It make take a little while to get your mealie colony off to a start. I remember when I started my mealworm colony almost 4 years ago it took a little while.
IMO I dont think you need any cardboard. Just forget about that. All I use is my substrate which is the gut-load and plastic boxes, and of course the beetles. 
I have a care sheet on my website about breeding the mealworms if you are interested in looking at it. 
Over time you will begin to see baby mealworms moving about. They will be extremely tiny but if you look closey and dig about in the substrate you'll find them. 
Or if you dont see anything by now maybe you dont have any males, or females so they cant reproduce. I'm not sure how big your colony is or how many beetles you have, but it was just a suggestion. 
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Shayna
www.LeoLoversLizardLounge.com
