Box Tree Moth (BTM) (Cydalima perspectalis) has stolen the thunder of another non-native boxwood nemesis: Boxwood Leafminer (Monarthropalpus flavus, family Cecidomyiidae). However, the European expat remains a significant boxwood pest and should not be ignored.
Of course, highlighting another boxwood pest risks stoking the flames of boxwood discontent, causing the uninformed and misguided to advise against planting these venerable pillars of landscape design, or worse, to yank out established plantings. We don’t apply the same thinking to other trees and shrubs. Think of the number of pests and diseases that affect oaks, maples, junipers, etc., etc. Yet we plant them.
Boxwoods have long played a significant role in landscape designs, with records of their use in the U.S. dating to the late 1600s. According to the 2024 USDA Census of Horticulture Specialties, Ohio ranks 7th in the U.S. with the number of boxwoods in nurseries and garden centers (570,857) and 5th in annual wholesale and retail sales ($9,894,000).
Deer will not eat boxwoods; their foliage can even make goats sick. They thrive in a wide range of sites, including the alkaline soils of western Ohio. They are broad-leafed evergreens with inconspicuous early-season blooms that support pollinators. Boxwoods are exceptionally responsive to shaping and sculpted topiary. Few, if any, woody ornamental plants could replace boxwoods in form and function.
A Meany Miner Life Cycle
Boxwood leafminer adults are flitting around their namesake host in central Ohio. The delicate adults superficially resemble miniature mosquitoes except for their bright orange abdomens. They may be seen swarming around their boxwood hosts, and large numbers commonly collect in spider webs, which is a disastrous outcome given that the adults only live for about a day.
Boxwood leafminer females use their needle-like ovipositors to insert eggs between the upper and lower leaf surfaces. Oviposition only occurs on new growth. Each leaf may contain multiple oviposition sites with several eggs per site. These sites will become individual leafmines producing blister-like leaf symptoms.
Eggs hatch in early summer, and the resulting maggots spend the remainder of the season consuming interior leaf tissue as they develop through the 1st and 2nd instar stages. There is some evidence that the maggots pause in their feeding and development during the summer; a physiological condition known as aestivation (= estivation). They resume feeding in late summer to early fall.
Winter is usually spent as 3rd instar midge maggots inside the leafmines. The maggots resume feeding in the spring and develop through a 4th instar stage. Much of the leaf damage occurs in early spring, with the ravenous maggots rapidly delaminating the upper and lower leaf surfaces as they expand their leafmines.
The maggots use their hook-like mouthparts to scour the tissue near the lower leaf surface just before they pupate, leaving a thin layer of epidermal cells to create a windowpane-like effect. This odd feature will become visible on boxwood leaves in southwest Ohio when most of the leafminer maggots have pupated. The pupae change colors as they mature from the light-yellow color of the maggots to orangish-yellow and finally reddish-orange.
Eventually, the pupae wriggle through the “windowpanes,” giving the delicate adults unencumbered access to the outside world. Adult emergence is heralded by pupal skins (exuvia) hanging out of the leaves. This occurs around the time red horsechestnuts (Aesculus × carnea) and doublefile viburnums (Viburnum plicatum) are in full bloom.
The Chronology of Major Miner Damage
The aforementioned leaf delamination, produced by the feeding activity of the miner maggots, progresses slowly in the summer. New leaf damage is subtle throughout the growing season, appearing as slightly raised blisters. The faint damage is easily overlooked.
However, once the maggots resume feeding in the spring, the delamination accelerates, causing leaves to turn brown. The damage is very apparent in early spring and is commonly mistaken for winter injury, but it rapidly becomes masked by new growth. The similarity to winter injury, coupled with the new growth cover-up, is one reason boxwood leafminer may fly below our radar.
On a side note, in 2021, I began noticing peculiar damage to overwintered leafmines, with the thin tissue covering the mines showing obvious peck marks or being entirely torn away. A search of the literature revealed that predatory birds, primarily titmice (family Paridae), will readily dine on the fat, mature leafminer maggots. As shown in the images below, the bird damage can rival or surpass damage produced by even heavy leafminer infestations.
