Fungus gnats on ZZ plant: how to identify and treat

How to identify fungus gnats on ZZ plant

Adults: small dark flies that flit around the pot and the air just above it. Larvae: tiny translucent worms with shiny black heads, found in the top layer of wet potting mix.

Damage signs to look for

Slowed growth in seedlings and young plants, mushroom-like off smell from the soil, occasional yellowing of lower leaves on small plants. Mature plants tolerate gnats with little damage.

Treatment ladder

1. 1Let the potting media dry between waterings, especially the top 1–2 inches — Colorado State notes this reduces egg and larval survival and makes the soil less attractive to egg-laying females.
2. 2Apply Bti (Bacillus thuringiensis israelensis) as a soil drench (mosquito bits work) — it kills larvae. Bti doesn’t persist indoors, so UC IPM notes repeated applications at about five-day intervals may be needed.
3. 3Use yellow sticky cards near the pot to trap adults and monitor population.
4. 4For severe cases, repot in fresh, well-draining mix and dispose of the old soil.

Prevention going forward

• Avoid bagged potting mix sitting open in a humid room — it can come pre-infested.
• Use a well-draining mix with chunky perlite or bark so the surface dries fast.
• Bottom-water plants prone to gnats so the surface stays dry.

Life cycle

Egg to adult takes three to four weeks at typical room temperatures (65–75°F). During their seven- to 10-day life span, females may lay up to 200 eggs. Larvae usually stay in the top 2–3 inches of media, which is why surface-drying breaks the cycle.

Source

Identification, treatment, and prevention guidance per Colorado State University Extension — Fungus Gnats, with other figures from UC IPM — Fungus Gnats as named in the text. Always follow the product label when applying any pesticide or horticultural oil.