How to - Pests & Diseases

How to deal with fungus gnats and shoreflies

Take control of watering and humidity levels to keep these annoying pests in check.

Many kinds of insects, especially those from the of the Diptera order can reproduce in the soil of indoor plants. The shorefly (Scatella stagnalis) and the fungus gnat (Bradysia sp.) are the most common. They appear suddenly, uninvited, and rapidly multiply. In large numbers, their presence can be bothersome and unpleasant.

How fungus gnats and shoreflies infest indoor plants

These bugs feed on decomposing organic matter as well as microscopic algae that develop on the surface of humid soil. At the adult stage, these small flies (between five and seven mm), and similar to fruit flies, will do little damage to plants. The same cannot be said, however, for their larvae—whitish or translucent maggots between four and six mm in length that emerge from their eggs in the rich and humid soil of household plants. These larvae can ravage the seedbed and impede the development of young plants by feeding on their roots. The corridors that they dig in the most tender parts encourages the growth of mould and the onset of “damping-off” disease.

Pest control

To limit the damage, the key is to reduce the amount of humidity in the soil and to then hunt down the creature.

  • Make sure to use a quality potting soil that drains well. Avoid gardening soil or black soil. For the seedbed, use only sterile soil.
  • Near the end of the summer, stagger the watering schedule in order to let the first few centimetres of soil dry out; larvae absolutely need humidity to survive.
  • Install yellow sticky traps to capture the adult bugs. You can buy these in garden centres or make them yourself by laying out glue (Tanglefoot MC) or petroleum jelly on a piece of yellow cardboard. Secure the trap on a garden stake using a clothespin. Inspect your traps regularly and change them when necessary. Spread a layer of gravel or coarse sand on the soil’s surface so that the female insects will hesitate to lay their eggs there. Do not apply any organic material at the surface.
  • Carefully clean any draining dishes and regularly remove the organic debris that fall in the soil.
  • Insert pieces of potato into the soil. The larvae will enter to feed themselves and a few days later, replace the eaten pieces.
  • In the event of a severe infestation, generously spray the soil, pots and dishes using insecticidal soap solution. Repeat the treatment several times, every five days. Any untreated parcel of soil can lead to re-infestation.

This article was translated from a version that appeared in Fleurs, Plantes et Jardins.


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