The small, white and slightly flattened larvae of the birch leafminer live within the birch leaves. They can be easily seen when the leaves are held up to the light. The adult is a black fly-like insect about 3/16″ inch long.
Habitat: Mature larvae overwinter in the soil under host trees. When the soil warms in the spring these prepupae pupate, usually in April, to transform into the adult stage. Within a few weeks the adult sawflies dig out from their earthen cells and fly to the newly expanding foliage of birches. The small black adults prefer to mate and oviposit on the upper leaves, especially in sunny areas.
Life Cycle: Adults overwinter in the soil and begin to emerge in early to mid-May. They congregate on birches and mate; females lay their eggs in newly-developing leaves. The eggs hatch in seven to ten days and the larvae begin feeding, making mines which are small and somewhat serpentine in form. As the larvae grow, feeding increases and the serpentine mines often run together to form the characteristic blotches and blisters on the birch.
The larvae mature in one to two weeks, drop to the ground, and enter the soil to pupate. New adults appear in about 15 to 20 days to start the cycle over again. During a normal year, a life cycle can be completed in five to six weeks. There are three generations per year. Only the first generation is considered destructive, because adult females prefer to lay their eggs in soft, young tissue and there are very few new leaves available late in the season.
Damages: Larvae feed on the mesophyll of the leaves, leaving the outer epidermis intact. Having the epidermis intact provides a nice protective house for the leaf miner to feed safely. The upper surface of the leaf can appear transparent to allow viewing for your own leaf miner at work. Areas where leaf miners are active appear as blotchy brown leaves.
Due to the fact that these leaf miners can produce several generations through the season, damage to your birch tree can appear severe as fall approaches. During a normal year, trees tolerate this severe damage because they will soon drop their leaves and go dormant for winter.
Control:Prevention is key since it is difficult to control the leafminer once inside the leaf.Species reported to be resistant and rarely affected by birch leafminer include Whitebarked Himalayan Birch , Dahurian Birch (Betula davurica), River Birch Schmidt Birch, Black Birch , Yellow Birch and Japanese Cherry Birch.
Keeping the soil in good condition with the use of organic matter and properly watering during times of drought will reduce the host plants susceptibility to secondary pests. Mechanical Handpicking and destroying the infested leaves is effective, but time consuming. Chemical Systemic insecticides for the control of larvae can be applied to the foliage by soil injection or injected into the tree.