BIRCH LEAFMINER - Fenusa pusilla adults are 1/8 inch long, minute, black wasps.The larvae of these wasps feed between the upper and lower leaf surfaces.  They eat the plant tissue and cause irregular brownish blotch mines that may turn whole leaves and trees brown during heavy infestations.  Look at newly expanded leaves for adults and brown spots indicating new mine formation.This imported sawfly prefers gray and paper birch.  Occasionally, it will attack black, yellow, European white or river birches.  Some more resistant birches include:  Betula davurica, B. maximowiczana, B. schmitii and B. castata.  There are three to four generations of leafminers in a year.  Mature larvae overwinter in the soil under the trees. Prevention is the key because once the leafminer is inside the leaf, nothing works to kill the pest.  Deep tilling helps to destroy the larvae in the soil.  If the first generation is controlled, damage for the rest of the season is greatly reduced.  A residual insecticide can be used at budbreak to prevent adult egg laying.  A systemic chemical might help to kill larvae in newly developing mines; however, hand-picking and destroying the infested leaves is the most effective, but tedious method.  If your plant needs to be replaced, use one of the more resistant species as noted above.