They haven't got my (Chinese Elm) leaves yet, but they remain here and there in the soil. I have used organic insecticide but they persevere. Should I re-pot? Tree was delivered last Mother's Day.

Red spider mites are not insects. Insecticide aint going to kill them. They will only be on leaves not on the soil so it sounds like you are not identifying correctly. Why would you re pot? It will not get rid of them. If the tree is in a sunny spot in the garden and the humidity is there they should go, but wiping the leaves with water will help.

Try A mixture 2 tablespoons of olive oil,1 tablespoon of dishwashing liquid, A . Goodluckmild brand like fairy liquid with about 2 litres of water,mix well and than spray, than I recommend you repeat after A few days. Make sure you drench the whole tree especially under the leaves and the trunck, this is perfectly safe and wont harm the tree or any pets and safe for you especially. Goodluck.

Thank you for your reply back! I use the All Seasons Horticulture & Dormant Spray Oil from Bonide. It seems to work. For a couple of weeks, I haven't seen one. Except just recently, I saw just one. Do they ever go away? Thanks again!

Sorry for the delayed reply but was away for A week,The best is to spray every few weeks to keep them away on the affected plant and if you use the organic mixture I gave you it is safe. I have the same problem with 1 particulare plant and constantly keep an eye on it, at first sign of them I spray before any damage is done, thats the best system I found,some plants are just more susestible to these things.

Thank you for getting back to me! I've been using the Organic All Seasons Spray Oil and I drench the soil with it to smother them. Then I wrap the pot with a plastic bag, so they don't go up to the leaves. It seems to work. My leaves are untouched. Thank goodness! I do spray the soil when I see one. I don't wait! Thank you so much for your help! Much appreciated!

Why drench the soil with soap and oil?
Really, the advice given lately is quite absurd. Spider mite does not live in or on soil. And drenching soil with soapbwater will not help you.

Find out what you have. I doubt it is spidermite if they are not on the tree and if you van spot just one. Then treat in a suitable manner. This is just flining something in the air hoping to hit.