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Birch Leafminer

Spots that form on leaves foolowing egg laying by leafminers.

Image Courtesy US Forest Service

A troublesome pest for a birch tree is the Birch Leaf Miner (Fenusa pusilla) . Like the lethal bronze birch borer, it is the larvae of the birch leaf miner which causes damage by burrowing into birch leaves in search of nutrients. The first indications of birch miner infestation are small green spots on the leaf surface, usually noticeable in May or June, which develop into ugly brown splotches. A birch leaf miner infestation rarely kills birch trees on its own but it is disfiguring and can weaken a tree making it susceptible to borer attack.

The leaf miner season begins in late April to mid May with eggs laid in slits on the upper surfaces of birch leaves. The larvae, small, flat whitish grubs (see image at left) in appearance, emerge in late May or June to begin feeding on leaves. In June or July, the larvae drop onto the ground to pupate.  The adults, small black fly-like insects called a sawflies, emerging two weeks later. There can be up to four generations per season depending on the climate.

Advanced leafminer damage

Image Courtesy US Forest Service

Birch leaf miners can often be effectively controlled by using systemic insecticides, but it is more difficult if the larva have already penetrated inside the leaf. If blotches on the leaves are noticed in the spring, spray the tree with the [Borer-Miner Spray] in early July to control the young larva from the second generation. A second spraying of the Borer-Miner insecticide should be repeated at the end of July. If spraying the tree, due to its height is not possible, you can also control the leaf miner with a special insecticide that is injected into the tree’s trunk. The product recommended is [Pointer Insecticide]. It should be applied at the first sign of the leaf miner blotches on the leaves or as a preventative treatment early in the season.

The following spring the tree should be treated again to prevent the reoccurrence of the leaf miner, this can be through a spray, a soil drench or injection into the tree trunk. If you are treating the tree with the Borer-Miner Killer Spray, it should be applied in 3 separate sprayings beginning about 1 week after bud-break and then every 10 days thereafter. If using the Systemic Insect Control Soil Drench or Pointer Injectable Insecticide, they require only one treatment, which should be applied at the time of bud break.

If the tree is suffering from an ongoing leaf miner infestation, it is also important to help the tree maintain its vigour by ensuring it is regularly watered and it is properly fertilized. For general tree maintenance and proper fertilization, apply the TreeHelp Annual Care Kit for Birch trees. These kits contain a fertilizer specifically formulated for a birch tree, as well as birch tree mycorrhizal fungi and biostimulant which help promote root development and long-term vitality.

Next: Bronze Birch Borer »


Grub Image Courtesy Canadian Forest Service

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