Every autumn I bring in select outdoor plants and find annoying flying insects all over the place, I assume hiding in the soil? I believe they are whiteflies and fungus gnats. Houseplant (Yellow) sticky stakes from Safer Brand seem to work but is there something I can place in the soil?
Pretty blurry photo but I would say those are fungus gnats. There are lots of products out there for fungus gnats but the easiest one is to allow the soil to dry an inch or so between watering. A layer of grit keeps the female from laying eggs into the soil which is very effective but you must do it to all your plants to reduce the population. I used insecticidal soap in the distant past but it is only temporary so must be used for a few weeks to break the cycle. The other thing to do is ignore them as they usually disappear over time only to come back the next year. If you are doing seeds though, it is another ball game.
Whiteflies are more difficult to control in my own experience - the best thing I've found is to remove the leaves they hide under and lay their eggs on (the plant should send out new leaves in a few days/weeks). Spraying means you must drench the plant on top and bottom of the leaves and stems. I do this for spider mites too in winter (on elephant ears).
