Leaf beetles feed on plant leaves sometimes stripping the whole plant of its foliage and destroying it.
Interestingly not all of them are categorized as pests because each species feeds on very specific plants, some of which are weed, thus leaf beetles can be used as biological weed control.
I had a hard time identifying the specimens on the pictures above. After looking at various resources on the Internet I have decided that they were in fact two different — although similar looking — species.
Leaf Beetle – Trirhabda virgata.
Goldenrod leaf beetle – Wildlife of Alberta Wiki.
Trirhabda canadensis – Goldenrod Leaf Beetle on BugGuide.Net.
Trirhabda virgata on BugGuide.Net.
Insects of Alberta – Skeletonizing Leaf Beetle – Trirhabda canadensis.
Toronto Wildlife – Leaf Beetles.