As a lapsed horticulturist, I am well aware of the leaf miners and the damage they can do to trees. I see the symptoms of them often on my nature wanderings or when working in the garden: patches of leaf that are discoloured, sometimes with tunnels visible, created by little larval miners excavating between the leaf surfaces. These larvae are usually either from moths, flies or–on birch leaves–by sawflies. Damage to the leaves can be extensive at times, and reoccurring attacks can weaken the tree and cause branch dieback and eventually death.
We have three Paper Birch (Betula papyrifera) in our garden in Edmonton, and while prowling through the front garden for photo op’s I came across some birch leaves that showed the typical damage of a leafminer, visible at eye level on the lowest branches of the tree. I decided to photograph the leaf, mostly with the thought that I would be able to share the images on a new garden blog that I have been planning. Because I could see that there was something under the papery damaged area (above) and because I had been playing with backlit leaves recently, I decided to try the technique on this birch leafminer damage. I simply pressed the leaf directly against the flash lens and adjusted the flash power settings manually until I had the right exposure.
I was somewhat surprised to see some detail in the larva, including some prominent dark marks on the segments behind the head. I emailed the images to Greg Pohl at the Northern Forestry Centre, who was able to identify it as the larva of Fenusella nana (Klug), the Early Birch Leaf Edgeminer, as determined from the key found in the paper Current status of invasive alien birch-leafmining sawflies (Hymenoptera: Tenthredinidae) in Canada, with keys to species by Digweed, S. C., et al (2009).
Searching online, I found Creative Commons images of the larva:
I could not find a CC image for the adult, but there are a variety of photos at BugGuide that give a good idea of its appearance and the damage it causes.
There are five species of leaf mining sawfly (not a true fly but a Hymenopteran related to wasps, bees and ants) that effect birch trees across Canada, and all are alien invaders. In Canada, two species (F. pumila and P.thomsoni) have been successfully controlled with releases of parasitoids, however, F. nana has yet to have that pleasure.
The life cycles of birch leafminer sawflies are all similar. F. nana adults are active mid-May to June, when the females use their saw-like ovipositors to lay up to three eggs at the leaf edge. The larva hatch and go through five or six instars between the surfaces of the leaf, overwintering in the last instar to pupate in spring.
Digweed, S. C., MacQuarrie, C. J. K., Langor, D. W., Williams, D. J. M., Spence, J. R., Nystrom, K. L., and Morneau, L. (2009). Current status of invasive alien birch-leafmining sawflies (Hymenoptera: Tenthredinidae) in Canada, with keys to species. Canadian Entomologist, 141(3), 201-235. DOI: 10.4039/n09-003.
Langor, D.W.; Digweed, S.C.; McQueen, R.L.; Spence, J.R.Where have all the birch leafminers gone? Forest Insect and Disease Notes, August 1996, A-033