in potting mix
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?
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.
Gardener, Issue 189, 2005, Page 24
Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.