You may encounter a fungus gnat infestation if you use any form of manure in your soil mix or you could easily carry them in from outside. The sticky tape works great for the flying versions of these tiny two-winged critters, but you should use a powdered Bacillus thuringiensis product to get them in the soil mix in their larval state; apply a very small amount to the top of your soil in all containers and anywhere else you store soil that is in use recycling or ready to use.
Your soil simply has to be inoculated with Bacillus thuringiensis, and it only takes a minimal amount to do so. Simply sprinkle a little into the top soil of each pot then spray it into the soil with a hand sprayer. The significant drop in the fungus gnat population will take about two weeks to notice, but the good news is that because Bacillus thuringiensis is alive, it will stay around and work indefinitely.
A powdered Bacillus thuringiensis available from Bunnings
Now that the gnat larvae are gone, the Bacillus thuringiensis appears to leave the soil as well, so you may easily reapply it if you see any symptoms of the gnats returning in the future. Another approach is to combine about 1/8th of a teaspoon of Bacillus thuringiensis powder with about 1 litre of fresh water and spray every plant in your garden with a small amount of this water. Very simple and very effective.
While the Bacillus thuringiensis takes care of the gnat larvae (in the soil). Hang a few of those sticky and twisty flypaper things to help knock out the flying versions.
For things like spider mites, you can utilise living predators, but the difficulty is that the predators that eat the spider mites can’t withstand conventional grow room environments, which are typically hot and dry. Predators prefer temperatures of less than 23 degrees Celsius and more than 60% humidity. Spider mites, on the other hand, prefer hot and dry circumstances.
Before you buy any predators, make sure to think about these things. These bio-predators operate considerably better outside (or in greenhouses that don’t get too hot).
Also, don’t overlook the intake vents. To prevent rogue parasites from being sucked into the garden, these will need to be screened off (if they draw air from the outside). Nylon stockings have remained a favourite for many still to this day. They allow airflow for passive intakes while also acting as a filter.