Birch leafminer: what the agencies record

This page reports what the official source says. It is not legal advice: ask the office named here. It does not identify what is wrong with any tree and does not say what should be done about one. The birch leafminer, Fenusa pusilla, is a European sawfly whose larvae feed inside birch leaves, and none of the three pages read lists it as regulated or notifiable. Each section names its source, all read on October 9, 2026. The parent page, insect pests of trees, compares it with the other pests on the list, and a tree it seldom feeds on has its own profile on the page for yellow birch.

Names and origin

Natural Resources Canada, called NRCan, gives the scientific name Fenusa pusilla (Lepeletier), the French name petite mineuse du bouleau, and places it in the sawfly family Tenthredinidae. Its fact sheet covers the three leaf-mining sawflies that attack birch, all accidentally brought from Europe early in the 20th century, and says they are widely distributed in Canada and the northern United States. It names the other two as Profenusa thomsoni, which does most of the damage to ornamental birches in the Prairie Provinces, and Heterarthrus nemoratus. Penn State Extension (updated June 24, 2026) says Fenusa pusilla comes from Europe, with its first United States record in Connecticut in 1923. University of New Hampshire Extension says it reached the United States before 1923 and became one of the most important nursery insect pests in New Hampshire.

Hosts

NRCan says all native and exotic birches are susceptible to at least one of the three species. Penn State calls the birch leafminer a key pest of gray birch and paper birch in Pennsylvania and says it seldom feeds on sweet birch, yellow birch or European white birch. It lists whitebarked Himalayan birch, Dahurian birch and river birch as reported to be less susceptible. New Hampshire Extension says the insects prefer sunny places and that birches tolerate attack best in areas that are shady, cool and moist.

What the sources say to look for

Stage or sign	What the sources record
Adult	A small black, wasp-like sawfly: about 3 mm long (Penn State), 3 to 4 mm with a wingspan of 6 to 7 mm (NRCan), about 1/8 inch (New Hampshire)
Larva	Whitish and slightly flattened, 6 mm long when mature (Penn State), 6 to 7 mm (NRCan); New Hampshire adds black spots under the thorax
First signs on the leaf	Small light green or gray spots where eggs were laid, noticeable from early June, turning into brown blotches (NRCan)
Later signs	Mines that overlap into large blotched or blistered areas, with the foliage at the top of the tree turning brown first (Penn State); larvae and black frass visible when a leaf is held up to the light (Penn State, NRCan)

Life cycle as the sources state it

Penn State gives this order. The pest overwinters as mature larvae in the soil under the tree and pupates in early spring. Adults emerge, mate and lay eggs in early May in southern Pennsylvania and mid to late May in the north, inserting the eggs one at a time into the soft, expanding leaves. The eggs hatch after seven to ten days, the larvae feed for two to three weeks between the upper and lower leaf surfaces, then cut a hole through the leaf and drop to the ground to build a pupal cell. First-generation adults come out two or three weeks later, and second-generation adults emerge in mid to late June in the south and about a week later in the north. New Hampshire Extension gives a larva buried 1 to 2 inches deep in a cocoon, pupating in May, an adult emerging in 8 to 15 days, a female laying several hundred eggs that hatch in 6 to 10 days, 1 to 20 eggs per leaf, and two generations a year in New Hampshire. It calls the first generation the most damaging. NRCan says two of the three species emerge from the leaves to overwinter and pupate in the soil, while Heterarthrus nemoratus pupates in fallen leaves.

Damage as the sources describe it

NRCan says a healthy tree can come through many years of light to moderate attack and that it knows of no deaths from this cause alone, but that the strain of the mining plus dry conditions and other insects may lead to branch and top dieback, and that the unsightly look is what people mainly dislike. Penn State says a healthy tree can have part or all of its foliage affected without serious injury, but that repeated defoliation may seriously weaken or kill a tree and may let secondary pests such as the bronze birch borer in. New Hampshire Extension says the feeding reduces the tree's ability to photosynthesise and leaves it more open to other insects and diseases, that severe damage, sometimes fatal, was common in the 1960s through 1980s, and that imported parasites keep the pest under control.

Regulated status and reporting

None of the three pages names a regulation, quarantine order or report line for the birch leafminer. For the agencies that take reports of regulated pests, see the guides on reporting a pest in Canada and the United States.

Management as the sources list it

NRCan lists watering, feeding and keeping birches healthy, and raking up fallen leaves for the species that pupates in them. Penn State lists planting the less susceptible birches, natural enemies that may have cut outbreaks, and registered insecticides timed to adult activity. New Hampshire Extension names two parasitic wasps present in the Northeast and says chemical controls are unnecessary for this pest in that state. This page gives no product names, rates or timings. For who may apply pesticides, see pesticide licences.