Where Do Insects Come From?

I recently read the Q&A about how to get rid of spider mites and aphids on indoor herbs. I too am growing herbs indoors with HID lights, and just recently noticed that my coriander plant is completely infested with aphids. Luckily, none of the other plants have any insects on them, and the coriander has been sprayed using a chemical spray.
Where do these insects come from? Are their eggs prelaid in the soil purchased from nurseries? If so, should the soil be treated before planting seedlings?

Yes, eggs are laid in the fall, but the rest of the year aphids bear live young. Usually aphids are brought in as some tiny new-born on a plant and when it finally grows up it starts to pop out new babies without benefit of sex!

How do the commercial growers control and prevent these insects? Are they too using these insecticidal soap sprays and dipping them every two weeks? That seems like a lot of work for a commercial grower!

We spray all our plants once a week with pyrethrum soap spray and that keeps the numbers down. Flooding a greenhouse with lady bugs controls aphids very well. We use that method in June.
Other commercial growers use Enstar a juvenile hormone to control aphids- it works quite well and is apparently quite safe.