Holly leafminer
Adult emergence ActivePhytomyza ilicis
17 host plants
Last updated
Holly leafminer larvae create pale, serpentine tunnels within holly leaf tissue, leaving mines that follow a random, winding pattern across the leaf surface. The damage first appears in summer and becomes increasingly visible as mines enlarge through fall. You see the pale, papery trails within otherwise green leaves on Ilex species. Heavy mining can cause some leaf yellowing and drop, though leaves tolerate light mining.
Remove and destroy heavily mined leaves to reduce overwintering populations. For valuable ornamentals, apply horticultural oil in late winter to target pupae. Once mining is visible, spinosad or neem oil applied to leaf undersides targets young larvae. Apply every 7 to 10 days during mining season. Maintain plant vigor and adequate moisture.
Quick Reference
Holly leafminer Adult emergence typically begins around 1625 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: Herms (OSU) phenological tables: 375 GDD₅₀ adult emergence, Secrest Arboretum OH 1997-2001 (Table 4). UMD IPMnet catalog corroborates. About GDD₃₂ →
Monitoring & Action
Inspect holly leaves in late winter (January-March) for yellow-green blotch mines. Hold mined leaves to light to confirm larva inside. In May, watch for tiny black flies hovering around new holly growth. Sticky cards near holly hedges can intercept adults.
Treatment is rarely necessary. Damage is cosmetic and does not affect plant health. Intervention justified only when mines are aesthetically unacceptable on formal hedges or specimen plants. Most landscape holly tolerates the damage without issue.
Larvae mine between the upper and lower leaf surfaces, creating yellowish-green blotch mines that become conspicuous by late winter and spring. Mines appear as raised, irregular yellowish patches on the upper leaf surface. Heavily mined leaves may have multiple blotches that merge. Leaves may drop prematurely. Damage is primarily cosmetic on established plants but can make formal holly hedges and specimen plants look unsightly. Does not threaten plant health.
Cultural Controls
- Pick and destroy infested leaves in fall or winter before adults emerge in spring. Practical on small hedges; labor-intensive on large plantings.
- Pinch infested leaves to crush larvae inside the mine.
- Prune infested branches during dormancy to reduce local population.
- Accept cosmetic damage on established plants. Holly leafminer does not threaten plant health.
Host Plants (17)
Holly leafminer is extremely common wherever English holly (Ilex aquifolium) is planted in the Puget Sound lowlands. English holly is one of the most widely planted broadleaf evergreen hedging and specimen plants in the region, and the leafminer is present on the vast majority of plantings. Mines are most noticeable in late winter when yellowish blotches stand out against dark green foliage. Most homeowners notice the damage in February-March and mistake it for disease. Adult emergence typically occurs in mid-to-late May in this region. The damage is purely cosmetic.
— Chris Welch, ISA Certified Arborist
Sources & References
Primary: PNW Insect Management Handbook
- Herms (OSU) phenological tables: GDD emergence data
- UMD IPMnet Pest Predictive Calendar
- Cornell Cooperative Extension: Holly Leafminer
- RHS: Holly Leaf Miner