Goldenrod Gall fly and Gall- Eurosta solidaginis
Also known as the goldenrod ball gall maker, is a fly species native to North America, especially in Simcoe County. This diminutive fly is about 5 mm long and quite hard to spot and does most of its travelling by walking. But these little flies are part of an interactive intra species ecological relationship of evolutionary biology, in which research into its biochemistry has inspired studies into anti freezing properties.
Now, that seems like a lot for a little fly, so let’s break it down to what really happens. In the spring, male and female gall flies breed for about two weeks and the female, by using her ovipositor, an egg laying organ, inserts her eggs in the buds or tips of the goldenrod. At this point the flies instinctually mate just after when the goldenrod begin to grow. Gall flies only breed on goldenrod, and in particular, specific species. Solidago Canadensis and Solidago gigantea are preferred with the former, Canada Goldenrod, being most popular. This species of goldenrod is native to Canada and can be found everywhere in large bunches.
After 8-10 days the larva hatches and burrows its way into the stem of the goldenrod. At this point the larva induces the plant’s tissues to create a gall around it. A gall is a hard bulbous, spherical like chamber, which forms around the larva. It’s thought that the chewing action and saliva of the larva induces plant hormones to create a gall. Parasitic insects are known to cause plants to react this way. In turn the gall provides a protective and nourishing habitat for the larva. The grub that resides in the gall in the fall is the third stage of larval stages and will over winter in it until it pupates into an adult in the spring. Now, at this point, right before sub zero temperatures set in the larva will excavate a burrow to the surface of the gall. It will stop short at the epidermis of the gall (the skin) and leave the burrow closed until the spring when the adult flies breaks through.
Now the most intriguing aspect of this little fly begins to take over. The gall offers protection but can’t protect against very cold weather like what we experience in Canada. So, cold weather stimulates the gall fly larva to produce glycerol in high concentrations as an intracellular anti-freeze. For almost 8 months the larva body freezes and the cells stay liquid, and the grub remains inactive. As the spring arrives the warm weather triggers the larva to pupate, or creates an enclosure, where an adult Gall fly hatches. At this point the adult will go up the excavated tunnel and pump liquid into a special component in its head. The swelling then bursts the thin outer wall of the gall allowing the fly to escape. Then it searches for a female and starts the cycle all over again.
There are perils of being in a gall. And this is where the gall fly evolution comes into play. The larger galls are great for protection from small predators but are easily spotted by larger predators, where smaller galls have less protection against smaller predators but are not easily spotted by larger predators. In particular, other insects such as wasps can predate gall fly larva. Female wasps can parasitize the gall larva by using their ovipositors to place eggs on the larva and consuming it when they hatch. This usually occurs on small galls. Throughout the winter, larger predators relay heavily on gall fly larva for nourishment in the cold. Black-capped chickadees and downy woodpeckers can peck into the galls to extract the energy rich larva. These birds can arguably survive cold winters because of the abundance of frozen gall fly larva. Greater understanding of how these tiny creatures evolved to freeze themselves for survival and species continuation can only be beneficial to the survival of the human race; or at least find us something warmer to wear while blowing snow.
Andrew Major is the Owner and Operator of Georgian Bay Wildlife Tours
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