Ah yes, the enemy of the Birch Tree! These crafty little larvae are originally native to Europe, but migrated to certain parts of Canada sometime in the mid-1900’s. Most entomologists agree that it’s not reasonable to expect the eradication of Birch Leaf Miner here in Canada, but many of our own native species have adapted to become predators of the larvae, helping to keep the population under control.
Why they’re good:
Birch Leaf Miners don’t have a lot going for them, but they do provide an important source of food for other predatory insects or birds. Though they only survive thanks to the shelter of the trees they invade, B.L. Miners are part of the natural ecosystem and usually aren’t detrimental when they ‘move in.’
When they’re bad:
The good news is that these insects mostly cause aesthetic damage, turning their ‘host leaves’ brown. It doesn’t look the nicest, but in most cases the affected tree will go on living it’s life as normal.
In extreme cases (like an entire birch tree covered in B.L. Miner), it’s possible that the leaf damage is enough to prevent the tree from completing a sufficient amount of photosynthesis, causing stress or death. We don’t see these cases very often, and even if a tree has a large infestation, it will likely be fine.
Where & When You’ll Find Them:
B.L. Miners have several stages in their life cycle. Eggs are initially laid in the leaf tissue in spring/summer, but the newly hatched larvae don’t stay long before dropping to the soil beneath the tree to over-winter. In the following, early spring, these larvae crawl back up the trunk of the tree, and nest into a leaf of their choice – as they begin to consume more leaf tissue, we begin to see the obvious signs of damage from Birch Leaf Miner. Typically these signs become obvious in May, June and even July.
How They Impact Your Plants:
During their nesting phase, B.L. Miners burrow into the tissue in the middle of the leaf, in between the outside surface of either side of the leaf. As they consume the inner tissue, they create little puffy “pockets” of depleted tissue in the leaf, where they are sheltered from predators and, unfortunately, insecticides. They live here until they are ready to mature into adults (sawflies), at which time they chew an exit hole and fly away.
The damage caused by these tiny pests is mostly cosmetic, and shouldn’t impact the overall health of the tree.
Identifying Leaf Miner Damage
As you might have guessed, Birch Leaf Miners specifically inhabit birch trees. At first glance, you may notice that your birch tree has several (or many) leaves that are turning brown, sporadically throughout the tree and likely concentrated closer to the bottom half.
Upon closer inspection, you’ll notice the typical ‘puffy pouches’ created as temporary homes for the larvae, and you may even notice the larvae wiggling around inside. These larvae will leave behind insect frass (poop) from the leaf material they’ve been eating, which will look like little dirt particles trapped within the leaf. It’s a good visual, we know.
How To Deal With Birch Leaf Miner:
Unfortunately, there aren’t really any remedies for Birch Leaf Miner once they’ve nested into your tree. If you’ve noticed a handful, you can always pick the brown leaves off and compost them, to prevent that generation from spreading. But if the infestation is substantial, you may have to ‘wait it out’ as the miners are too protected within the leaves to effectively use organic or pesticidal insect killers.
Water & Fertilize Your Tree: The best thing you can do for a tree with Birch Leaf Miner is water and fertilize. Think of these bugs as the “common cold” for your tree – it won’t kill your tree, but some proverbial chicken soup and vitamins will help promote overall health and some new growth to help counter-balance a pesky nuisance.
Tanglefoot: You can proactively address birch leaf miner by helping to prevent the larvae from crawling onto your tree in the first place! Tanglefoot is a sticky, non-drying paste that can be applied around the trunk of a tree as a ‘bug trap’ for B.L. Miners and crawlers looking to make their way up the trunk. Don’t apply Tanglefoot paste directly to the tree trunk (it will soak into the bark, become ineffective quickly, and leave behind a mess), but instead wrap the trunk with a 6-10” of Tree Wrap, and apply the Tanglefoot paste on top. This contains the mess and makes it easy to remove and re-install later. For best results, apply tanglefoot as early in the spring as possible (April!), and refresh your trap throughout the spring and summer as needed.