An exposed larva on the lower side of the leaf lamina of Populus tristis causing a circular green spot and a slight raised papilla on the upper side. Observed from southern CA up to Washington. There may be multiple generations per year; fresh galls with larvae have been seen in May and September.

This gall can only be distinguished from the similar p-balsamifera-leaf-gall by host ID. There are places where both hosts occur, and distinguishing the two plants can be difficult. It is possible that the two midges are in fact conspecific.