Predatory Nems for Fungus Gnats

  • Thread starter Soilfoodwebguy
  • Start date
  • Tagged users None


Indoor organic grow and a basic organic IPM which has worked well over the past several years however I have recently experienced a significant fungus gnat invasion which I have chalked up to using a horse manure-based compost top dress over my usual use of EWC. This along with my standard practice of maintaining consistent moisture under my mulch layer resulted in a significant outbreak.

I am on week 5 of flower so I looked at other options instead of neem oil. I first removed the mulch layer and the first couple of inches of soil from my 10-gallon fabrics. Set up fans across the pots and allowed the first couple of inches of medium to dry. I then powder applied DE across all pots soil surfaces and placed sticky traps in all containers. At first this approach knocked down the hatched gnats significantly. I then replaced my top dress with a EWC based blend and applied a new mulch layer. And they then they came back with a vengeance as the recently pupating larva hatched over a week later.

At week 2 of this challenge, I decided to bring in some warriors through the addition of beneficial nematodes. I applied Steinernema feltia at a rate of 3,000,000 active nematodes per 50 gallons of soil 4 days ago but continue to identify a medium level of hatched gnats on the sticky traps.

If anyone could share insight on these issues as perhaps, I am expecting quicker or more positive results going into day 5. Should I consider applying another round of nems after 7 days or is the pupating stage longer than the 4-5 days?

At this stage, my flowering crop is not showing any significant signs, but I want to take these bastards out before they do. Any suggestions would be appreciated.


Using this brand and species, It takes about 2 weeks, and then the flies will be knocked back to almost nothing. But there will still be the occasional fly. I have not used the species you're talking about.

Top Bottom