I’ve only observed heavy bird predation on boxwoods in Spring Grove Cemetery and Arboretum. I have not observed the damage elsewhere in Greater Cincinnati, nor have I received reports of birds damaging infested boxwood leaves elsewhere in Ohio. The published observations I found in the literature were made in Maryland. However, I have found no research on the overall impact of bird predation on boxwood leafminer populations.
Miner Management
There is a wide range of susceptibility among boxwoods to the depredations of the boxwood leafminer. Evaluations continue, but a listing of less susceptible boxwoods may be found in the “Selected References” below. Choosing resistant or less susceptible boxwoods means less attention and resources must be dedicated to managing boxwood leafminer.
Clearly, plant selection provides the best long-term solution. Research is also progressing on discovering boxwoods that are resistant to box tree moth. Stay tuned.
Of course, insecticide applications remain an option to manage boxwood leafminer on older, more susceptible boxwoods. This is particularly important if the boxwoods cannot be easily replaced owing to their size and importance in landscape designs.
Ideally, where boxwoods are being attacked by both boxwood leafminer and box tree moth, a two-fer could be realized if insecticides targeting one suppress the other. Unfortunately, it’s sometimes apples to oranges.
Some insecticides, such as chlorantraniliprole (e.g., Acelepryn) are very effective against box tree moth but not effective against the leafminer. The opposite is true for nicotinoid insecticides such as imidacloprid (e.g., Merit, Xytect, etc.).
Boxtree moth caterpillars are currently feeding on boxwoods, and adult leafminers are on the wing. Topical insecticides such as pyrethroids (e.g., bifenthrin) can kill both as long as the leafminers have not yet inserted eggs into the leaves. Acephate (e.g., Orthene) has systemic activity and will also kill both pests. Abamectin (e.g., Avid) applied during leafminer adult emergence provides another option. Abamectin is not systemic like neonicotinoids; however, it does have translaminar activity, and the leaf penetration presumably kills the first instar maggots. This natural fermentation product also has good activity against spider mites.
Selected Resources
Askew, S., Chamberlin, L., Day, E., Del-Pozo, A., Dellinger, T., Derr, J.F., DeWitt, K., Fearer, C., Frank, D., Hong, C., Langston, D., McCall, D.S., Nita, M., Parson, R., Peer, K., Pfeiffer, D.G., Rice, K.B., Richardson, R.J., Salom, S.M., Wilson, J., Yang, S., and Zeng, Y. (2026). Horticulture & Forest Crops, 2026 Pest Management Guide. Virginia Cooperative Extension, Virginia Tech, Virginia State University, Pub. 456-017.
www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/456/456-017/ENTO-635.pdf
Census of Agriculture (2024) Census of Horticulture Specialties (USDA Publication No. AC-22-SS-3, Vol 3, Special Studies, Part 3). U.S. Department of Agriculture, National Agriculture Agricultural Statistics Service.
d'Eustachio, G. J. (1999). Integrated management of the boxwood leafminer (Doctoral dissertation). https://drum.lib.umd.edu/bitstream/handle/1903/24726/d'Eustachio,%20G.J..pdf?sequence=1
d’Eustachio, G., & Raupp, M. J. (2001). Application of systemic insecticides in relation to boxwood leafminer’s life history. Arboriculture & Urban Forestry (AUF), 27(5), 255-262.
https://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2937&Type=2
Saunders Brothers Boxwood Guide, 7th Edition. (2024) Saunders Brothers Inc.
www.saundersbrothers.com/_ccLib/attachments/about/Boxwood+Guide+-+7th+Edition.pdf
Thurn, M., Lamb, E., & Eshenaur, B. (2019). Disease and Insect Resistant Ornamental Plants, Buxus – Boxwood, NYS Integrated Pest Management Program, Cornell Cooperative Extension.
