I am pretty new at this so I am wondering what you mean when you say the beetles aren't feeling right.
Hm...the best way I can think to put it is that when you have a colony of whatever--chickens, goats, rabbits, mealworms, redworms--going well, you tend to 'get the feel' of things going well and smoothly. I'm not getting that from my mealies, and I can't figure out why.
Maybe it's just the learning curve..?
Re Giant Mealworms:
When I started, I accidentally got a batch of some 75 'medium untreated' worms that just grew...and grew....and GREW. Huge pupae, huge beetles...but once I saw the same progression in *guaranteed* untreated mealies, I knew they had to be the treated ones, and despite my desire to have more BIG BIG mealworms through breeding, fed them out. You can't select for an artificially-enhanced trait, darn it!
The chooks just loved the treat, and I've been growing the 'regulars' since.
As for selection for larger...I've been wondering about that, too...get a batch all the same age and cherry pick the ones that develop fast/large out into their own container, then grow them out for breeding, and do the same again. Have to do it with lots of recordkeeping, though--need to know if you're making actual progress, after all.
You know...at X days these were 'this long' in the first batch; at X days the second batch was 'that long'; and at X days the twentieth batch was....OMG RUN!!
