminer is my primary suspect, and I have recently seen boxwoods looking just like you describe, which had high populations of this common insect pest.
With only one generation per year, this species of fly is currently in its larval (juvenile) stage inside the leaves. Since larvae feed on tissues between the upper and lower leaf layers, you need to peel the leaf apart to find them to confirm their presence. Outwardly, affected leaves will become blistered-looking and have a blotchy yellow or orange-yellow color. Leafminer damage can also cause the shrubs to shed some leaves prematurely, so the branches will have more sparse growth towards the plant’s interior than a healthy shrub would have.
Boxwood Leafminer larvae are tiny orange-yellow legless maggots, only a few millimeters long. However, they are readily visible if you open a leaf and examine it with a hand lens, since the rest of the leaf interior will be light green. They will pause feeding as winter approaches and overwinter in a state of suspended development, resuming more voracious feeding in early spring. Larvae pupate and transform into orange gnat-like adults by late April or early May (depending on temperature trends).
Some boxwood cultivars have greater resistance to leafminer (as well as to fungal diseases) than others. When the insects are too abundant for their damage to be tolerable, prune out the infested branch tips or, as a last resort, consider using an insecticide to reduce their numbers. Keep in mind, though, that insecticide use can have unintended consequences: it might facilitate an increase in boxwood spider mites (by killing mite predators); harm pollinators visiting the boxwood’s flowers; or harm insect or spider predators wandering its foliage that would be eating pests.
You can learn more about leafminer as well as other common boxwood ailments on Extension’s boxwood diagnostic web page (go.umd.edu/boxwood]).
By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.
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