Irregular bead galls alone or in scattered groups of 2-4 along the midrib (occasionally other veins) on the upper side of Acer negundo leaves. They typically appear gray due to a thick glaucous coating but are green darkening to purplish-red beneath.
Observations are collected in this page on BugGuide.
Three midge galls have been described on Acer negundo: a club-like bud gall, a succulent leaf vein swelling, and a small bead on the upper leaf.
According to Gagne 1989, Contarinia negundinis is considered responsible for two distinct gall symptoms on Acer negundo. On leaves, it causes succulent pouches along the midrib, sometimes affecting the entire leaf. On stems, it causes the growing tip and the petioles of attached leaves to swell and harden below the bud. The bead gall is considered an undescribed species. However, usage in the literature does not uniformly align with this practice.
Gillette gave the name Contarinia negundinis to the bud gall in 1887. Felt reared a midge from a leaf gall (no description given) and named it C negundifolia as early as 1905 (though the species isn't considered formally described until 1908), speculating that it might be a synonym of C negundinis. In 1906, Jarvis attributed the vein gall to C negundinis. Felt's 1917 Key attributes both the bud and vein galls to C negundinis and a gall that seems to match the bead gall to C negundifolia. His 1917 description of C negundifolia is ambiguous but also seems to best match the bead gall.
All later sources use either C negundinis and C negundifolia to refer to the vein gall or sometimes both the vein and bud galls. Only one source (Stein and Kennedy 1972) mentions the bead gall at all after Felt 1917, and they call it C negundifolia. However, this source also incorrectly calls the vein gall Dasineura communis, so its nomenclature may not be reliable.
At some point between 1993 and 2010, C negundifolia was officially made a junior synonym of C negundinis. While it would be most convenient to think that C negundinis and C negundifolia always referred to distinct galls that were combined by this synonymy, this doesn't seem to be the case. Without seeing the source in which the two species were made synonymous, it isn't clear which galls are actually included in the synonymy.
