Not sure what are the bugs, very possible they are springtails. They 'love' organic conditions; they are actually beneficial - but in a compost heap, not in your containers. Probably because you added worm castings? and perhaps you are keeping soil too wet. If they have good conditions (and most likely they do, otherwise you won't have them), they will multiply. I also wonder about organic fertilizer.

I wouldn't discard plant because of sprigtails, but would definitely eliminate what they like.

rina, thank you. I initially thought that they were springtails too, but I looked at many videos and photos of springtails and they definitely don't appear the same. The shape and their moment is very different (these are rounder and slower).

I think it's very likely that the eggs were in the worm castings, but there are no visible bugs in the bag of castings itself. The fertiliser I used is Ecothrive Charge (a dry powder), so I don't think they originated there.

I don't think that the soil is too wet either, the plant has only been watered once since it's been re-potted (a week ago) and the top layer of soil is now dry (I had planned to water it within the next few days). The other plants that were affected (before I discarded them), I allowed to almost completely dry out, and the bugs were still there.

Do you have any advice on how I can stop them without re-potting?

Hi Louise. Can I ask where you found neem oil and Castile soap in the UK? And how you came up with the mixture you potted in and where you sourced the ingredients?

Louise

Soil could be wet deep inside, even if top is dry. it all depends on what it consists of. Best way to find out is to use bamboo or other wood skewer or chopstick, that will reach deep down into pt. Extra water accumulates there, and it could keep your soil mix wet.

It is impossible to guess what the bugs are without photo and sometimes even with photo :) I understand that it is very difficult to photo very small bugs. Could be springtails (they come in many different sizes, colors and shapes), soil mites, root aphids...and many. many more. It is possible for bagged soil to be infected, but bugs still need certain conditions they like to thrive and multiply. More organics in your mix and more water + higher temps and/or humidity, better environments there is for many bugs. Common advice is not to use things like worm castings in potting mix. They are great for gardens, not so much in containers.

(from wiki): Springtails commonly consume fungal hyphae and spores, but also have been found to consume plant material and pollen, animal remains, colloidal materials, minerals and bacteria.

ETA: I just looked up Ecothrive Charge. It is beetle poop - so you are adding more poop to your pots, together with worm poop. ..."Laboratory tests have shown that Charge consistently contains more than 450 million colony forming units of beneficial rhizosphere bacteria per gram! "...

It also states: ..."Charge is a 100% naturally-produced potting soil enhancer ..."

Soil enhancer is not fertilizer IIRC.