Flying en masse for a week or two, they're an incredible nuisance for rollerbladers, runners, bikers, walkers, or anyone else who likes to breathe. Or dislikes tiny insects stuck to your eyeballs.
Blue ash aphid, Smokey-winged ash aphid, and several other colloquial terms describe the pests that plague Eastern Washington and Northern Idaho every fall.
Even with sunglasses, I once spent 5 minutes trying to dislodge one of the little insects from my retina. This final batch represents the end product of months of reproduction, culminating finally in the presence of some egg-laying females.
This is when you see the inch-thick congregations on ash trees. The females lay their eggs in cracks in the bark. Next spring, they'll hatch out, begin reproducing without mating, boogie over to an evergreen tree, and spend a few weeks snacking on a fir. It's a fairly complex insect (info sheet)--moreso than you'd think based on their size.
They can do real damage, so the best thing you can do is actually a non-pesticidal approach: Dormant spraying. A dormant oil applied in the next couple weeks will help smother live aphids on the trees & shrubs as well as prevent oxygen from reaching the eggs in the bark cracks. A late winter dormant (Feb - April) takes care of many more.