FUNGUS GNATS – Why are they so difficult to control?

Anybody who owns a house plant is keenly aware of these annoying black flies that hover around the soil surface. In small numbers they are an irritation, but when an infestation develops they can make your house an unpleasant place to be, and severely damage your house plants’ roots. For commercial growers of pot plants, fungus gnats are even more bothersome as they harm profits by reducing growth rates and plant quality.

While fungus gnats are a major pest of both amateur and commercial horticulturists, no silver bullet cure exists for controlling them. In this blog post I explain what are the main barriers for control, and then reveal our new take on a solution that is more holistic and biological in its approach.

Barrier #1 – Subterranean enemy

While we associate fungus gnats with them flying close to the soil and circling plant pots, they actually spend the vast majority of their life cycle in the growing media (soil / compost). This is a major barrier to controlling fungus gnats.

Why? because the soil interferes with any attempt to use pesticides to kill them. It does this in a number of ways:

• The soil acts as a barrier preventing the pesticide solution reaching the eggs and larvae (maggots) which will be huddled around your plant’s tasty young roots.
• The water already in the soil dilutes any pesticide application, thus reducing its effectiveness further.
• The microbes and complex natural compounds in the soil will deactivate the active ingredients in the pesticide. This is normally a good thing for a pesticide as it prevents soil and ground water contamination. However, when trying to kill a soil-dwelling pest it is a disadvantage. As such, many pesticides applied to soils will quickly be deactivated (some will be inactive in well under 24 hours).
• Any pesticide that is highly effective and persistent at killing insects in soils is unlikely to be authorised for use by governments. This is because the opportunity for poisoning the soil, it’s biology, the wider environment, water sources, and the surrounding human population is usually deamed too great.
• The soil delays you detecting the problem until the infestation has established. A female gnat can lay hundreds and hundreds of eggs. However, unlike pests that eat leaves, you will not be able to detect the eggs and maggots. Instead you will have to wait until either the plant starts to suffer (wilting), or the adults emerge from the soil surface. The resulting hundreds of adults will themselves be on the hunt for fresh soil to infest! In fact, even before the adults emerge, they could be laying more eggs before they reach to the top of the plant pot and you see them.
• As the eggs and larvae live in soil/compost, you can actually be the reason for bringing an infestation into your growing environment. This is because most bags of compost will contain fungus gnats already; especially if they have been stored outside exposed to the elements. Hydroponic growers using grow tents know this fact only too well. These growers use grow tents with fine mesh filters to prevent any flying insects from entering their grow rooms. However, they will invariably suffer fungus gnat infestations when using coco fibre (coir) as a growing medium as the fungus gnats will be brought into the grow room at the start of the grow in the bigs bags of coco fibre.

Barrier #2 – Not a single enemy

To the untrained eye all fungus gnats look the same, but there are actually 1700 known species worldwide. Furthermore, it is estimated that there could be as many as 18,000 more species yet to be described! These all belong to the insect family called Sciaridae, which gives rise to the other common name for fungus gnats, ‘sciarid flies’. While they are all different species with differing biologies and morphologies, virtually all of these species will be a pest of young delicate plants.

However, control methods so far used have treated all fungus gnats as one species, and tried to shoehorn in a ‘one size fits all’ approach to their control.

The wide number of species of fungus gnats also means that no matter your local environmental conditions or the particular plant you culitvate, there will always be a species of fungus gnats ready to colonise your soil.

Barrier #3 – Fungus Gnats don’t bother farmers

While a severe annoyance to house plant growers and horticulturists, fungus gnats have no economic impact on arable crops on farms. Arable crops are the major revenue streams for multinational pesticide companies, and as such, that is where they put all of their funding for research and development. This is another reason insecticides targeted against fungus gnats, and more importantly, effective against fungus gnats are non-existing.

Barrier #4 – Plants love wet soil, so do gnats

The first thing people tell you when you look for methods to control fungus gnats is ‘don’t keep the soil moist’. However, if you want optimum levels of plant growth, this is not an option. In fact, in some crops like tomato, not keeping your soil well-watered is a recipe for even more damaging problems, such as ‘blossom end rot’.

How can Eutrema’s help?

At Eutrema we manufacture a product called Lime Sulphur. This acts as a soil sterilant. Once a diluted solution is applied to the growing media generated hydrogen sulphide gas in the air pockets. This gas is poisonous to all soil life except the plant roots. This hydrogen sulphide smells of rotten eggs and you need to wear a Type B filter mask when using, however, Lime Sulphur quickly breaks down to harmless calcium and sulphur plant nutrients. Because of its non-residual action, it is not considered a pesticide and can even be used on organic farms under EU regulations.

We have seen good results against fungus gnats on house plants using Lime Sulphur, however it obviously will not kill the annoying adults flying around your plants. Therefore we still recommend you pair its use with the use of yellow sticky traps, and apply regular doses at weekly intervals to kill eggs and larvae added by the flying adults after the first dose.