How small are they? Are you sure the larvae was from these insects that you see or have you noticed anything else (like flying insects)?
Worm castings are great. They impart nutrients, microbiology and organic matter. Where-ever there are microbes and organic matter other organisms will want to show up to live and feast, including pest insects and harmless arthropods. Virtually every organism in the outside world has a predator, including pest and harmless insects. Organisms which are harmless to plants might actually be predating on the organisms which
would harm plants. Your grow room isn't much like the outside world, in the sense that an indoor environment most typically lacks the bio-diversity that we find in nature. This is of great advantage to any insect that finds a suitable environment like your grow room because there simply wouldn't be so much (if any) competition and no variety of predators (if any) to keep populations in check. As a result the populations of any organism which finds such suitable conditions will very likely explode.
So, there is a possibility that what you have is a pest insect but it is also possible that the bug is actually predating on pest insects or at least isn't doing any harm. You'd have to identify the bug in question. There are lots of resources that can help you with this, there are also lots of possibilities. It helps to narrow it down by getting a good description of your bug in question: size, color, general appearance, apparent behaviors, etc. If it has jointed limbs, a segmented body and an exoskeleton then it is some kind of arthropod. If it is less than a couple millimeters small it would be a microarthropod.
Some microarthropods (mites, springtails) are among the more desirable\tolerable insects to have in an indoor grow room; you usually don't notice them. Many soil mites are harmless (they are eating dead organic matter, microbes, or pest insects).
http://gardening.about.com/od/gardenproblems/a/GardenInsects.htm
Keep in mind there are some really good bugs out there. These are my Hypo-defenders: shiny little orange saviors that hold a perimeter and eat fungus gnat eggs and larvae (other insects, too).