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Well, as long as you spot clean any boluses, and there are no problems like mold, mites, or any other issue that might necessitate a change, then there is no real need to change the substrate much at all. Mine tend to get cozy on their substrate as they lay down silk and create burrows and such, and changing the substrate just stresses them out. For adults, spot clean any waste or nastiness. For juveniles and slings, you'll eventually change the sub when you rehouse them.
Ah, I see. I'm assuming that the stirmi enclosure is warm and moist, so there is an increased potential for mold and decomposition of organic material. Did they just appear? You may be able to remove the a bit of the substrate and waste in that area to diminish the numbers.
Unfortunately, moist enclosures make for very hospitable environments for gnats and flies, and once they get a foothold, they can be difficult to eradicate. If there is organic material in there, they could be phorid flies (are they hump backed?) or fungus gnats if the sub is too moist. Isopods are a wonderful way to keep moister cages clean, as they remove the waste and materials that can attract the flies in the first place.
I've heard some folks completely rehouse a T when they spot gnats or flies, as they worry about the risk these little buggers pose. I've also read accounts by keepers who are not particularly bothered by these pests, and only work to limit the numbers present (and don't rehouse or change substrate). Hopefully, some folks who have wrestled with these nuisances will chime in with their experiences.
Adding isopods and springtails will help combat pests in humid enclosures. If you already have alot of gnats and what not I would change the sub and add a clean up crew to the fresh sub
They are fungus gnats. No threat or concern, just annoying. They flock to moist areas, plant soil, sink drains, and the sort. You need to decrease the moisture level of the substrate and increase air flow, this get rid of the gnats. I use isopods in all my enclosures, they really are your best choice for pest control.
I have tarantulas that have been on the same substrate for more than 4 years.