Depending where you live. Gnsts can't be an issue. Or where you get your soil.
Mainly it comes from inert larva that lay dormant already in your sub., and you've no way of knowing. So you must treat 100-200L of sub by either baking your soil, (methods on line, or boil treat your soil with hot water, or a hot bath/shower water. Which all takes time. And then, you might still end up getting them especially if you live in a humid country were they thrive all Yr round. So, it's not anyones fault.
It's not as though they ring the doorbell and you invite them to snack on your fungi or lay eggs in your soil.