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How to identify it
- Mines begin in the center of the leaf, usually near a major side vein, and expand outward. The leaf's upper and lower layers separate so the mine looks like a flattened or collapsed blister filled with dark frass.[1]
- Damage first shows in July as small, light blotches on the upper leaf surface. By late July and August mined leaves turn a sickly yellowish brown rather than the clear gold of natural fall color.[2][1]
- Opened before the larvae leave, a mine holds flattened yellowish larvae about 6 to 7 mm long; several larvae can share one mine, and at heavily infested sites as many as 40 larvae have been counted on one leaf.[1][2]
- Adults are all female and emerge in late June and early July. The eggs are inserted under the upper epidermis in the middle of the leaf and hatch within about 12 days.[1][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Birch leafminer (Fenusa pumila) | The earlier-known birch miner that the amber-marked species was confused with until 1955. In Alaska Fenusa pumila is present but not currently a problem, and amber-marked birch leafminer damage peaks late in the season, in late July and August.[2][1] |
| Late birch leaf edgeminer (Heterarthrus nemoratus) | Often in the same tree or even the same leaf. Its mines start at the leaf edge as a reddish ring expanding inward and contain little frass, because the larvae cut slits to eject it; amber-marked mines start in the leaf center and are full of dark frass. The edgeminer also pupates in fall inside the fallen leaf rather than in the soil.[1] |
| Birch casebearer | Amber-marked birch leafminer larvae never feed openly on the leaf surface: they live between the leaf layers and leave the leaf only to drop to the soil and spin a cocoon.[1][2] |
Where it lives
Long known in Europe and probably introduced, it was first reported in Canada in Quebec in 1959 and now occurs throughout Canada. In Alaska it was first reported in Haines in 1991, identified in Anchorage in 1996, detected in Fairbanks in 2002 and has since spread through Southcentral and Interior Alaska, reaching Delta Junction by 2018. Lathrolestes parasitoids have been linked to its decline in Alberta and the eastern United States.[2][1]
Biology and life cycle
Hosts are birches, including grey, white (paper) and yellow birch in Canada and Alaska paper, Kenai, resin, dwarf arctic and western paper birch plus horticultural varieties in Alaska.[2][1]
There is one generation a year. Adults emerge from soil cocoons in late June and early July. All are females that lay viable eggs without mating. Eggs are laid in slits cut into the midsection of the leaf and hatch within 12 days.[1][2]
Larvae mine between the leaf's epidermal layers through six instars over about 24 days, feeding during the first five. Mature larvae leave the leaf, drop to the ground and spin cocoons in the soil or litter, where they overwinter before pupating the next summer.[1][2]
Females prefer to lay on leaves that already carry eggs, so in heavy years leaves become overcrowded; as the count nears ten eggs per leaf, almost all larvae starve. This crowding, which quickly exhausts the food in a mined leaf, helps explain why infestations collapse and why heavily hit patches in Anchorage were followed by light infestation the next year.[1][2][3]
It prefers shaded or semi-shaded foliage on small trees; open-growing ornamental birches are generally less susceptible. In Alaska both it and the edgeminer attack leaves throughout the canopy.[2][1]
Damage and signs
- Damage is mainly aesthetic. In outbreaks more than 90 percent of a tree's leaves can be mined, but most of the feeding happens late in summer after the tree has done most of its growth, and Alaska surveys have recorded no birch deaths from these insects.[1]
- Heavily mined trees are weakened and may become more susceptible to other insects, but generally do not die from the leafminer.[2]
- In Anchorage, infestation has been patchy, occurring in neighborhood-sized patches that shift from year to year. Aerial surveys in 2025 mapped roughly 123,000 acres of birch leafminer damage in Alaska, with the most active defoliation in and around Fairbanks.[3][1]
Health risk
These are stingless sawflies. The sources describe only leaf damage and mention no risk to people or pets.[1]
Control
Prevention
Because the damage comes late in the season and even a severe infestation seldom lasts more than a year, control is generally unnecessary.[2]
Open-grown ornamental birches in full sun are generally less susceptible than small trees in shade or partial shade.[2]
Biological control
The parasitic wasp Lathrolestes thomsoni, linked to the collapse of leafminer populations in Alberta and the eastern United States, was released in Alaska from 2004 to 2008 (more than 3,000 wasps, mostly around Anchorage). It is now established and spreading from release sites. It lays its egg in the sawfly larva, which still spins its cocoon; the wasp, not the sawfly, emerges the next spring.[1]
Natural crowding also ends outbreaks: females lay on already-occupied leaves, and larval mortality approaches 100 percent as eggs per leaf approach ten.[1][2]
Scientific name
The birch leafminer is named here as Fenusa pumila, the name used by the U.S. Forest Service Alaska Region and GBIF and by a 2014 checklist of British and Irish sawflies that treats the older use of Fenusa pusilla as a misidentification. The ITIS database still calls the birch leafminer Fenusa pusilla, while iNaturalist now uses that name for an elm leafminer.[1][5][4][6][8]
Common questions
- My birch is turning brown in early August. Is it fall already?
- Probably not. Look closely: leafminer-browned leaves are a dull yellowish brown, the two leaf layers are separated into a flattened blister, and the inside is full of dark frass, sometimes with flat yellowish larvae still present.[1]
- Will it kill my birch?
- Alaska forest health staff have recorded no birch mortality from these leafminers, and Natural Resources Canada says attacked trees generally do not die, though heavy mining can leave them more open to other insects.[1][2]
- How do I tell it from the late birch leaf edgeminer?
- Edgeminer mines are a reddish ring starting at the leaf margin and hold little frass; amber-marked mines start in the leaf center near a vein and are packed with dark frass.[1]
Sources
- USDA Forest Service, Alaska Region, “Birch Leafminers (Forest Health, Alaska Region).” www.fs.usda.gov/r10/natural-resources/forest-health/birch-leafminers (accessed 2026-11-11)
- Natural Resources Canada, Canadian Forest Service, “Ambermarked birch leafminer, Trees, insects, mites, and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/7926 (accessed 2026-11-10)
- Entomological Society of America (Journal of Economic Entomology), “Spatial dynamics of the invasive defoliator amber-marked birch leafminer across the Anchorage landscape (Lundquist, Reich and Tuffly 2012, Journal of Economic Entomology 105:1659-1667).” doi.org/10.1603/ec11406 (accessed 2026-11-10)
- Biodiversity Data Journal, “Checklist of British and Irish Hymenoptera - Sawflies, 'Symphyta' (Liston, Knight, Sheppard, Broad and Livermore 2014).” doi.org/10.3897/BDJ.2.e1168 (accessed 2026-11-11)
- Global Biodiversity Information Facility (GBIF), “Fenusa pumila Leach, 1817 (GBIF Backbone Taxonomy).” api.gbif.org/v1/species/4490761 (accessed 2026-11-11)
- Integrated Taxonomic Information System (ITIS), “Fenusa pusilla (Lepeletier), TSN 152814.” www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=152814 (accessed 2026-11-11)
- iNaturalist, “Profenusa thomsoni taxon page (order and family).” www.inaturalist.org/taxa/552336 (accessed 2026-11-10)
- iNaturalist, “Fenusa pusilla (elm leafminer) taxon page.” www.inaturalist.org/taxa/1028796 (accessed 2026-11-11)