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 Goldenrod Gall Fly Eurosta solidaginis
The aptly named Goldenrod Gall Fly has a parasitic relationship to the goldenrod plant, which its entire life, and death, is centered around.
Appearance – Like other flies in the family Tephritidae, or fruit flies, they are about a quarter inch long and usually have dark, speckled patterns covering their clear wings. They have a wide head and large, iridescent eyes.
Range and Habitat – They are found in the Northeastern and Midwestern regions of North America, wherever there is goldenrod.
 Gall – The Goldenrod Gall Fly overwinters as a larva inside of a gall on a goldenrod stem. These round galls are a deformation of plant growth caused by either the physical actions or chemical secretions of the fly.
Galls can be found among newly dried weeds during the winter in fields where there was goldenrod in bloom during the fall. This fly’s galls are the most easily seen galls on the goldenrod during the winter, and are usually about three quarters of an inch in diameter and appear on the upper half of the stem. If the gall is opened, the larva, which is white with a dark head, can be seen in the center.
Life Cycle - Due to their poor flying abilities, Goldenrod Gall Flies spend much of their adult lives traveling up and down goldenrod stems. Before mating, a male will chose a spot on a particular plant and wait for a female to arrive. He then attempts to attract her using a courtship dance. If this dance is successful, the female will find a spot on the plant to inject her eggs.
The fly larvae will hatch in about 10 days and immediately start to eat the stem of the goldenrod. The larvae’s saliva has a chemical in it that alters the growth of the plant, resulting in the creation of a gall. The larvae spend a year inside the gall before becoming adults. In spring and summer, they continue to eat and the gall continues to grow. In the fall, each larva will dig a tunnel that will later be used in spring as an exit. Before winter arrives, each larva prepares its body for the winter by producing a chemical that prevents it from freezing in the cold months. In the spring it will become active and chew a tunnel through the gall to its edge, only to turn back around to the center in order to pupate. Later in the spring, it emerges as an adult and proceeds to mate.
Predators – The gall acts as protection against many predators, but in some cases they can show animals that have learned what’s inside the gall where there is something to eat. The Downy Woodpecker and the Carolina Chickadees look for galls in order to break them open and eat the larvae inside. Particular beetles and wasps are also known to prey on these flies.
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