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Pests & Problems Part 3 - Sciarid Flies

Pests & Problems Part 3 - Sciarid Flies

Sciarid flies are also known as "dark fungus gnats". They live in and around the compost of your pots and like damp, moist conditions.

Sciarid flies

Whilst the adult flies are just an unsightly nuisance**, the young larvae can damage your chilli plants by attacking the root structure.   Mature plants are unlikely to be affected but the larvae tunnel into the stems of seedlings and young plants.  Lower leaves drooping onto the compost will also be eaten.  This can cause plants to wilt or lead to poor growth. At worst, it will kill them off.
**The adult flies can transmit diseases so worth removing them all if possible.

The adult flies are about 5mm long, black with long antennae. They can be found around the compost surface. Their larvae around 1cm long, legless and translucent  with dark, shiny heads.  They are harder to spot but can be found just under the soil and around the roots
Each adult female can lay up to 300 eggs after mating, so if you have Sciarid flies, you need to control them.

The traditional method of controlling Sciarid flies is by hanging sticky traps. However as we said in our Aphids article, we would strongly suggest NOT using these traps as you are likely to trap and kill the very insects which you actually want  to attract.

Basic control  : 
  • Do not overwater your plants
  • Remove any infected plants from your growing area
  • Practice good plant hygiene by removing any old, dead leaves and fungal growth from the top of pots.
  • Improve ventilation in your greenhouse

There are four main biological controls which can also be used : 
Hypoaspis miles
1) Hypoaspis miles - A ground-dwelling predatory mite which feeds on Sciarid fly eggs and larvae. These are also known as flour mites. Whilst not great for your self-raising, they are brilliant for controlling larvae.
2) Steinernema feltiae - Normally used to control the larvae of the Crane Fly, these Nemataodes are also effective in controlling Sciarid fly larvae. Water them into the soil of infected plants and they will infect the larvae with bacteria and kill them
3) Coenosia attenuata or 'hunter fly' is a predator of various pests, including Sciarid flies, Shore flies, Leaf Miners and Whiteflies.
4) Atheta coriaria or "Rove Beetle" is a predator of Sciarid fly larvae and thrips
(For biological controls we recommend www.ladybirdplantcare.co.uk)


Other ways of controlling Sciarid flies include : 

1) Vermiculite - A layer of Perlite, Vermiculite or fine grit on the compost around the stems of susceptible plants will also help to reduce sciarid problems.  It provides a dry barrier that prevents Sciarid flies from laying eggs into moist compost which is required for them to hatch.
2) Garlic candles - Fumigating a greenhouse at the end of the season with a sulphur candle has been the traditional way of clearing pests.  However we recommend Natural Garlic Candles instead.. They are used in the same way as a sulphur candle i.e. simply place in your greenhouse, light the fuse and leave overnight. HOWEVER, unlike sulphur candles you do NOT need to remove the plants before using and they are safe to use in aluminium greenhouses and polytunnels. Garlic candles will not taint any fruit and / or vegetables.
3) To deter Sciarid flies, Cedar wood chips can also be used.  Fill your plants with a small layer at the top of the compost.