Deer flies suck. Nobody likes them. When I lived in southern Maine, I knew when to expect them to start annoying us trail runners, and when they'd start to leave us alone. They'd usually start around the first weekend in June, sometimes as early as Memorial Day weekend, and sometimes not until mid-June. Starting around mid-August, they start to have a hard time catching up and are less numerous, and by Labor Day weekend at the beginning of September, you could kiss them bye-bye until next year.
Their season doesn't really appear to be temperature dependent, at least not when they end. I can remember a few especially hot and humid summers like this one and despite heat waves in late August lasting well into September, the flying nuisances would thankfully still be gone when expected.
As a result, I've theorized that if temperature doesn't seem to matter, it must be the waning daylight hours.
Now that I'm in the Downeast region, the comfort of knowing when they'd leave is somewhat out the window, and everyone I've talked to so far has only had vague notions. My initial thought was their season would end sooner up here, but then I remembered the days will remain longer here than southern Maine until it's officially autumn. That doesn't bode well. But... if the deer flies sense the loss of daylight on a daily basis, which is greater the further you travel north, then there's hope.
Time will tell. This season has also been different in that we're in a drought. Deer fly larvae develop in water, and there hasn't been too much of that around. With that, they've only been around in key areas. But hopefully those spots will also see a decrease as we continue to lose well over two and a half minutes of daylight a day, and see that loss continue to rise each day until autumn. Ahhh, cool dry air and no bugs... not far off now.