Steinernema feltiae

How it works

What arrives is the infective juvenile, the one stage that lives outside a host. Mixed into water and drenched onto compost, it moves through the film of moisture around the particles, enters a fungus gnat larva and releases symbiotic bacteria that turn the host's tissue into food. The larva dies within one to two days (UConn) or three to four (Colorado State); the nematodes feed and breed inside it, and a new generation of juveniles leaves the cadaver to search for more hosts.

Two limits follow from that. It needs moisture to move, so a drench onto dry compost, or compost that dries out in the following days, is mostly wasted. And it only attacks what lives in the compost — the larvae, and the soil-dwelling stages of thrips — so the adult gnats keep flying until the ones already in the pots have emerged and died. Judge it on the potato slices, not on the flies.

Is it established?

What a working drench looks like, in order. Read the larvae first; the adults lag by a generation.

1. DAY 2Fewer live larvae under potato slices — UConn says infected larvae die within one to two days
2. DAY 4Most larvae infected at the drench should be dead by now (Colorado State: three to four days)
3. WEEK 2UConn says activity lasts about 10–14 days in the media; drench again if larvae return
4. WEEK 2–3Adult catches on yellow cards fall, once flies already pupating have emerged and died (BASF: pupae take 3–7 days, adults live about 8)