The Birch Leafminer is a serious pest of Gray Birch, Paper Birch and White Birch in Pennsylvania. It may occasionally attack Black, Yellow, or even River Birch. The emergence of this pest in the Spring may initially be recognized by the presence of the adult black wasps approximately one-eighth inch in length on newly opening leaves. Larvae may reach one-fourth of an inch in length and feed in yellowish-brown blotch mines. There are two generations per year. Mature larvae overwinter in soil under trees.
The Birch Leafminer adult is a small, black sawfly with a wingspread of approximately one-fourth of an inch. In early Spring, adults emerge and start to lay eggs. Eggs are inserted singly inside the soft, newly expanding leaves. After 7-10 days, the eggs hatch into small, whitish, slightly flattened larvae which feed for 2-3 weeks between the upper and lower leaf surfaces. The larvae and black fecal matter are easily seen in the mines when infested leaves are held up to the light. Two generations of Leafminers are common in Pennsylvania. Adults of the second generation usually emerge during the latter part of June.
Larvae consume the tissue between upper and lower leaf surfaces. Injury appears in the late Spring as irregular brown “scorched” areas on the leaf surface. At first the mines are separate and small, but eventually they overlap to form large blotched or blistered areas. Some Birch trees are almost completely browned from top to bottom because of the feeding activity of this insect. The tops of the trees are normally the most seriously affected.
Use a systemic pesticide to kill larvae in newly developing mines. If the first generation is controlled, damage for that season is greatly reduced. Effective control is dependent on timing that can best be determined by careful observation of the tree foliage. Thoroughness of the spray coverage is also of equal importance. Formulations of several systemic insecticides are labeled for Birch Leafminer control.* Insecticides should be applied in May when the leaves are fully extended. Repeat in mid-late June. Follow all label directions for specific information on host plant label clearance, phytotoxicity information, safety precautions and dosage information.
* See Woody Ornamental Insect, Mite and Disease Management, The Pennsylvania State University (2005) for more details.
WARNING: Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of reach of children, pets and livestock. Dispose of empty containers right away , in a safe manner and place. Do not contaminate forage, streams or ponds.
For professional assistance with tree and shrub problems contact a Keystone arborist
Penn State Fact Sheet on Birch Leafminer