Birch trees are native to Alberta and are common in our forests and urban landscaping. Favoured for their great winter aesthetics and manageable size, Birch trees have paper white bark and rich green leaves that rustle in the wind. Sometimes the leaves turn brown and seem to rattle like miniature maracas. This browning is due to Birch leaf miner.
Birch leaf miner is a sawfly that targets the various Birch species in Alberta. There are three species of Birch leaf miner in Alberta: Fenus Pusilla (Lepeletier), Heterathrus nemoratus (Fallen), and Prefenusa Thomsoni (Konow). All three are similar, with the main differences being in how many generations they have each year, as well as where they overwinter. Lepeletier leaf miner has two or three generations each year, depending on how warm the fall is. Fallen and Konow leaf miners, on the other hand, have only one generation. Lepeletier and Konow spend the winter pupating in a cocoon in the soil beneath the tree, while Fallen spend the winter pupating in a cocoon inside the leaf mines. Adult sawflies are tiny black flies, and the larvae are even smaller, light coloured worms with no legs.
Adults emerge from their cocoons in late May and the females cut small slits into leaves where they will lay their eggs. The eggs hatch within a couple weeks and feed for eight to twelve days. The larvae feed on the inside of the leaf, creating distinctive mines. After feeding, the larvae will drop to the ground to build their cocoon, all except for the Fallen miner, which builds its cocoons inside the leaf mines. There will be three or four generations throughout the summer, with the last staying in their cocoons over the winter.
Birch leaf miner damage is mostly superficial and does not significantly harm the tree, but will damage the trees’ aesthetic and make it more difficult for the tree to deal with other ailments at that time. Birch trees with only leaf miner will survive, but it is rarely just leaf miner attacking the tree.
There are three native species of Birch trees in Alberta: paper, river, and bog. Paper Birch is a medium sized tree, reaching about 40ft tall and 25ft wide at maturity. It is deciduous and, once matured, has distinctive white bark that peels off, while younger bark is chocolate brown. It takes Paper Birch bark about 10 years to turn white. Paper Birch leaves are dark green and serrated and grow alternating off the twig.
River Birch is found natively on our riverbanks. It often grows with many slender stems and does not develop white bark like paper birch but instead becomes greyish-brown.
Bog Birch is a small deciduous shrub found in the northern Midwest. It prefers arctic, tundra, and sub-alpine climates and is rarely found near Edmonton.
What Can Be Done
There are insect controls introduced in Alberta that naturally control leaf miner. Parasitic wasps that target the miner were introduced by the government and are effective natural controls. Avoid spraying insecticides on birch trees as it will harm the parasitic wasps rather than effectively manage the leaf miners.
Keeping your trees healthy is another great way to help them better cope with leaf miner infestations. Have deadwood and structural issues pruned and ensure other ailments are treated as they arise. Keep your tree properly watered, fertilized, and ensure it gets enough sunlight. Systemic insecticide treatments are available to treat leaf miner infestation.
What Kentucky Blue Grass Can Do
We can professionally assess your trees, treat ailments, prune for health, water and fertilize, and remove trees, as necessary. We provide leaf miner treatments that are best applied in the spring or fall.
Denise Allen, ISA Certified Master Arborist, March 19th, 2021
Hugh Philip & Ernest Mengerson, University of Alberta, “Insect Pests of the Prairies”
W.G.H Ives & H.R Wong, Government of Canada Canadian Forestry Service, “Tree and shrub insects of the prairie provinces”