Several species of insects feed on birch tree leaves in the spring months of every year. The damage these insects can cause ranges from minor aesthetic browning or yellowing of leaves to severe infestations of the foliage and bark. Homeowners can treat the majority of these pests with insecticides and hand removal of any larvae or insects.
Bronze Birch Borer
The bronze birch borer is a type of flatheaded borer that eats birch tree leaves in its adult form. Damage initially appears as yellowing or dropping of the leaves from the top of the tree in late summer. By the next spring, an infestation could be in progress, with full-grown borers feeding on leaves on the lower branches. The presence of bronze birch borers usually means the tree's bark and heartwood are infested with borer larvae.
Spittle bugs, so named because the insect covers itself with a protective layer of spittle, feed on the sap of birch tree leaves. The presence of these bugs does not pose as significant a threat to the tree as do borers, though the damage a spittle bug can do is unsightly. Affected leaves will brown or turn yellow, may drop from the tree in patches and will generally appear misshapen due to the bite patterns of the insect.
Dusky birch sawfly
Larvae of the dusky birch sawfly feed on birch tree leaves. Fully grown larvae can appear similar to caterpillars and are yellow-green with black spots along their sides. The Maryland Cooperation Extension refers to the larvae as "gregarious" and notes that the insects-to-be generally feed in groups, with the edges of leaves being devoured first. Young trees are at greatest risk of an infestation of dusky birch sawfly larvae and can become defoliated if left untreated.
Aphids are a form of pest insect that swarm over trees like the birch and devour its leaves. You can find these insects on the underside of tree leaves feeding on the sap in a fashion similar to that of spittle bugs. Leaves being eaten by aphids will twist, yellow or brown over time. Infestations of these insects occur most often in the spring months, when tree leaves are just budding.