It turns out, it takes about 2.5 weeks for the larvae to hatch (which is consistent with the literature). I first noticed they hatched when I saw ants congregating around the egg mass locations, pulling the newly hatched larvae out (see photo below).All this activity on the aspen did not go unnoticed. Soon, bald-faced hornets moved in, displacing the ants, feeding on the larvae. And, if you recall, this is where the whole story began, with bald faced hornets chewing bark to mix saliva to make their nests.
It did not take long before any of the surviving larvae burrowed deeper into the cambium, protected from ants or bald-faced hornets. However, still shallow in the trunk or branches of the aspen, they are easy prey for woodpeckers. Last night a hairy woodpecker showed up. You can see in the photo at the beginning of the post and below this male hairy quickly drilled a hole into the aspen and spent the evening eating the borer larvae.