The larvae hatch in the second instar and bore to the vascular differentiation zone, where each makes small chamber, with the number doing so successfully increasing with shoot size. This may aid the export of surplus sugars produced by larval feeding. Most of the nutrient loss to the plant occurs during the gall-growth stage, but the larvae barely grow. By the end of its growth, the gall is woody with chambers lined with nitrogen-rich cells on which the larvae feed and grow for a month, increasing their dry weight to 7.7 mg. Gall size varies widely.
Large galls minimize larval parasitism which is high in Europe.
However, the gall stretches the stomata on its surface, preventing them from closing and causing a high water loss to the plant. Thus, large galls are detrimental on dry sites, so the best sites are shaded and moist. This may indicate that the fly in prepared for high losses as stems have more larvae than they can nourish. Mature larvae pupate in 8-9 days when exposed to air. In nature, the tunnels to the larval chambers fill with undifferentiated cells.