Here's what they looked like later. I totally lost track of instars as they got more and more staggered, but there seemed to be seven. We found that they weren't picky eaters, but prefered their Sandbar Willow to all else. After the first batches were found, we found them in mid August scattered from New Mexico to near Cheyenne Wyoming, all up and down the Front Range through Colorado. Look for Sandbar Willow, and you often find Io Moth - at least this year.

We ran out of Sandbar Willows a few times, and here is what we tried when that happened. The results were identical for larvae from Sandoval County, NM and Larimer Co, CO. A list of what they ate (and didn't) includes:
Oak assorted species (all acceptible - some were found wild on Q. gambelii in New Mexico)
Gray Alder (nibbled, but not a favorite)
River Birch (didn't touch it)
Robinia (not a favorite, but will nibble on it)
Redbud (love it)
Serviceberry (love it)
other Willows (they tasted them when young, but won't touch them when older, tried Blue-stem, Weeping, Black, Gooding, White, Peachleaf and a few unidentified mountain species)
Quaking Aspen (love it)
other Populus (including deltoides, wislizenii, acuminata, nigra, alba - they won't touch them)
Ulmus pumila (will eat it willingly, but not a favorite)
Apple (wouldn't touch it)
Prunus (would nibble on it, but didn't like it)
Rose (wouldn't eat any tried, cultivated nor wild)
Rhus, assorted species (a few nibbles and no more)
Acer (would eat it if nothing better was around)
Golden Rain Tree (sort of the same as Acer)
Mulberry and Paper Mulberry (love them)
Trifolium and Baptisia (would eat them, but it's a pain to supply enough)
Hosta (they nibbled on it, but the leaves were old and yellow; not a good test)
Grass (Would eat soft wide-leaved annual weedy types, none identified, but several species. Would not eat tough fibrous perennial species such as Ravennah and Pampas.)
Sandbar Willows (Salix exigua / interior - all time favorite, and prefered foodplant in habitat)