Darkwinged Fungus Gnats

Fungus gnats, also known as mushroom fly, the literal translation of their Swedish name would be “grief mosquitoes”. And yes! These flying little marauders for sure will cause you a lot of sorrow! They should not be confused with the yellowish small fruit flies, which are often seen in the kitchen over the fruit platter. Fruit flies are easily caught in a mixture of water, detergent (soap, washing-up liquid, etc.), and vinegar in a glass. The fungus gnats, however, are playing in a completely different league that requires significantly stronger measures for extinction than a simple vinegar trap!

I often hear comments like ”I never had problems with fungus gnats and I have been growing indoors for 30 years!". To those people I say: Congratulations! Sincerely, congratulations to you. But the day you get an invasion of these little buggers, that totally ruin the carefully planned seedlings, you might want to read this article.

ABOUT DARK WINGED FUNGUS GNATS

This is how Sciaridae - Darkwinged Fungus Gnats work. The gnat itself is about 3-4 mm long, black with transparent wings and two antennae. There are many different species and some may have different colors and appearances. It is characterized by its peculiar, jerky, and incredibly annoying way of flying. It is not a particularly good flyer for longer distances, but it is skilled at running fast on the earth's surface. It thrives very well in moist, fresh peat soil. The mosquito lives for about 1 week and during this time can lay about 70-300 eggs. The eggs are laid on the soil surface in rows of up to 30-40 eggs per row, preferably near the stem of a plant.

The eggs are extremely small, oval, white, and a bit semi-transparent. The eggs are very durable and can withstand freezing. They hatch after about 4-6 days and become larvae that feed on decaying plant parts, fungi, roots, or seeds. Indoors the larvae feed on our seeds and our seedling's tiny root parts. They then eat their way into the stem of the plant. The larva is initially small but grows to a size of about 6mm. They are white-transparent and if you look very closely you can see their digestive tract through the skin. The head is black. The larva spends about 12-14 days feeding and then develops a pupa of silk and other available material. The pupa hatches after about 4-6 days. The whole cycle from egg, to larva to a new gnat, depends a bit on temperature and species, at about 20 degrees the cycle is about 22-30 days. Fungus gnats thrive less in environments where the temperature is above 22 degrees. An interesting parenthesis is that all offspring of a specific gnat tend to be of the same sex, i.e. females or males.

Sometimes the larvae of the fungus gnats are known to aggregate and migrate en masse, forming a snake-like line crawling atop each other. This phenomenon has many names, sciarid snake, among others. The snake is about 2-4cm wide and can be as long as 3 meters! In the past, it was considered dangerous to touch it, and it forebodes bad luck and war.

FUNGUS GNATS IN LARGE-SCALE CULTIVATION

In large-scale greenhouse cultivations and mushroom cultivations, fungus gnats are a major problem, not least financially, as the larvae can do great damage to the cultivation in a short period of time. The problems are often greatest during winter and spring when the temperature is most favorable, i.e. below 22 degrees. In areas with nearby natural areas, lawns and soils with a large proportion of organic material, especially during periods of rain, can appear in large numbers. Preventive measures are used, such as controlling irrigation, using cultivation substrates other than peat soils and sticky fly traps as a control. There are also many pesticides against fungus gnats, which are also widely used. In large-scale cultivations, it has unfortunately shown that fungus gnats have developed up to 47 times resistance to pyrethroid insecticides. Instead of pesticides, you can use nematodes. Nematodes have been shown to fight fungus gnats to 85% in commercial fungal cultivations. There is also a predatory mite and a bacterium available on the market that is comparable to the nematodes in effectivity.

FUNGUS GNATS IN SMALL-SCALE CULTIVATION - INDOORS

Fungus Gnats are common in nature but are usually not a problem as there are plenty of natural enemies and a lot to eat outdoors. Indoors, however, we do not find plenty of natural enemies and nothing else to feed on other than our tiny seeds and seedling.

Often eggs already exist in the peat soil we buy in sacks and they are very difficult to detect. Fungus gnats are particularly fond of peat soil, which is often used in soil production. When the soil is taken indoors, you sow and water, a favorable environment is created and the eggs hatch after a few days.

The larvae of the fungus gnats do not only eat roots, it also eats peat, fungi, seeds, etc. The larval stage lasts for about two weeks. After that, the larvae pupate, followed by the gnat sometime after. The gnats are attracted by light and you often notice them first by a window, hovering over seedlings or potted plants. Once you discovered the flying gnats, you most likely already have a big problem. The gnats themselves do not pose a danger, but before the gnat stage, the larvae were feeding for about two weeks. And that is the real problem. Larger plants and potted plants usually survive an attack, but it is worse with small plants. You notice that the seedlings are not doing really well and begin to wither. If the attack continues, the plant will die, since it has no root system any longer. If you notice that a plant has been attacked, it is not enough to just treat that plant. All plants in the same room, within flying distance, must be treated. If a fungus gnat colony gains a good foothold indoors, the reproduction takes place continuously and overlaps with eggs, larvae, and gnats in all stages at the same time.

