After bringing my houseplants back indoors when summer ends, tiny flies will appear after a week or two. Is there a insecticide solution that can be watered in and not harm the plants?
Bringing Houseplants Back Indoors at End of Summer
Certified GKH Gardening Expert
This sounds like fungus gnats. They are not harmful in any way, as they only eat dead or rotting material. They can be a bit of an annoyance, but they don't venture out of the soil often. The only time you will see them is if the soil is too wet, which forces them to the surface. I would recommend watering much less indoors.
You could even boil some sliced hot peppers in the water, then strain and use this water to mix rosemary oil in. This will prevent any insect from entering the soil.
Here is a link for more ways to create your own pest control: https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm