Birch Leafminer: Appearance, Territory, Damage and Life Cycle

Latin Name: Fenusa Pusilla

Appearances: Adult birch leafminers are about an eighth of an inch long, black, and resemble flies. Larvae are yellowish white in colour, have no legs, and can grow to be up to 14 inches long. Brown areas on birch leaves indicate that caterpillar-like larvae have been feeding inside the birch leaf. When an injured leaf is held up to the light, it appears hollow and contains black frass (larval excrement). It’s also possible that the larva is still feeding inside the leaf.

Host Plants: Gray birch, paper birch, and European white birch are the primary hosts of the birch leaf miner (Fenusa pusilla). Leaf miners prefer broad leaf plants, including ornamentals like the columbine, which I adore. They also have citrus trees and blackberries on their menu. Beans, cabbage, lettuce, tomatoes, and peppers are the most commonly infested vegetables in the vegetable garden.

Territory: Birch leafminers are European sawflies belonging to the Tenthredinidae family that were introduced to Canadian woodlands between 1920 and 1960. Birch leafminers are found in all provinces and territories except Nunavut, and they attack all kinds of birch. Natural enemies include parasitic wasps and other parasitic wasps.

Damage Caused: Healthy, mature birches are not seriously harmed by first-generation leafminers. Leafminer damage often does not affect more than 40% of the leaves. Even if more than 60% of the leaves are affected, well-watered birch trees can withstand the harm. Because second-generation leafminers do not attack fully developed leaves, the damage they inflict is less severe.

Life Cycle and Habits: The citrus leafminer has four stages of development: egg, larva, pupa, and adult moth. Adults do not harm plants and only survive for one to two weeks. Birch leafminers in Minnesota typically produce two generations per year. They spend the winter as pupae in the soil. Adults emerge in central Minnesota around mid-May, when the leaves first begin to grow. Females lay one egg at a time in slits cut into the leaves. A leaf can have several females laying eggs in it. Legless, worm-like insects develop from the eggs. These young larvae form kidney-shaped tunnels by feeding between the leaf surfaces. They eat for around two weeks before dropping to the ground to pupate. The pupae give birth to second generation adults in around 2-3 weeks. These adults deposit eggs in newly emerging leaves around the middle of June. After two weeks of feeding, the larvae drop into the earth and convert into pupae. These pupae will remain in the soil until the next spring.