It can also be the case that you think that some seeds have a poor germination rate. But that the seeds have in fact been eaten and didn’t even have the chance to germinate. Tomato seeds generally germinate fairly quickly and usually have time to germinate and sprout before the gnats get started. It is worse with seeds that take longer to germinate, for example, chili, peppers, and eggplant that have a germination time of 10-30 days. If there are eggs and larvae in the soil, the seeds can very well be eaten before they have had time to germinate.

PREVENTIVE MEASURES

So what can be done do to avoid problems with dark-winged fungus gnats? Heating up, or sterilizing, your sowing soil and potting soil before sowing is a good start. The eggs are very robust and can withstand freezing. But they cannot withstand heating. There are many ways to sterilize your soil.

• In the oven on a plate, baking bag or other.
• In the microwave in micro-resistant plastic boxes.
• On the grill outdoors.
• On a fireplace.
• Any other way of choice. The important thing is that the soil is heated to about 120 degrees Celsius for about 30-40 minutes. Then it is guaranteed egg-free.

We use a large baking tin in the oven, pack it thoroughly so as much soil as possible is sterilized in each run. Turn on the oven at 120 degrees. When it is hot, set the soil and let it bake for 30-40 minutes. Personally, I am not afraid of soil management and food handling in the same place. I am very careful and of course clean everything thoroughly afterward. This is of course up to each and every one. If you are uncomfortable with soil in the oven, maybe it fits you better with a microwave or grill? Alternatively, use a roasting bag in the oven if you do not have a microwave or grill. After the soil has been sterilized, it is often very dry. Water and mix until the soil is moist enough for sowing. If you sow in dry soil and water afterward, seeds and soil will float away. An additional recommendation is to be more careful when handling soil with manure from animals than you need to be when handling peat soil with sand/mineral/perlite etc.

A common protest against sterilizing soil is that this would "kill" the soil and all the important microorganisms. Unfortunately, it seems to be some kind of confusion between a cultivation substrate and a well-functioning natural soil. A cultivation substrate could for example be peat or coconut fibers. Natural microorganisms are not present in these substrates and are not needed for a seed to start germinating and sprouting. It is possible, for example, to germinate a tomato seed on a damp kitchen paper, or in a glass of water. How many natural microorganisms are there in paper or water? We use cultivation substrate to be able to create a good and protected environment for the seeds to germinate and during their first time in life have something to anchor their roots in. With a little fertilizer, it will grow well without microorganisms. When the plants are of suitable size, we plant them outdoors in the garden, pots, etc. And then it is important to have naturally living soil, full of life, symbiosis, and microorganisms! An ecosystem with decomposers that works well and creates a healthy, fine, nutritious soil of organic material.

If you use peat briquettes, there may be eggs of fungus gnats there as well. Do the same with the briquettes. Alternatively, place the briquettes in boiling water for sterilization. Boiling water can also be used for soil. However, the risk is that it will be a bit gooey. You can also use other cultivation substrates. Fungus gnats are particularly fond of peat, but not at all as fond of bark soil or coconut fibers.

If you buy ready plants, it may also be wise to quarantine them for a while. Alternatively, change the soil or water with nematodes. Potted lettuce and potted spices you buy at grocery stores can also contain fungus gnats. If you want to save a potted lettuce, you should be careful with this as well. Maybe rinse the soil from the roots and plant in new soil.

The fungus gnat thrives in temperatures below 20-22 degrees. At these lower temperatures, in combination with moist peat soil, a very favorable environment is created. 20-22 degrees is thus normal room temperature. At temperatures above 22 degrees and with drier soil, it is not as favorable. If possible, place the seedlings in a place where it is warmer than 22 degrees. Then the crop is not hit as hard in a possible attack. It is also beneficial to water from below so that the surface of the soil is kept as dry as possible and to let the soil dry up between watering. You can also cover the surface of the soil with, for example, sand, perlite, decorative stone, leca, or bark and thus make it much more difficult for the gnats to find their way down to the soil.

An often recurring scenario that takes place in various gardening groups is whether it is worthwhile to sterilize potting soil or not if you have potted plants at home. It is claimed that the gnats will be transferred from the potted plants to the seedlings and that it, therefore, does not matter. And yes, it is true that fungus gnats can get transferred from the potted plants to the seedlings. But never the less, it is worth the effort to sterilize the potting soil. I will try explaining my point in the example below.

