Goldenrod gall flies are also known as goldenrod ball gall maker, these flies live their entire life on goldenrod plants. It is a species of fly native to North America.
Goldenrod gall flies lay their eggs in the goldenrod plant. The fly larvae cause the plant to form a gall, a swollen growth of plant tissue. Large galls tend to be pecked open and their larvae eaten by birds. Small galls are more frequently parasitized by tiny wasps, whose larvae eat the fly larva and take over the gall. This complex situation implies that Stabilizing selection causes no change in the average of the population; extremes (large and small galls) become less common is acting on gall size.
