They're different than what are commonly called earthworms - usually smaller, thinner and redder. If they're teeming in your compost pile, it's almost certainly those.

But it begs the question, if you have them in your compost bin, why do you want to start a separate worm bin? There can be good reasons to do so, of course - or just for fun.

If they're surviving and thriving in the 'wild' of your compost bin though, clearly the conditions are good for them. The fact you're seeing them at certain times of year just indicates that when it gets too hot or too cold for them, they 'go to ground' (retreat to safer conditions where you may not see them) or lay their cocoons which hatch when conditions are better. No need to mess with worrying whether it's too hot or too cold.

That's safer for them, too, and how they deal with weather in nature. A bin that's 'separate' esp in hot sun can indeed kill them off (probably because there was nowhere sufficiently cool for them to retreat to and/or for the cocoons to survive). They know how to 'retreat' and may indeed go a bit underground where temps are more moderate.

The only disadvantage is that obviously they're much less active in cold. But my outdoor pile freezes solid most winters and they always come back. On the positive side it requires no effort or thought to keep the bin at correct temperatures or conditions.

>But it begs the question, if you have them in your compost bin, why do you want to start a separate worm bin?<

Because I want to generate worm casting for my garden. My compost pile doesn't seem to get hot enough to kill weed seeds. This year's seed starting effort was plagued by random tomato seedlings. So while I'll continue to compost in my compost pile, I want a source of worm castings. Are you implying there's a way to retrieve worm castings from the compost pile? If so, do fill me in. :)

If you have a compost pile, there'll be worm castings in there. No easy way to separate though. I don't think they are particularly magic, but if you want to try, by all means. At least you can do so without having to buy worms.

Not particularly magic? Huh. They seem to be all the rage this gardening season. I keep encountering articles and videos extolling the virtues of worm castings, despite their low NPK ratio.

Gardening is filled with promises and rumours of magic compounds that will make everything grow fabulously, etc. Maybe more this year because more people are gardening and stuck at home, and these fashions go in cycles. Every new group of gardeners discovers something 'new' that's as old as the hills.

I'm a big fan of worms, worm composting and worm castings. But just physically, I fail to see how regular compost mulch plus separate worm castings is going to be radically different than compost that has worm castings spread throughout because the worms live in the pile. (And unless your garden is quite small, you're going to add other compost anyway)

But hey, vermicomposting is fun and if you want to, why not. Just note that it does require additional effort and thought. And the thing I think most vermicomposters find is that the physical amount of the 'pure' worm castings you get at the end will not be very large. (And of course some of the stuff in the worm bin is not actual worm castings but other stuff that's also just compost). So I'm not at all trying to discourage you, worm composting is good, just actual miracles are few and far between.

My parents used to vermicompost and sell the worm castings at church rummage sales (for charity). Everyone swore by them as the Absolute Best Thing for their garden. But this was in a mostly urban environment where few had their own good quality compost, and most of the customers wanted to spend money on the charity anyway.