Local feeder (Diptera: Tephritidae) in stems of Ambrosia

Spindle-shaped to ovoid stem galls belonging to this tephritid graced plants growing in a brushy area I explored in late July. The galls occurred in the middle to upper part of the plant, with at least one example formed near the base of the stem of a side branch. Inside a typical gall were one to two central cavities, each with smooth inner walls that were the same color as the surrounding pith; the cavity interiors were clean and contained no solid frass. In one case, there were two cavities in a single gall, and the cavities were separated by a wall of pith. The larva observed inside one cavity was a plump maggot, fat and rounded at the posterior end and tapered at the anterior end, with a smooth white integument decorated with a series of striking black transverse markings on the abdominal segments. By early- to mid-August, larvae had formed black puparia in the galls. Puparia overwintered and adults emerged at the beginning of June the following year.