The example in the table below takes place in an ordinary home at a normal room temperature of 20-22 degrees Celsius. There are some ornamental plants in the windows. The soil used in the example is a standard peat-based potting mix. The seeds used are tomato seeds with a normal germination time of 5-8 days. Not a single glimpse of fungus gnats have been seen hovering, so it was assumed that everything is fine. But little did we know! Some time ago during the regular spring-clean-up, the ornamental plants were given some new soil. This soil was not sterilized. And in this soil, some eggs of fungus gnats were dormant. There had never been any problems before with fungus gnats, so this household did not keep a backup of nematodes in the fridge. In the example, we assume that this type of dark-winged fungus gnats has a life cycle of 30 days.

• In scenario 1, you choose to sterilize the potting soil, just for the sake of safety, even though it takes a little extra time and effort.
• In scenario 2, you choose not to sterilize the potting soil. No fungus gnats have been seen so far, so what could possibly go wrong?

CHEMICAL CONTROL

If the accident were to occur and you would be smitten by an attack of dark-winged fungus gnats, despite careful sterilization, planning, and control, what to do then? You can spray with any insect spray containing pyrethrins or any kind of spray "against flying insects". Pyrethrins are harmless to mammals, however, can cause some irritation if you get it on the skin. But this is a short-term solution and only kills the flying gnats, not the larvae.

There are also a variety of pesticides to fight fungus gnats, such as Diflubenzuron, Bifenthrin, Cyfluthrin, and Kinoprene. However, this is not commonly used by small-scale farmers, it is commonly used in large-scale cultivations. Unfortunately, fungus gnats develop resistance to some of the pesticides.

Stop-It Fly Pen can be used by us small-scale farmers. You just draw an “invisible” line with the pen, for example on the edge of the pot or on a window. Insects that come in contact with the poison die. The toxins are effective for about 3 weeks, then the treatment must be repeated. Unfortunately, this is also a short-term solution and only kills the flying gnats. The fly pen contains a poison called cypermethrin. It is a synthetic pyrethroid, just such an insecticide that fungus gnats resistance against in large-scale crops.

MECHANICAL CONTROL

It´s always a good idea to keep some sticky fly-traps over seedlings, to easily keep track of any fungus gnats getting stuck. Every dead gnat is a good gnat that will not lay 100 eggs! Should you find gnats on your sticky fly-traps, you should act immediately! The fly-traps are often yellow because insects are often drawn to yellow.

You can also arrange potato traps. You simply split a potato in two, or take slices of potato and put them on the soil. The larvae of the gnats are drawn to the potatoes. Every day you lift the traps and check if there are any larvae. If there are larvae, these are quickly spooned out from the soil!

There are also some theories out there that coffee grounds and cigarette butts should act as repellent. However, I have not seen any scientific evidence of this, nor have I made any experiments of my own. Personally, I don’t want to water my seedling with cigarette water. But this is of course up to everyone to decide for themselves.

BIOLOGICAL CONTROL

A decorative version of biological control is carnivorous plants. I have seen this from time to time. Unfortunately, it is doubtful how effective this method is. There are a number of different species, such as Venus Flytrap. If they are potted plants that are otherwise appreciated, they have at least no negative effect. Unfortunately, they only have an effect on the gnats, not the larvae.

A more effective method of biological control is nematodes. The nematode ‘Steinernema feltiae’ fights fungus gnats with 85% in mushroom cultivations. The nematode 'Steinernema carpocapsae' also works well. There is also a predatory mite, 'Hypoaspis miles', and a bacterium, 'Bacillus thuringiensis var. israelensis', available which are comparable to the nematodes in effectiveness.

For small-scale farmers, there is a nematode available in suitable packaging. The nematode ‘Steinernema feltiae’ and the product is called NemaBlom. The packages cost around 10€ each and can be bought in well-stocked flower shops and can also be ordered on the Internet. The packaging is stored in a refrigerator and then has a shelf life of about 1 year. If the packaging is stored at room temperature, the shelf life is only 3 months. They must not freeze, be exposed to sunlight, dehydration, or temperatures above 30 degrees, then the nematodes die. The nematode is thus a living organism that literally eats up the larvae.

It is easy to treat with nematodes. All pots to be treated should already have moist soil, so do not treat dry soil. Then empty the contents of the bag in 1 liter of cold water and water a smidgen in each pot. Of course, it depends on how big the pots are, but it is estimated that one bag is enough to treat about 20 normal-sized potted plants. As long as there are larvae left, the nematodes continue to feed on the larvae. About 2 weeks after treatment, all gnats and larvae should be gone.

SUMMARY

• Always sterilize the soil you are sowing in and transplanting little seedlings in.
• Put sticky fly-traps over seedlings to keep track.
• Keep nematodes at home on standby.
• If possible, have seedlings in a room where it is over 22 degrees Celsius.
• Water from below to keep the surface of the soil dry. Possibly put sand on the surface.