Poplar Petiole Gall Aphid

Normally, when I walk by this Poplar Tree, I see its leaves shimmering and fluttering in the wind. Today, I saw leaves rather weighed down and stunted in size compared to the ones way higher in the tree canopy.

On closer examination I saw these:

These growths are galls. Masses of plant tissue that grow over nesting insects. I wasn't sure if it was an aphid or mite which caused this. Turns out it's an aphid. 

Unfortunately, this tree has other issues - some dying branches and peeling bark. The leaves show a bit of stress with perhaps a form of rust. These galls won't kill off the tree, but certainly don't help matters.

The best info I could find on this aphid is on Manitoba's Agriculture Food and Rural Initiatives site.
There you'll find the contents inside the gall and the aphid's life cycle. Well done, MAF&RI!