French common name: Mineuse (petite) du bouleau
Scientific name: Fenusa pumila (previously Fenusa pusilla)
The birch leafminer is found on the island of Newfoundland, in the Maritime provinces and Quebec, and in Ontario as far north as Sudbury and Sault Ste. Marie. In the west it is found in Saskatchewan, Alberta and southern British Columbia and on the southern shore of Great Slave Lake in the Northwest Territories.
Almost all species of birch can be attacked by the birch leafminer. Yellow birch is attacked but larvae may fail to develop, suggesting some resistance in the tree to the leafminer. Green alder has been recorded as a host in Europe.
Hosts in Canada
Principal hosts in Canada
Paper birch, gray birch, weeping birch
All native species and all introduced species of birch are likely susceptible
Occasional or potential hosts
The birch leafminer is active between May and mid-September, and in most of Canada has two or three generations per year. Some locations may experience as few as one generation per year or as many as four.
The first generation of birch leafminer adults emerge in the spring at the same time as new leaves are being formed on birch trees. The female will deposit her eggs on these new leaves as they are expanding. The eggs hatch in 4 to 14 days. The newly hatched larvae feed within the leaf for 8 to 12 days before emerging from the leaf as mature larvae. These larvae drop to the ground and create a cocoon in the soil, emerging 1 to 2 weeks later as a mature adult. A complete generation can take 5 to 6 weeks, or slightly longer at higher latitudes. The final generation overwinters in soil cocoons as mature larvae, with pupation occurring just before emergence in the spring.
The distribution of the birch leafminer is similar to that of birch in North America. When outbreaks occur, they tend to be located in urban areas, but there have been incidences of significant defoliation in forests. The birch leafminer prefers leaves on newly formed shoots. Trees with an abundance of new shoots—for example, small trees, trees that are browsed by animals, or trees that are frequently pruned—may suffer more damage from the leafminer. Mature trees experience less damage because new shoots tend to form only at the ends of branches.
Climate influences the number of generations the insect has during a year, with warmer regions of the country tending to have more generations per year. The birch leafminer does not seem to be affected by cold temperatures, likely because of its habit of overwintering underground.
Birch leafminer is often found in association with other species of leafmining sawflies that attack birch (e.g., the ambermarked birch leafminer and the late birch leaf edgeminer). Its habit of attacking leaves early in the spring and its preference for newly formed shoots may be an adaptation to avoid competing with these other species.
A number of natural enemies attack the birch leafminer. Eggs are killed by predators and parasites, and occasionally by other birch leafminers. This occurs when eggs are deposited in leaves that already contain larvae. Larvae can be killed within the leaf, and are also vulnerable to predators when they exit the leaf to pupate. A number of native parasitic wasps are also known to attack the birch leafminer. However, none of these factors are known to limit the size of populations. Effective control of the birch leafminer is achieved only by two introduced parasitic wasps, Lathrolestes nigricollis and Grypocentrus albipes.
Attack and damage
Damage appears as a small brown or reddish-brown, irregular-shaped patch (a “mine”) on the upper surface of the leaf. If the mine is occupied, the larvae can be seen when the leaf is held up to a light.
Leaves that are attacked by birch leafminer larvae will often appear curled at the edges. The leafminer attacks before the leaf has fully expanded, which interferes with normal development and results in a deformed leaf. There may also be a darker brown or reddish-brown patch near the centre of the mine, close to where the egg was laid.
On immature trees, or trees with many new shoots, if many leaves have been attacked the damage can be easy to find. On older or more mature trees, damage may be less apparent or restricted to leaves on the outside of the crown. Mined leaves will stay on the tree for some time, often not dropping until early fall.
There are many insects that create mines in birch leaves and often these species can be found on the same tree at the same time. This overlap in species can make it difficult for the untrained eye to determine which insect is responsible for damaging a tree. Careful inspection of the shape of the mines can help to determine which species are present.
The birch leafminer is an aesthetic pest. Homeowners and others do not like the brown and curled appearance of leaves damaged by the leafminer in urban settings and on ornamental trees. Small populations can be tolerated and often go unnoticed. The damage does not kill the tree because it occurs later on in the season. However, if the same tree suffers heavy defoliation for a number of years it may be weakened and more susceptible to attack by other insects.
Status in Canada
The birch leafminer is not native to Canada. It was first recorded in Ontario and Quebec in the 1920s and had spread to Alberta by the 1960s. It is one of five species of birch leafmining sawflies that were introduced to Canada between the 1920s and the 1960s. The birch leafminer was a significant pest of birch in Quebec and on the island of Newfoundland in the 1960s and 1970s, and in Alberta in the 1970s and 1980s. It was controlled by the introduction of two stingless parasitic wasps. Today, while the insect is still found in many parts of the country, it is much less abundant.
The Canadian Forest Service introduced two parasitic wasps, Lathrolestes nigricollis and Grypocentrus albipes, to parts of Quebec and Newfoundland in the 1960s and 1970s and parts of Alberta in the 1990s, to control the birch leafminer. Both wasps are native to Europe, where they are common parasites of the birch leafminer. L. nigricollis established in all three provinces and was important in suppressing outbreaks of the birch leafminer. Today, partly because of the introduction of these parasitoids, outbreaks of the birch leafminer are rare.
The birch leafminer has no known impacts on native forest ecosystems. The damage is aesthetic and the species has not been implicated in the death of trees. In the past, homeowners and municipalities spent significant resources on insecticides to control leafminer outbreaks. Today, most populations are controlled by parasitic wasps.
Digweed, S.C., C.J.K. MacQuarrie, D.W. Langor, D.J.M. Williams, J.R. Spence, K.L. Nystrom, and L. Morneau. 2009. Current status of exotic birch-leafmining sawflies (Hymenoptera: Tenthredinidae) in Canada, with keys to species. Canadian Entomologist 141: 201–235.
MacQuarrie, C.J.K., D.W. Langor, S.C. Digweed, and J.R. Spence. Fenusa pumila Leach, birch leaf miner, Profenusa thomsoni (Konow), Ambermarked birch leaf miner (Hymenoptera: Tenthredinidae), in P.G. Mason and D. Gillespie (eds.), Biological Control Programmes in Canada 2001–2012. CABI Publishing. In press.