What are fungus gnats?
Fungus gnats are the tiny black insects you might see buzzing around your African Violets and indoor plants at times. Actually, there is not a single insect known as a fungus gnat, but it is a generic term applied to a family of insects, including Sciaridae, Diadocidiidae, Ditomyiidae, Keroplatidae, Bolitophilidae, Mycetophilidae. These insects are a common pest of indoor plants and feed on root hairs, decaying plant matter and fungi.
Are fungus gnats a problem?
There are differing opinions on whether fungus gnats are just a nuisance or whether they pose a real threat to your plant collection. Adult fungus gnats don’t damage plants or bite people. However, the presence of gnats in significant numbers is likely a sign that your growing conditions are not optimal.
Gnats love conditions where there is high moisture, humidity, decaying plant matter, and still (poorly circulating) air. Their presence might be an indication that you’re overwatering, that there’s insufficient air circulation, and that you your plants need a grooming.
A small infestation can quickly get out of hand because fungus gnats multiply profusely. An adult gnat will live only about a week, but in that time, can lay up to 300 eggs. Within 4-6 days, the eggs hatch into tiny larvae that move around your potting mix feeding on plant roots and decaying matter.
A healthy plant that is actively growing may not be affected. However, in larger numbers, gnat larvae can cause root rot and fungal infections. Adult gnats can carry disease or fungus spores in their mouths and on their bodies. Since they are mobile, they can easily spread disease to your whole collection.
In 2019, my collection of African Violets was turned into a ghost town. I lost more than 200 plants to a rapidly spreading fungal infection. Although there were several causes, fungus gnats were a key player in spreading this infection.
What to do about fungus gnats
Controlling or eliminating gnats requires a sustained effort (at least a couple of months) and a multi-pronged approach that targets that gnats at all stages of their life cycle. The key to winning your battle against the gnats is to break their life cycle at all stages of their development.
Multi-pronged approach for eliminating Fungus Gnats
Gnats at the larval stage live under the surface and can be controlled with soil drenches or water additives. For a natural remedy, you can try watering with hydrogen peroxide (3%) mixed at the rate of 1 part peroxide to 4 parts water or insecticidal soaps. Another effective remedy is a Neem Oil drench at the rate of 5ml per litre of water. For wick watering, CX Tanlin (NilNat) drops are added to your water supply and are highly effective.
Young gnats will be found crawling across the surface of your mix. They are best controlled with surface sprays or barrier treatments. For a natural remedy, try pyrethrum or neem oil sprays. Alternatively, there are many effective insecticides such as Bayer Confidor (Imidacloprid), Yates Mavrik (Tau-fluvalinate), Yates Success (Spinetoram), and Brunnings Bug Kill. Household fly sprays should never be used as they cause chemical burns to your plant foliage.
Adult gnats will fly from plant to plant looking for moisture, fresh sources of food, and places to lay their eggs. The best defence against them is to lure them away from your plants and trap them. Ribbon fly papers and yellow sticky insect traps a
Improving Growing Conditions
Any efforts to eliminate gnats can only be effective in the long-term if they are supported by attending to growing conditions that are contributing to the problem
- Moisture – Be sure to fix any leaky pipes or moisture areas you might have around your plants. Improve poor drainage. Ensure you are not over watering your plants. If you are using wick watering, ensure the growing mix contains at least 50-60% coarse perlite.
- Air circulation – Adult gnats are not good flyers and do not like moving air. Ensure your plant collection is not over-crowded and that their is sufficient air circulation around the plants. Powdery mildew might be another sign of poor air circulation. For large plant collections, use a small low voltage electric fan (available from hydronic shops), ceiling fan or computer fan.
- Introduced sources – Commercially grown house plants and potting mixes that contain pine bark are common sources of fungus gnats. Think twice before buying that lovely maidenhair fern from Bunnings, or treat any new plants that you bring into the house with soil drenches. Ensure your African Violet potting mix is free from pine bark.