Many times, their eggs are already included in unsterilised store-bought soil. Fungus gnats don’t consider the roots of your cannabis as their favourite food. They’d rather devour fungus and decaying matter in the soil. But, they do become a problem when organic matter in the soil is depleted. In that case, fungus gnat larvae devour and damage root hairs and the tender cannabis roots themselves.
Symptoms of fungus gnat infestation: • When roots are damaged, plants can show various signs of disease including yellow leaves, wilting, spotting, and drooping. • Symptoms of nutrient deficiencies can occur (sickly growth, leaf discolouration, etc.) despite correct pH and proper nutrients. • Seedlings weaken, fall over, and die (“damping off”). A fungus gnat infestation is relatively easy to diagnose. Fungus gnats are small, but you can see them if you get close to your plants. Here’s how to spot them: • Tiny “flies” crawling and jumping on the soil around your plants. Not as easy to spot as they are very tiny and may be hidden in the soil. • Otherwise unexplainable plant issues including, but not limited to, pale leaves, spots, brown edges on leaves, and drooping plants.
Signs can vary and may appear as other plant issues and diseases. HOW TO ELIMINATE FUNGUS GNATS FROM CANNABIS PLANTS. In a certain sense, fungus gnats can be a good thing, as they alert you to issues with your watering regimen. When they appear, it’s almost always because your plants are being overwatered. Fortunately, getting rid of them is relatively easy: • Water less frequently. The first, and most important, thing you should do is rethink your watering routine. Allow the soil to dry out fully between waterings—it will be fine. Most of the time, if you do that, the fungus gnat problem will go away on its own. Get yellow sticky traps from the garden store and position them around your plants. These traps attract the fungus gnats, which will stick to them. Know that yellow traps alone will not entirely eradicate an infestation. But, they are a good indicator of whether you (still) have fungus gnats and can prevent many of them from harassing your soil. If you have a pest infestation, you don’t need to get all hardcore with chemical insecticides. Neem oil can help get your infestation under control. Treat the top layer of your soil with it to eliminate the larvae. Diatomaceous earth is another way to get rid of bugs naturally. Sprinkle the fine powder on the soil around your plants. A simple standing fan gently blowing air across your soil has lots of advantages. It will also help dry out the soil quicker to eliminate the gnats for good. Some growers know the Bacillus thuringiensis bacteria for getting rid of caterpillars. There is a variety of these bacteria, Bacillus thuringiensis israelensis, that works for fungus gnats. Prevention is always better than treatment—here’s what you can do to keep those nasties away. More than anything else, pay attention to the amount of water you give to your plants!
Wet, warm conditions are practically begging for fungus gnats. If you are not sure when to water, you can do the “lifting test”. Simply lift up your plant with its pot and compare the weight when fully watered versus dry. Since fungus gnats live and breed in soil, you can cover it with sand, gravel, or perlite to prevent them from getting cosy. Keep in mind, however, that while covering the soil may prevent the gnats from appearing, your plants may still be water-logged. Sometimes, you simply can’t prevent bugs from appearing, for instance if they were already present in the soil. Do this by putting some soil in a flat, oven-safe container like a baking pan and cover with foil. Place a meat thermometer into the centre and “bake” for at least 30 mins at 82–93°C. While you’re at it, ensure your pots and growing area are clean as well. This way, you can minimise the risk for all manner of diseases, fungi, and pest infestations.
If you find your cannabis plants are frequently "bugged", consider growing in an alternative medium. When you grow weed in coco, perlite, or hydroponically, pest infestations are less likely. So Pyramid seeds has done an excellent job is showing me they suck!!