Mystery solved

When I came home yesterday evening, I noticed that the fluffy, white insects were not limited to the backyard.  They were also drifting around in front of the house, as well as up and down the street.  I decided to make another effort to identify them.  I was able to scoop several out of the air with my hand, and I captured one in a glass jar for closer inspection.  Going through the insect key from the beginning, I found that what I had was a woolly aphid (Aphididae, subfamily Eriosomatinae). 

A woolly aphid migrating to a new host plant.  For scale: this aphid is about 3mm long.

These aphids alternate between feeding on alders and silver maples.  While on one host, they form dense colonies of wingless clones.  Then, when it is time to migrate to the next host, they produce winged offspring like the one pictured above.