Mining or tunneling in the leaf tissues by the larvae produces conspicuous blotches or blisters on the leaves which later turn brown. Some birches are almost completely browned from top to bottom due to the feeding activity of this insect. However, the tops of the trees are often the most seriouslt affected portions. There can be up to four generations of the insect in a season depending upon the locality and climatic conditions. The first generation of the insect causes the most damage.
Adults begin egg laying in the spring when the leaves start to unfold.
At this time of the year, all leaf tissue is soft and fovorable for larval development. Additional damage may be caused by the second generation, but attack by succeeding generations during the same season is of little importance. These subsequent generations may affect sprout growth where foliage is soft and new. Old foliage is not attractive to sawflies during ovi- position. Normally a healthy tree can lose part or nearly all of the current crop of leaves without being seriously injured.