crickets not critters. Any crickets let in the cage over night can bite your dragon while he sleeps. They can chew large wounds over a period of time which is why it is not recommended to leave uneaten crickets in the cage.
Mites could come from other dragons, especially any recent additions or already been on your dragon when you bought him. Typically you can see dark small oval bugs crawling on your dragon, in the substrate or if you soak your dragon for about ten minutes or more in luke warm water, you can see them in the water (they drown and fall off).
The spot where you say there is a scale missing..is it red or just normal skin color? Any sign of bleeding or scabbing? It could be nothing at all too.
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PHLdyPayne