The gall, caused by several larvae, develops by extensive hypertrophy of the leaf tissues; it is an irregular, elongate, globose swelling along the midrib or along the larger lateral veins of the leaflet. The size varies considerably, depending largely on the number of larvae within, and galls on lateral veins are often smaller than those on the midrib. Usually only one gall forms on a leaflet.
A young gall is green with bright crimson areas, and a mature one is mostly green. Degenerating ones become yellow, then turn brown. Galled leaves degenerate and fall from the tree shortly after larval departure. The mature gall is turgid; the degenerate one is flaccid and rubbery.
Felt described the gall and stated that the opening is on the under-surface of the leaf. Actually, the gall develops while the leaf is still partly closed so that at maturity the opening is a narrow slit formed by that portion of the upper leaf surface at the edge of the gall.”
- Louis Wilson: (1966) Life history, habits, and damage of the boxelder leaf gall midge, Contarinia negundifolia Felt (Diptera: Cecidomyiidae) in Michigan©