What can I do to get rid of big white grubs from potted plants

you have to re-pot them in fresh soil.What soil did you use to begin with ?I have never heard of grub in houseplants.Did you use soil from outside.http://homeguides.sfgate.com/rid-grubs-planter-boxes-80452.html

Ask at your nearest quality store that sells plants and lawn treatments. There are chemicals that will kill the grubs, but you want to be sure they're safe for your plant.

This question about "curl grubs" was answered on another website that I found. I do think that you should get a true diagnosis though. You should have a Cooperative Extensive office somewhere in your area. Ours is run by our county government but I think it's a federal program. If not, look for someone who can identify plant bugs - maybe there is a local nursery that is open. I don't know where you live so I don't know if it's winter and they are closed. If you live in a warm area, they should be open. Read the link below if you think they are "curl bugs" to see what they have to say. Wishing you the best & Happy New Year!

https://www.yates.com.au/garden-expert/answers/pests/7551-curl-grubs-in-pots-how-to-control

Sprinkle soil with Sevin Dust, then water. This has always gotten bugs out of my plants.

If I had grubs in my pre-bonsai plants, I'd remove all soil from the pants and repot. If your soil harbors grubs, who knows what else it is contaminated with. It sounds like you have a long term project going and I'd not want anything to hinder that.

Sounds like might be vine weavil grub , there is a solution you can get from the garden store. I would remove all grubs you can see because if you don't all the time they will be eating your plant roots and will soon turn into adult beetles and start laying more eggs that will turn into grubs over and over again...........