Numerous small pouch galls along the midrib or major lateral veins of Populus tremuloides. They are pale green to pink-red, hairless, thin-walled, sometimes spotted with white inclusions, and typically seem to lay flat against the leaf more than protruding downwards. When growing densely they become confluent. They are similar to Harmandiola helena, which also occurs in numbers along the midrib of P tremuloides in similar patterns, but differ in having the majority of the gall on the lower side of the leaf and a slightly protruding slit opening on the upper side of the leaf. Further, H helena galls are typically oriented more or less perpendicularly to the leaf, while the galls of this species seem to be oriented at an angle. Observed from Nova Scotia to Manitoba and south to Michigan from July to September.

This gall is similar to the galls induced by Harmandiola populi on Populus alba and tremula in Europe.

This gall morphotype can be distinguished from other midge galls on Populus leaves by the following traits:

• Larva concealed within galled tissue, protruding entirely on the lower side of the leaf
• Only a slight protrusion on the upper side around a linear slit opening