Fungus Gnats: Control and Prevention for Carnivorous Plants
Fungus gnats are a common pest that can plague carnivorous plants. They are small and dark in color and can often be seen flying near the surface of damp soil. While adult fungus gnats aren’t harmful to carnivorous plants (and may even be a free source of food!), the larvae can cause severe damage.
Fungus gnat larva feed on fungi and organic matter in soil, but they also feed on plant root, stems, and leaves. The larvae have also been known to spread pathogenic fungi. Infested carnivorous plants may become unhealthy and slow their growth while seedlings and Butterworts can be lost altogether, yikes!
Fungus Gnat Larvae
Fungus Gnat Prevention
There are a several of ways to combat fungus gnats, the best of which is to prevent them taking up residence in the first place. Preventative measures include:
Ensuring there is adequate airflow around plants to discourage fungal growth. This creates an unattractive location for fungus gnats to lay eggs.
Covering soil with at least 1/4 of an inch of coarse sand or gravel to create a physical barrier between the gnats and the soil.
Allowing the soil to dry out between waterings. Caution: Only use this method for plants like Mexican Butterworts that can tolerate dryer conditions.
Isolating infested containers to keep uncontaminated plants from becoming infested.
An adult fungus gnat preparing to lay eggs in damp soil
How to Fight an Infestation
If preventative measures have failed, there are several ways to combat an infestation:
Physical removal
One way is to replace the soil in the pot. Remove the plant from the pot and discard the contaminated soil. Rinse the roots and container with clean water to knock off remaining larvae. Also try to kill any adult gnats. After removing all of the contaminated soil and clearing the area of adult gnats, repot the plant with fresh soil and then follow ongoing preventative measures.
Organic pesticides
If repotting isn’t an option, natural pesticides are another way to manage infestations. Products that target larvae systemically like Cold Pressed Neem soil drenches or biological larvicides like Gnatrol or Mosquito Dunks are all options known to be safe for many carnivorous plants.
Chemical pesticides
Chemical pesticides like Bayer 3 in 1 are very effective against fungus gnats and safe for most types of carnivorous plants. Here is a whole article dedicated to it. The down side to chemical pesticides is that they can be harmful to beneficial insects like bees so it’s best to use them as a last resort.
In order to use the methods above effectively, it helps to understand the life cycle of the fungus gnat.
Life Cycle of a Fungus Gnat
Adult female fungus gnats live about 7 days and can lay up to 1,000 eggs during their lifespan. Once the eggs are laid, they will hatch in 4-6 days. After hatching, the larvae begin tunneling through the soil in search of food. The larvae stage lasts 10-14 days. Once the larvae are old enough, they spin a cocoon in the soil and enter the pupa stage which lasts 4-7 days. At the end of the pupa stage, the adult exits the cocoon and begins mating and laying eggs. Then, the cycle starts over. A full cycle takes 25-34 days to complete.
Life Cycle of a Fungus Gnat
How does this help?
From the life cycle breakdown, we can see that it becomes more effective to treat an infestation from multiple angles. To the extent possible, remove adult gnats as they appear to reduce the number of eggs being laid. At the same time, take action against the larvae with natural or chemical pesticides and employ better preventative measures. Doing all of these throws an effective one-two-three punch.
The most important factor though is to keep up with treatments for 1-2 full life cycles of the gnat (about 2 months). This not only ensures unhatched larvae are killed as they emerge, but it also factors in pupae that make it to adulthood and manage to lay more eggs. With consistency and proper treatment length the persistence will pay off and the cycle will be broken, eliminating the infestation.
Sources:
Fungus Gnat Larvae by John Brittnacher, International Carnivorous Plant Society
Fungus Gnatsby G.R. Nielsen, Former Extension Entomologist, Plant and Soil Science Department
I tried a sundew for the handful of fungus gnats that occasionally get in my house. It caught a few, but only a few.
I was amazed when I noticed that some–or most?–fungus gnats are so light-footed that the sundew couldn’t hold them.
Maybe I had the wrong kind of sundew? It believe it was a species native to South Africa.
I’m looking to try a butterwort.
It’s hard to say why the sundew wasn’t able to hold onto some of the gnats, it might be that there wasn’t enough dew to keep them stuck or that the gnats were somehow avoiding the sticky part of the leaves. Butterworts are pros when it comes to catching gnats, as long as the leaves are sticky, I’d be surprised if the gnats were able to escape.
Glad to hear it, Stacie! A tray of water isn’t necessary but can be very helpful in maintaining consistently damp/wet soil which is important for your sundew species.
There is an amazing way to beat them – with nematodes!
That is a good option as well!
I tried a sundew for the handful of fungus gnats that occasionally get in my house. It caught a few, but only a few.
I was amazed when I noticed that some–or most?–fungus gnats are so light-footed that the sundew couldn’t hold them.
Maybe I had the wrong kind of sundew? It believe it was a species native to South Africa.
I’m looking to try a butterwort.
It’s hard to say why the sundew wasn’t able to hold onto some of the gnats, it might be that there wasn’t enough dew to keep them stuck or that the gnats were somehow avoiding the sticky part of the leaves. Butterworts are pros when it comes to catching gnats, as long as the leaves are sticky, I’d be surprised if the gnats were able to escape.
I just received my gorgeous little sundews. Do those need to be in a tray of water after the travel shock instructions?
Glad to hear it, Stacie! A tray of water isn’t necessary but can be very helpful in maintaining consistently damp/wet soil which is important for your sundew species.