Fighting Fungus Gnats
Note: To clarify, the words soil and potting mix are used interchangeably in this post. Technically indoor gardeners use potting mixes (compost, perlite, bark, etc), but soil just sounds nicer.
Fungus gnats are probably the number one pest that novice and long-time plant collectors share an equal amount of frustration with. They are relatively harmless in small numbers, but become quite a nuisance if left unchecked. To manage fungus gnats, or better yet, avoid them all together, it is best to examine what allows them to thrive.
According to the University of Massachusetts Floriculture Program, the fungus gnat lifecycle begins with a female gnat laying up to 300 eggs in fertile soil. The ideal soil is compost or peat rich, which hold water well and allow for fungal growth, perfect for gnat larvae. After hatching, the larvae eat for up to 12-14 days. Larvae typically eat the fungus that is present in the soil. In its absence, they transition to eating healthy plant roots. By doing this, larvae can cause more damage to otherwise healthy houseplants plants than adult gnats, contrary to common belief. Larvae transition to pupae within the soil, and emerge in their adult form 5-6 days later. Adults can live for around 10 days.
Interestingly, the life cycle which would typically take around 4 weeks, will speed up in warmer conditions. Therefore, when dealing with gnats in in sunny spot, or with the heat on, that may be exacerbating the problem. Understanding the life cycle of gnats and their preferences is the key to combating them.
PREVENTION
There is no bullet proof method to preventing the presence of gnats in houseplants. They tend to show up on occasion, seemingly out of nowhere. To better understand how to avoid them, look to the life cycle to understand what is helping them thrive.
The adult females look for ideal places to lay eggs. Fungus gnat larvae thrive in soils, or potting mixes, that are peat heavy. Fortunately, eggs are laid within the top inch of soil, making them accessible, and the most immediate steps to avoid gnats are within a gardener’s control. Some easy to follow preventative measures include:
Ensure that plants have enough air circulation around them. This also helps to keep the temperature cool. Gnats love warm environments.
Clean all dead leafs from the surface of the soil and trim off any soon-to-be-dead lower leaves.
Occasionally aerate the soil, or churn up the top one inch or so, allowing it to dry out.
For plants that are often planted in a peat-heavy mix, like Calatheas, Alocasias or African Violets, try bottom watering rather than watering from the top.
There are products that can be laid on the surface of the soil to create a physical barrier, preventing the female from even accessing the surface. An example would be Gnat Nix.
A popular biological prevention method is the use of BTS, short-hand for Bacillus thuringiensis israelensis. This bacteria can be commonly found in Mosquito Bit products. Leaving the Bits in a full watering can overnight then watering as usual is very effective at destroying existing larvae. The bacteria is toxic only to the larvae for 48 hours, and not at all to the eggs or adults. Therefore, if adults are present, the solution must be continuously reapplied until the adults are absent for… * refers to life cycle image *… about two weeks. Using BTS regularly as a preventative measure is common and non-toxic to plants and humans.
TREATMENT OF AN INFESTATION
The ways in which to treat an infestation vary. Using a two-pronged approach that targets both the larvae and the adults is considered best practice. Once fungus gnats are noticeable, using a sticky trap will remove some of the adult population. It is best to assume that they have laid eggs, so in addition to using the cleanup and soil churning measures mentioned above, using BTS will help kill the larvae-BUT be sure to continue for over ten days, which is the time it takes for larvae to hatch. Additionally, insecticide can be used to target the larvae, but the soil has to be soooaked. The insecticide must reach a depth of one inch, evenly. This is the target area for eggs.
PERSONAL ANECDOTE
Those who have had plants around for a while may have developed their own methods. I personally prefer not to deal with any chemicals, etc. I am on a pretty regular cycle when it comes to gnats. As soon as the first sunny false spring Saturday hits around March, all the plants go into the shower. It is a major cleaning/watering event. Then the sun disappears for weeks, and my plants are soaked. Gnats emerge. Happens almost every year.
When I first notice gnats, I immediately crack open a window. Then I go through the plants in that area and clean up any dead leaves, but heavily focus on the base of the plant and top of the soil. I also use my finger or a chopstick to mix up the top 1” of soil. This helps to dry it out and keep it loose, making it a less than ideal spot for gnat eggs. In terms of prevention, I tend to water my plants deeply and avoid frequent watering, giving the top of the soil time to dry out. My home jungle only has a handful of water-loving plants like ferns and Calatheas, and those I tend to bottom water.
Simple measures can prevent fungus gnats from establishing. However, their emergence should not invoke panic. It should be stressed that fungus gnats are a product of the right environment. It can be taken away as easily as it was created, as long as consistent attention is paid to the plants and their care, specially around water and soil moisture.