Answer: It really isn't all that unusual to find insects in potting soil. Generally they are harmless to plants and to people; they feed on decaying organic debris, which is what is in the potting soil to begin with. If you find insects, and you don't want them in your houseplant soil, the best action is to unpot the plant, rinse off all the old potting soil and repot in fresh, new potting soil. This will effectively get rid of the insects.
Bifenthrin is a synthetic pyrethin and is very effective against soil-borne insects. It should not harm your plants. I suspect that your new plant is developing dry ends because it is trying to adjust to the light levels, temperature and humdity in your home, which is probably different than those where it was previously growing. You can isolate the plant (keep it away from your other plants) until it recovers and then set it out with your other houseplants.
Best wishes with your new plant!
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