Plant Doctor Archive

Maggots in potting mix

I'M HAVING trouble with the roots of seedlings in my greenhouse being eaten by small maggots in the potting mix. What are they and what can I do?

IF THE maggots are 3-5mm long with a whitish body and dark head they're almost certainly the larvae of tiny twowinged flies known as fungus gnats or sciarid flies. The adult female fly lays eggs in wet potting mix and the larvae feed on microscopic fungi and decaying organic matter in the mix. They thrive in warm, moist greenhouse conditions and are seldom a problem outdoors. They usually don't harm plants, but if, for some reason, seedlings or cuttings are already weakened, the larvae can seize the opportunity and feed on tiny roots, making an already bad situation worse.

The presence of fungus gnats is usually a sign the potting mix is too wet. One of the best ways to avoid the problem is to use a very free-draining potting mix and allow it to dry out somewhat between waterings. To control existing larvae you could drench the mix with a solution of an insecticide such as Carbaryl or Maldison, or use a diazinon-based product like Liquid Diazinon or No Insects Lawnguard Prills. Try it on a small number of plants first in case some seedlings have an adverse reaction to the chemical.

The yellow sticky traps sold in the garden centres to help catch whitefly are also quite good at catching the adult flies, which should reduce the number of eggs laid. Tie the traps to stakes or hang them just a few centimetres above your seedlings.

Some commercial nurseries use a biological control system, which involves releasing a predatory mite into the mix to feed on the fungus gnat larvae, but unfortunately it's not available in small packs for home gardeners.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.