The areas of leaves that are consumed by the amber marked birch leafminer larva turn brown. Because people often do not see the early signs of birch leafminer feeding, it often appears the tree has suddenly dried up or become diseased. This browning is caused by the outer layers of the leaf drying out after the leaf miner larva has consumed the green tissue between the outer layers of the leaf.
Early mines appear as light green or whitish discolorations on the leaves. Larvae sometimes can be seen easily when leaves are held up to sunlight, especially as the mines and larvae grow larger. Feeding over several weeks causes the blemish to take on a blister-like appearance. A single leaf can contain as many as 40 larvae whose mines may merge to destroy the total photosynthetic area of the leaf. Heavy infestations of leafminer larvae can seriously affect a tree's photosynthetic capacity. Repeated attacks will generally cause stress which may induce susceptibility of the tree to other injurious agents.