Of course, they were still alive.
There were hundreds of them. Overnight, they crawled out and covered the bin with webs of silk.
That pale brown isn't the plastic fading from sunlight, it's the mass of webs.
I gave them a dose of fly spray. It turns out that they really don't like it, so most of them are now dead. However, my wife stopped me before I did the final side of the bin, in case I killed any bees that might have been nearby. As a result, the caterpillars may have been able to reach the ground along the single-line vertical web they'd built. I guess we'll find out next year if our roses are covered in the little blighters.
Do your job, birds!