Keeping birch from the lurch: Combating the leafminer
I recently received an email from Scott Digweed at the Northern Forest Centre, Natural Resources Canada, about the birch leafminer. I wanted to share it with my readers:
I read with interest your article on birch leafminers in last week’s Edmonton Journal, and just wanted to pass along some updated info that I thought you might find helpful. Unfortunately, the city and NRCan info sources are out of date relative to work I and others have been involved in during the early 2000s with these insects.
Our work on birch leafminers is summarized in a recent paper. Some key points from that paper:
There are five species of birch leafminer in Canada. Four of these now occur in Alberta (possibly five by now). Life histories of all are summarized in the paper.
The main species attacking birches in Edmonton has, for at least as long as I’ve worked on this system (since the early 1990s), been Profenusa thomsoni. This species has one generation per year, causing damage to leaves throughout the tree crown later in the summer (July). It is attacked by a parasitoid, Lathrolestes thomsoni (formerly L. luteolator) that was not introduced as a biological control agent, but was simply found attacking P. thomsoni here in the 1990s. The parasitoid does a good job of regulating populations on most trees most of the time, as far as we can tell. Population levels of P. thomsoni sometimes increase locally, but, within a few years, parasitoid numbers also increase, and the leafminers seem to come back under control.
There are two other species that have been in Edmonton for decades. Fenusa pumila attacks new leaves only, generally only at branch tips in Edmonton. It has multiple generations per year (this is the life cycle you describe in your article), but is rarely problematic here, and we have introduced highly specific parasitoids (one of which is a different species of Lathrolestes) against it from Europe. The third species that has been here for a long while, Heterarthrus nemoratus, is always rare here, and has not been the subject of a biological control program here.
During our recent work, we detected Scolioneura vicina in Alberta. This species can be locally abundant, and we found no specialist parasitoids attacking it. This could end up being the next problem species on our birch here.
We also detected the fifth birch leafminer species, Fenusella nana, in interior British Columbia. Given that much nursery stock moves into Alberta from B.C., I believe it is only a matter of time before F. nana is here as well. This species is the most prevalent species now in eastern Canada, and could potentially become pestiferous here once established.
I would suggest that eradication of birch leafminers is not possible here, nor is it a reasonable expectation in any biological control context. When new species are introduced, populations are brought under control either by native enemies attacking the new hosts, or by introduced enemies brought over from the host’s home environment to effect control in the new one. Once host-parasitoid relationships are established in the new environment, populations of the host and parasitoid will fluctuate relative to each other, but very rarely does one disappear altogether.
I would not personally recommend treating birch trees chemically for birch leafminers. Topical chemicals or root drenches will affect parasitoids and predators as well as the leafminers, which dilutes their usefulness. All birch leafminer species found in significant numbers here attack birch later in the summer, once leaves have provided much nutrient value to the trees already. The leaves don’t look great, admittedly, but I suspect the effects of the leafminers on overall birch health are negligible.
Water stress is a much more significant issue for birch in the urban environment. Their shallow roots must compete with lawns for water. We used to recommend mulching under birches out to the dripline to help mitigate this stress. Also, birches are not a long-lived species, so what we perceive as early dieback in birches might just be the trees showing their age. In the forest setting, they are generally pioneer species in Alberta, thriving after fire or blowdown, but replaced within a decade or two by poplar and spruce.
I agree with you that the probability of birch leafminers being introduced to a tree by purchased mulch is exceedingly unlikely. The adult leafminers fly well, and we’ve recorded them travelling long distances by car(!), which they get into by accident after landing on our clothing, etc., and then escape from the car at new locations. I suspect this method, and almost certainly travelling in soil associated with nursery stock, are the main methods of long-distance dispersal. Over shorter distances, any newly planted birch can be expected to attract adult leafminers from nearby trees in Edmonton within a few years, depending on surrounding population levels.
Thanks for the great articles — you’re performing a very useful service getting info out to Edmontonians about insects in the gardening context!
If you are interested in reading the paper it can be accessed from Natural Resources Canada’s website.