I wrote about goldenrod ball galls back in the spring when the gall flies were dormant pupae inside gray/brown galls from the previous year. Now, adult flies have emerged from last year’s galls, mated, and laid eggs on fresh green stems. There, the larvae have induced the growing stem tissue to swell into a gall surrounding them.
The galls are large and conspicuous. At least on the hillside where I found them, the galled plants are close together in a few groups, with plants in between those groups having no galls. This distribution pattern could be the result of females distributing their eggs on one plant after another as they encounter them, moving as little as possible between plants, until they run out of eggs. Or perhaps some plants are more attractive and/or less resistant to the galls. Because goldenrods spread vegetatively, each patch of goldenrod contains genetically identical stems. If one of those stems resists gall flies, they all will. If one of those stems is vulnerable to gall flies, they will all be vulnerable.
There also is some level of interaction between plant and fly, resulting in variation in gall size. Warren Abrahamson has explored goldenrod gall flies in detail.
The galled plants have one gall each (though sometimes there can be more). The flies spend most of their lives inside their galls, feeding and developing from midsummer through winter and spring into the next summer. The adult flies serve to produce another generation of galls. And the production of galls is always essential for the production of a new generation of flies. Year after year, at least some manage to succeed.