It has become difficult to ignore: why is the foliage on our Birch trees (Betula papyrifera) turning brown across the landscape?
It is the work of an insect introduced from Europe to eastern Canada in the 1920s. From there it has since spread west to Saskatchewan, Alberta and southern British Columbia and on the southern shore of Great Slave Lake in the Northwest Territories.
The birch leafminer is a small black sawfly 3.7 mm long. Its life cycle involves almost any species of birch and possibly some alder. This group of sawflies has an ovipositor modified to be used as a saw to facilitate the deposition of eggs under the surface of leaves. It is active between May and mid-September, and in many warmer areas of Canada has two or three generations per year.
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 inside 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. 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.
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.
A number of natural enemies attack the birch leafminer. 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. Both wasps are native to Europe. L. nigricollis established in all three provinces and is important in suppressing outbreaks of the birch leafminer. Check in here if you are interested in biological control agents: https://naldc.nal.usda.gov/download/35126/PDF
The damage is mostly 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. [Natural Resources Canada https://www.nrcan.gc.ca/forests/fire-insects-disturbances/top-insects/13371]