Since BTi does the job, using chemicals (especially a systemic) for fungus gnats is like using a gun to swat a fly. WAY more power than needed.
Imidacloprid is very controversial stuff. I don't want it in my yard, and definitely not in my house, especially if there is a completely organic way to address the issue.
http://npic.orst.edu/factsheet...
http://www.motherearthnews.com...
https://www.beyondpesticides.o...
One can also just repot to get rid of them. Unpot the plant, rinse the old soil off of the roots, wash the pot if re-using to make sure there are no eggs stuck to it, replace plant with new soil.
If one is using a chunky mix, bottom watering isn't an option, the water won't wick up. Bottom watering can also lead to a build up to toxic levels in the soil of chemicals like fluoride, chlorine, and minerals like lime. If gnats can reach drain holes, they will lay their eggs there.
