Early mines on left and late mines on right. Life Cycle and Habits Mature larvae overwinter in the soil under host trees. When the soil warms in the spring these prepupae pupate, usually in April, to transform into the adult stage. Within a few weeks the adult sawflies dig out from their earthen cells and fly to the newly expanding foliage of birches. The small black adults prefer to mate and oviposit on the upper leaves, especially in sunny areas.
Mated females use their needle-like ovipositor to punch a hole in the leaf and lay eggs.
Often the female withdraws the ovipositor without laying an egg. The damaged spot may turn brown as the leaf expands and hardens. The eggs are often visible in the tissues between the major leaf veins. They may appear as small raised spots on the leaf surface. Within 7 to 10 days the eggs hatch into tiny flattened larvae with wide front segments.