Boxwood leafminer
Monarthropalpus flavus
5 host plants
Last updated
Inspect boxwood leaves for blistered puffy appearance on undersides with orange or reddish tint; upper surface shows pale yellow or light green speckles where tiny larvae tunnel between layers. Heavily mined leaves turn brown and drop prematurely by late summer. Look for this damage on common boxwood throughout growing season.
Prune out and destroy heavily mined leaf clusters to remove developing larvae, particularly effective in spring when blisters first appear. Apply horticultural oil in late March to smother overwintering pupae before adults emerge. If mines appear later, remove and destroy infested leaves rather than spraying; boxwoods tolerate defoliation well. Repeat infestations may warrant resistant species.
Quick Reference
Boxwood leafminer Adult emergence typically begins around 1221 GDD₃₂. As of June 3, 2026, all seven Puget Sound stations have passed this threshold (2435.5–2672.8 GDD₃₂), so Adult emergence is likely underway across the lowlands.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Jun 3, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 2,673 | Adult emergence | — | — |
| Kent / Auburn | 2,665 | Adult emergence | — | — |
| Seattle / UW | 2,610 | Adult emergence | — | — |
| Olympia / Tumwater | 2,570 | Adult emergence | — | — |
| Tacoma / Puyallup | 2,535 | Adult emergence | — | — |
| Bellingham / Whatcom | 2,533 | Adult emergence | — | — |
| Sequim / Rain Shadow | 2,436 | Adult emergence | — | — |
Source: UMD IPMnet catalog. Sources: UMD: extension.umd.edu. Wisconsin Extension: Wisconsin Extension (landscape pest table). About GDD₃₂ →
Monitoring & Action
In April-May, squeeze boxwood leaves gently - blistered, spongy-feeling leaves contain overwintered larvae. Hold infested leaves to light to see larvae inside. In May, watch for swarms of tiny orange midges around boxwood hedges at approximately crabapple first bloom. This swarming is the most reliable confirmation of the pest and the key timing indicator for any spray applications.
Treatment is rarely necessary on casual boxwood plantings. Intervention justified when: (1) formal hedges where blistered, disfigured leaves are aesthetically unacceptable, (2) chronic heavy infestations causing branch dieback, or (3) nursery production. For most landscape boxwood, the damage is cosmetic and does not threaten plant survival. Consider replanting with resistant cultivars rather than repeated treatment.
Larvae mine between the upper and lower leaf surfaces, causing characteristic blistering and swelling of infested leaves. Mines appear as raised, puffy areas on the lower leaf surface. Infested leaves turn yellow-green, then brown, and may drop prematurely. In spring, heavily mined leaves from the previous year's generation become conspicuously blistered and disfigured as overwintered larvae resume feeding and pupate. Damage is primarily cosmetic on established plants but can be severe enough to disfigure formal hedges. Chronic heavy infestations reduce plant vigor and can cause branch dieback.
Cultural Controls
- Plant resistant cultivars: Buxus sempervirens 'Suffruticosa' (dwarf English boxwood), 'Pendula', and 'Argenteo-variegata' are reported to be seldom damaged. B. microphylla 'Grace Hendrick Phillips' also shows resistance. This is the most effective long-term strategy.
- Pinch infested leaves in late spring to crush larvae inside the mine. Practical on small hedges; labor-intensive on large plantings.
- Pick and destroy heavily infested leaves. Reduces local population but does not eliminate the pest.
- Prune boxwood after adult emergence (late May-June) rather than before, to avoid stimulating new growth during peak egg-laying.
Host Plants (5)
Boxwood leafminer is common wherever boxwood is planted in the Puget Sound lowlands. The mild maritime climate allows larvae to overwinter successfully inside leaf tissue. Adult emergence typically occurs in mid-to-late May, coinciding with crabapple bloom. The pest is often first noticed in spring when overwintered mines become conspicuously blistered. Most residential boxwood plantings tolerate the damage without treatment. For formal hedges where appearance matters, resistant cultivars are a better investment than repeated spray programs.
— Chris Welch, ISA Certified Arborist
Sources & References
Primary: PNW Insect Management Handbook
- WSU HortSense Boxwood Leafminer
- UMD IPMnet Pest Predictive Calendar
- Wisconsin Extension GDD Landscape Insect Pests
- Cornell Cooperative Extension: Boxwood Leafminer