I don't know if anyone is still interested in this, but I'm actually doing to be undergoing a similar project here soon! I've read several papers that mention using mealworms to degrade polystyrene, with notes that over 50% was completely destroyed, the remaining 50% is passed through, more or less, with a minimal amount of carbon from the plastic actually being retained. I found references that the mealworms can be used for other plastics as well, so my idea is to try and quantify how much the worms can eat, as a percentage of their body weight, possibly throughout their growth process, as well as see if they can do their entire life cycle on only plastic.
Superworms grow much bigger, but i remember reading that they actually are less efficient at it, though if mealworms can't do a full generation (beetle to beetle) on plastic, the superworms might be a better option just by virtue of lasting ~2 years as worms instead of mealworms constant pupating. The article I link below says that beetles were produced, but they didn't see if they were reproductive, and I had found others that said they had none survive till pupation. Some places say you need to acclimate worms on a rice diet first, then introduce to plastic. If my initial test doesn't work, I'll try inserting this step in before doing any significant changes.
(link for mealworms eating polystyrene: https://pubs.acs.org/doi/10.1021/acs.est.5b02661
The citations for it can lead you down an interesting path. One thing to note specifically is the mealworms themselves only physically break up the plastic, it's the microbes in their gut that actually process it, something proven by smearing mealworm guts over a petri dish and measuring the micro-divots the microbes ate. but the microbes by themselves are nowhere near as efficient as mealworms, likely due to the chewing up of material.
To test for microplastics in their waste, (of course i can't find mmy saved link here...) I'm going to use a simple set up of pvs with valves that works basically by making the plastics float. for heavier plastics, I'll have to use ZnCl I think, but for most just saturated saltwater should work. for this I'm not going to be too fusseed about following exact scientific handling, since for the most part it's going to be a yes or no. if yes, I'm going to use filters to try and catch enough fo the microplastics to get a sample size big enough to see if specific plastic eating microbes can handle it from there.
(this is not the way I was going to do it, but intrests me: https://www.sciencedirect.com/science/article/abs/pii/S0048969720328552
Ideally, I want to find out if using mealworms to eat plastic would result in safe to use frass, mealworm meal and happen with a minimum of input.
Sorry this is all sorts of rambly but as soon as weather in my area gets below 80F consistently, I'm going to order some mealworms to kick the testing off =)