It is thought by some that the larvae eat root hairs, thus damaging your plant and disrupting intake in the rhizosphere. Others believe the larvae are simply competing for nutrients with the roots. There are probably more ideas, but those the ones from the top of my head.
Last time fungus gnats tried to move in...
- Used extra DE powder for pool filter* and covered the top of the growing medium.
- Cut way back on watering. Waiting for the plants to wilt before they got water. Fortuitously, the strain wouldnt wilt for ~10-14 days.
- Turned the fan up a little. The plants could handle some extra wind, but I don't think the flying gnats could that easily.
- Pest strips. Manage to get passed the hurricane and the DE? GL escaping glue.
*It's commonly said that pool DE is a lower and grade (less CaCO2, more SiO2) and should not be used to fight gnats. I went ahead and did it anyway because I was a lot of the shit laying around, and I figured "extra SiO2 can't hurt; it's ubiquitous in the earth's crust/soil." I'm not usually a 'ends justifying means sort of person', but the pool DE powder along with the other steps worked.