At which stage does the Birch Leafminer primarily damage leaves?

The Birch Leafminer primarily causes damage during the larval stage. At this point, the larvae feed on the tissue between the upper and lower surfaces of the leaves, creating distinctive mines or tunnels. This feeding behavior leads to a reduction in the leaf's ability to photosynthesize and can result in aesthetic damage to the tree, as well as potential weakening of its overall health. The adult stage, which comes later, does not cause damage in this manner; instead, adults are primarily concerned with reproduction. The egg stage involves the larvae developing inside the eggs without causing any impact on the foliage. Similarly, during the pupal stage, the larvae are not actively feeding and thus do not damage the leaves. Thus, it is during the larval stage that the Birch Leafminer poses the greatest threat to birch trees.