Birch Leafminer (family Tenthredinidae)
History
Edmonton has seen a recent surge in Birch Leafminer in the last couple of years. It is a pest that was once under control for seemingly a couple of decades.
Three species of sawfly (Fenusa pusilla, Profenusa thomsoni, Heterarthrus nemoratus) introduced from Europe in the early 1970’s were responsible for the browning of birch leaves in Edmonton. The larvae feed on the inner green tissues of the leaf, which then causes discoloration in the leaf. Feeding over several weeks causes the leaf to look brown and look less than appealing to homeowners.
Control
Many years ago the City of Edmonton released a parasitoid wasp that helped drastically reduce the problem in Edmonton and made it a very minor issue again. This allowed people to plant Birch with confidence and once again enjoy their beauty that so many have come to love and enjoy. This lasted many years until recently it was discovered that a new type of leaf miner is now existing in Edmonton that this parasitoid wasp cannot control. The European Cutleaf Weeping Birch (Betula pendula ‘Laciniata’) is often the most affected by leaf miners, but recent introductions of the Asian Birch (Betula platyphylla), such as Dakota Pinnacle Birch and Parkland Pillar Birch which have become a quick favourite of home owners, have also proven to be a bit of a magnet for these pests.
Currently there are limited options to control this pest. Gone are the days where we can use Lagon or Cygon to control this pest by painting the chemical on the base of the tree. These were found to be far too toxic and since have been banned for use in Canada. There are less toxic chemicals available today that can be sprayed onto the leaves that may help to control the miner in a small way, but keep in mind that anytime we spray a chemical it has the potential to kill both beneficial and harmful insects. Using something like a lower concentrated version of Malathion or an insecticidal soap may help reduce some of the populations, but should be used as a last resort. Alternately, once the damage becomes apparent on the tree, it is usually past the point of treating in any significant method as the insect has already done most of the damage to the tree.
Conclusion
Keep in mind that it is very unlikely that this pest would ever kill your tree and should thus be considered a pest that will be more of an issue in regard to the appearance of the tree, more than anything else. Cleaning up leaf litter in the fall, not composting the leaf litter and making sure your tree is not stressed from too much or too little water, and keeping a good healthy soil around the tree is much more important and beneficial for the tree to fight off any insects or disease that may come its way. This is not a pest that will transfer to other species of trees, so you won’t have to worry about other trees in your yard getting this pest, that aren’t in the Birch family. This is also a reminder to us that having diversity of species in any ecosystem is something to strive for to avoid having a situation where all your trees are affected in a negative way. This will also help slow the spread of insects and diseases in our urban forest.
It is an unfortunate reality for many homeowners will have to live with this pest for the foreseeable future, until a more long-term solution can be found. Keep in mind that there is no tree that is immune to all insect attacks and diseases. Nature has a way of balancing things out and although this is major nuisance for homeowners right now, it is unlikely to kill a healthy tree and can be considered a cosmetic pest at the moment.
