I've gone 9 years now without a single fungus gnat or thrip.
I use no poisons, just a preventative.
Kirkland unscented dryer sheets laid on the soil surface repels pests the hunt by scent. That includes mosquitoes, cockroaches, "meat bees", and fungus gnats.
They are repelled by the surfactant waxes. The Thrips drop larvae to the soil where they go through 7 instars before emerging as adults. The dryer sheets catch, and trap the larvae.
The large white flies are repelled by a layer of worm castings on the soil surface under the sheets.
My remaining problem is WPM. I refuse to use Eagle-20.
Best, so far, is SM-90. It does not eliminate PM, but does control the blooms for a week or two. And the scent of coriander oil is much preferred to Neem.
Aloha,
Wee 'zard