First of all, welcome to the forum. As you will soon find out beekeepers are very opinionated, you will get half a dozen answers just by asking three keepers one single question
Is it necessary to treat again in the spring?
Depends on your mite count...
How do I determine varroa levels in each hive?
It is best to monitor your mite drop levels by using a screened bottom board, with a removable tray under the screen, upon which you can apply a layer of vegetable oil to trap the mites in. I don't recall how to calculate the amount of mites in the hive by the number of ones that are on the sticky board, but you can use the search button at the top of this page to search through all the previous posts for a certain word or phrase that you my find to have been discussed already.
Someone mentioned putting paper towels soaked in vegetable oil on the bottom board.
The white background will help to contrast the mites so you may see them better, and the vegetable oil will coat the mites and suffocate them.
Since your interested in chemical free hives, you might want to search through the forums for "Small Cell" discussions. You will find this topic to be quite controversial at times, but some of us are using it with much success, and no chemicals.
I personally don't believe that chemicals are at all "necessary", they are just evil.