Some flies migrate, just like some birds and butterflies, but it is rarely recorded. In fact, this might be the first record of a flower fly (or hover fly) migration for North America!
It took place on April 20th, 2017, at beautiful Montana de Oro State Park. I was walking with my dear friends Marianne and Gary Wallace when Gary said “look at all those bees!” I looked, and realized that they were not bees, but flower flies, thousands of them. They were flying against the wind, in a northward direction, not stopping at all. I actually got some video of this migration; have a look.
The flies look like bullets going by. We didn’t have any collecting gear (duh, it’s a park), but we tried knocking some down, to no avail. They were moving fast and avoiding contact. I tried taking some stills, and this is my best (bad) photo:
Why would syrphids migrate? These flies feed on aphids as larvae, and aphids are found on tender new growth. California’s rainfall comes during a short time in the winter, and then things dry up. An hypothesis is that as the more southerly vegatation dies off from lack of water (and thus also the aphid supply dwindles) the flies move northward looking for green, still tender fields, well stocked with their aphid food.
This was something I never expected to see, but wanted to record so others could keep their eyes open for it in the future. I’ll be back next year looking for them at the same time of the year, hopefully to learn which species they are.