Lilac Leaf Mining Moth
If you notice large brown blisters on the leaves of your lilac plants, it may be the result of the caterpillar larvae of this moth.
The lilac leaf mining moth (Caloptilia syringella) lays eggs in Lilac (Syringa), privet (Ligustrum) and ash (Fraxinus) leaves. When the caterpillars emerge, they burrow (mine) through the leaves. The second phase of their growth begins when they emerge from the mine and roll the leaves conically from the tip creating a protected shelter where they flourish.
The caterpillars pupate on the underside of leaves. There are two generations each year, with most leaf damage occurring in June and in August to September. The second generation typically overwinter as pupae.
Below, damage caused by caterpillars of the lilac leaf mining moth.