French common name: Tenthrède mineuse de Thomson
Scientific name: Profenusa thomsoni
The ambermarked birch leafminer is found in every province and territory except Prince Edward Island and Nunavut. (While it is present on the island of Newfoundland, it has not been recorded in mainland Labrador.) Most large infestations tend to be located in or near towns and cities, although outbreaks have been recorded in forests.
Hosts in Canada
Principal hosts in Canada
Paper birch, yellow birch, weeping birch
All native species and all introduced species of birch are likely susceptible
Almost all species of birch can be attacked by the ambermarked birch leafminer. In cities, populations are often found where paper birch (B. papyrifera) and weeping birch (B. pendula) are planted as ornamentals.
The ambermarked birch leafminer has a one-year life cycle in North America. Adults begin to emerge in late June or early July in most of Canada and can be found well into August in some locations. The female lays her egg into the upper surface of a birch leaf. Hatched larvae feed inside the leaf by mining the tissue between the upper and lower leaf surface. The mature larvae exit the leaf and drop to the ground, where they construct a pupal cell (cocoon) in the soil. They spend the winter as pre-pupae, pupating in the spring just before emerging as adults.
In northern Ontario, eggs can take up to 12 days to hatch, and development in the leaf takes about 24 days; in western Canada and colder locations, egg hatch and larval development can take longer. In all sites, development can be faster when the larvae are exposed to warm conditions—for example, when they inhabit a leaf on the south side of a tree.
The distribution of the ambermarked birch leafminer seems only to be restricted to the distribution of birch. Populations appear to experience occasional outbreaks that can range in size from a few trees, to a few hundred hectares, to an entire city. Anecdotal observations suggest that the size of populations is linked to weather, with hot, dry conditions being conducive to the growth of populations. The ambermarked birch leafminer does not seem to be affected by cold temperatures, likely because of its habit of overwintering underground.
Female ambermarked birch leafminers do not need to mate to lay eggs, and males are never found. It is assumed that populations are entirely female. This reproductive behaviour means that populations can build up quickly when conditions are ideal. It is also common to find multiple larvae within the same leaf, often of different sizes or developmental stages. This happens because in many locations the adult emergence period can extend over a few weeks, and multiple females will sometimes deposit their eggs in the same leaf.
When populations are large, competition among leafminers can cause mortality. Eggs can be killed when they are undermined by older larvae, and smaller larvae can die of starvation if all the leaf tissue is consumed before they complete their development. Predators and parasites also kill larvae. Birds will remove larvae from the leaves and parasitic wasps attack the eggs and larvae. One species of wasp, Lathrolestes thomsoni, has been implicated in the control of populations in Alberta.
Attack and damage
Damage appears as a small 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.
The number of mines on a leaf can vary—from one or two when populations are small to up to 10 or more mines when populations are large. When densities are high, the mines will coalesce and many larvae can be observed feeding inside a single large mine. The damage to the leaf will cause it to dry out and turn brown. Mined leaves will stay on the tree for some time, often not dropping until early fall. This can result in trees that look burnt for the latter part of the season, when other trees are still green.
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 ambermarked birch leafminer is an aesthetic pest. Homeowners and others find the burnt appearance caused by the larvae undesirable 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 is defoliated for a number of years it may be weakened and more susceptible to attack by other insects.
Status in Canada
The ambermarked birch leafminer is not native to Canada. It was first recorded in Ontario in the 1940s, and since then it has expanded to almost all of Canada. It is one of five species of birch leafmining sawflies that were introduced to Canada between the 1920s and the 1960s, and has the widest geographic distribution. Most outbreaks of birch leafmining sawflies that have occurred in western Canada in the last 30 years have been of this species, with recent outbreaks occurring in parts of Alberta, northern British Columbia and the Northwest Territories. It is commonly found in association with the birch leafminer and the late birch leaf edgeminer, which can exacerbate the damage seen on trees. Populations in the Northwest Territories, British Columbia and Alaska have been the target of biological control projects that have introduced the parasitic wasp Lathrolestes thomsoni, with some success.
The ambermarked birch leafminer has no known impacts on native forest ecosystems. The damage is aesthetic and the species has never been implicated in the death of trees. In the past, homeowners and municipalities spent significant resources on insecticides to control leafminer outbreaks.
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.
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