Psyllids are small insects related to aphids. They are small winged insects that are often green or brown in color and about 1/6 of an inch long. They are very quick moving and can jump quickly from a leaf, into flight. They have been referred to as jumping plant lice.
When in small numbers, they do little damage to a tree. Under favorable conditions, however, the Psyllid population can grow very rapidly and cause serious damage to birch trees during the growing season. The Psyllids, like aphids, attack the tree by sucking the sap out of leaves. The symptoms are very visible on the leaves in the form of multiple pucker marks, yellowing, and the twisting of leaves, giving the appearance of deformed leaves. As the severity of the aphid infestation increases, leaf drop and die-back of twigs and branches can be seen.
During an psyllid infestation, leaves often appear to be shiny and dripping sap. This sap is called honeydew and is an excretion of undigested sugars from the psyllids. It often drips onto other leaves, other plants, and onto the ground. The honeydew then becomes an attractant to ants, which feed on it. In most cases, the ants are only symptoms of the honeydew and are not actually attacking or hurting the birch tree.