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Hi I’ve just noticed my bonsai soil has fungus gnat larvae In it. little white worm type with little black head. It seems to be damaging my Chinese elm.

Do I repot in new soil or use some sort of spray to kill them off?

They are not gnats. the gnats do no harm. It is something else if it is causing damage

I have no experience with problem to the plants due to gnats. However a few resources online claims that the larvae chew on young roots.

"Fungus gnats are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots and can be a problem in greenhouses, nurseries, potted plants and interior plantscapes. Adult fungus gnats may emerge from houseplants indoors and become a nuisance."

From what you describe it could very well be fungus gnat larvea. These are thin worm-like animals with a black head. If you also have the "running fly" adults in your pot then yes..

Trick is to let the soil dry out more between watering. Which is tricky as you will get near the welting point of your plant. But that is how I get rid of them. To really let the top of the soil dry out. I do get rid of them because, especially with young seedlings, these can damage the roots with dead seedlings as a result.