I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
You can't sterilize the soil with plants growing in it. However if you have loose potting soils you can place them in a clear plastic bag about 4 inches deep and put it in the sun for a couple of days which will usually kill any bugs in the soil. There are some insecticides that can be drenched through a plant's root system to kill insects but such applications are usually not necessary as most bugs in potting mixes are not attacking the roots.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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