You have to get aggressive when controlling FG.
Control watering, no sloppy irrigation.. No excessively wet media. FG reproduces quickly if given the right conditions. Keep your greenhouse clean.
I think the more important idea is to target the FG larva, this is done with Bt or Bacillius thuregenesis treatments. AKA Mosquito bits.
Discard contaminated media, if you even suspect its contaminated, treat the entire bag with Bt.
But better to get something clean.
Whats good about BT is they cannot develop resistance to it. (EPA doc) I've tried several controls of imichlorprid/malathion/pyrethium by itself, or in rotation. They simply dont break the cycle of reproduction and the media can get recontaminated quickly. They just dont have that residual activity like BT has. Plus I'm a cheapskate. I have a few years supply of Merit 75. but how fast FG can develop resistance to it is frightening. Bt is pretty impressive in how the entire class of dipthera larva is controlled by it.
Mosquito bits when activated you can see mycelium shooting out of the bits and this is exactly what you want. My suggestion for Bt application to the top of soil is no less than 1 bit per sq/cm of soil surface dont be stingy with it. Bt is also harmless to humans and plants, in fact GMO corn "StarLink" cultivar has Bt toxins within the corn itself. Not approved for human consumption, but it has made it into the food chain.
Back to Bt control strategy.
If you are having FG problems, use Bt, Also, consider decontaminating the media itself, I've unpotted plants and washed top layers off, to remove the larva, and when I repotted, I put some on the bottom and I topped the ball off with Bt and then topped it off with clean media and applied the bits to the top.
What about the fliers?
I personally use a combination of traps and No Pest Strips. The traps are obviously an instant kill and is used for monitoring, but the NPS dont instantly kill, but I can tell that they have a very erratic flight pattern with NPS in the room. Yes I know dichlorvos are not exactly ideal but it seems to suppress them, you have to use them in a way that is not according to label directions, but when I see control I simply space them out more and more and eventually they go back into the plastic bag to be used if issues develop. In my experience NPS only covers about 5-10 square foot for FG control.
What I do like about BT vs. chemical controls is no worries about residuals, and I can use it even during flower, no worries at all. Ive seen some commercial inoculants using Bt in the formulas.