This was crawling along our wooden retaining wall mid-July here in Eastern Ontario.
This beautiful caterpillar is an American dagger moth larva.
This insect is found east of the Rockies. Larvae feed on leaves of alder, ash, birch, elm, hickory, maple, oak, poplar, walnut, willow and other deciduous trees.
According to Covell(Peterson Field Guide to Moths) this is the largest dagger moth in the East.
The straight line of the black tufts are those of the cottonwood not the American dagger caterpillar.
The cottonwood dagger feeds on trees of Populus sp trees such as trembling aspen, balsam poplar, and cottonwood, but may also feed on birch and willow.
These trees are found near riverbanks and floodplain areas. Cottonwood daggers are found all over North America tho not commonly seen.
Tho this dagger caterpillar is not listed as one of the known stinging caterpillars, and it is not as commonly seen as the American dagger (which do have stinging hairs), it would be best to NOT handle this caterpillar.