Many club members bring their indoor bonsai inside in September - before the furnace in the house comes on. I prefer to leave my indoor trees outside as long as possible. Before bringing your trees indoors spray them with an insecticide. Make sure they are insect free. I also use a systemic insecticide several weeks before I plan on bringing them indoors.
Some bonsai enthusiasts feel that no fertilizers are needed after later summer.
The use of fertilizers with a high nitrogen content is detrimental to the health of the tree. You want to avoid developing new shoots that will not harden before winter. On junipers add a small amount of agricultural lime to promote good color next year. Insects are as busy now as ever and diseases are ever present. Spider mite population can be controlled with Kelthane.