A fly with a black abdomen, brownish yellow thorax, yellow head and legs. The 3.1-4.5 mm long wings have dark bands forming two Vs on the leading edge. The ovipositor is much longer than the abdomen and is swollen from the base to the middle. An average of 100 eggs are laid singly into developing florets, but usually several are laid into the flower bud. Mines down the floret and into the receptacle which is galled.
Adjacent galls coalesce to form a multicelled gall which is woody when mature.
The larvae feed on special cells formed inside the gall and when mature are 3 to 4 mm long. Formed in the gall cells. Larvae completing development before the end of June emerge for a second generation, but most overwinter as larvae and emerge in the spring. Emerge over 6 weeks and live for several weeks, so they are present for much of the summer and able to attack early and late flower buds. Mature larvae in the gall cells.