Does anyone here put calcium or vitamin powder into their silkworm chow? Would this actually be beneficial to the cham?

you probably don't want to put vitamins in the chow. silkworms are very healthy as they are already with their chow. the only thing better than the chow are fresh mulberry leaves. just putting some calcium on the silkworms would probably be easier since you wouldn't really know how even the calcium would spread in that chow. all they need is a light dusting anyway.
it also depends on how you use your silkworms as a staple or a treat? most people use crix as a staple combined with high quality gutload. depending on the gutload, vitamin supplementation is not needed too often because the gutload already has it. once a week at the most, i only do once every 2 weeks. over supplementing is something i would be more concerned with rather than under.
too many vitamins can bring about many problems, more so than too little.

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I just thought that it maybe might make it so I could do less dusting, but you are right about not knowing how much your cham is actually getting. Right now they are just a treat, but when I get my tax returns, I wanted to look into maybe breeding them. Especially cause crickets freak me out!