Birch leafminer
Fenusa pumila & Profenusa thomsoni
Birch leaves going brown in blotches from the tip of the branch in early summer, as if scorched. It is a sawfly grub inside the leaf, and the tree survives it.
How to know it
Hold a browning leaf up to the light and you will see a flat, pale, legless grub inside it, between the upper and lower surfaces. The mine is a translucent blotch rather than a winding trail, and it starts between the veins in the middle of the leaf. The adult is a small black sawfly.
What you'll see
- Pale green translucent blotches appearing in birch leaves in late spring, mostly on the youngest leaves at the branch tips.
- Blotches turning brown and papery, so the tree looks scorched from a distance.
- Several mines running together until the whole leaf is brown.
- A grub visible inside the leaf when it is held to the light.
- A second, lighter round of damage in midsummer from the following generation.
Where it comes from
Two to four generations a year. Adults lay eggs into the tissue of newly expanded leaves in spring; the grub mines the leaf for a fortnight and then drops to the soil to pupate. Only the newest leaves are attacked, which is why the damage is at the branch tips and why an established tree keeps plenty of sound older foliage.
Gray, paper and European white birch are hit hardest; river birch is markedly resistant. Damage is worst on young trees and on trees that are already stressed by drought or by bronze birch borer.
Dealing with it
- Do nothing on a mature tree. The mining looks alarming and the tree carries on.
- Pick and bin mined leaves on a small tree while the grubs are still in them.
- Water and mulch the tree deeply in dry weather; the real risk to a birch is the borer that follows a stressed tree, not the miner.
- Do not spray a contact insecticide; the grub is inside the leaf and it will not reach it.
- Have a valued young tree treated systemically by an arborist only if it is being defoliated year after year.
Keeping it out
- Plant river birch or a resistant cultivar where the miner is common.
- Keep birches watered and mulched, since they are shallow-rooted and drought-sensitive.
- Avoid planting birch in hot, dry, exposed sites.
- Encourage the parasitic wasps, which have brought it under control in much of its introduced range.
- Rake and remove fallen mined leaves in autumn on a small tree.
What it goes for
Birches — gray, paper and European white especially; river birch is largely resistant.
Who hunts it for you
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com