I tried that, but nothing happened. I think The grubs might have disappeared, but they reappeared again at some point. Also tried leaving the room and coming back. I must have missed something. I'll try again after playing the other games.
Well I just played through it start to finish and the grubs were caught as intended. :) So it's definitely working. Not a bug which I was worried I'd somehow introduced.
When you drop the Moss in the alcove look again just to make sure they're nibbling the Moss. ;)
