When they are beginning to pupate in to the alien-like pupae, they will sort of become chubby and inactive. If you handle them it won't affect them in any negative ways. Mealies don't have coccoons, they turn in to beetles, not moths
They go from worm -> pupae (aliens, or as my sister got me in the habit of calling them, fetuses
) -> beetles. The amount of time they sepdn in each depends on the temperature they are kept at. They will take longer at cooler temps. It all depends whether you want to do the seperation and rotation of all the stages. I used to keep them all in one big container and never had any problems, but in the past few months production seemed to almost stop, so I began seperating them and the colonies were booming again. Sometimes it works, sometimes it doesn't.