Often dismissed as more of a nuisance in protected plant culture, fungus gnats can easily get out of control if not attended to. We’re not sure why, but growers tend to overlook the negative impact that feeding fungus gnat larvae can have on plant growth, health, and yield.
The larval stage actively feeds on the plant's root system. Fungus gnat larvae feed on fungi, decaying organic material, and live plant tissue. They thrive in wet, over-watered conditions, and tend to exist in the upper soil profile, but may live near drain holes. In addition to feeding on roots, the larvae will hollow-out the stems of cuttings in propagation environments.
Fungus gnat larvae and adults are also known vectors of plant diseases. Unfortunately, this negative trait is often underestimated or overlooked. Larval stages can directly transmit root rot from diseased to healthy plants. The list of transmitted diseases includes Pythium, Fusarium oxysporum, Verticillium albo-atrum, and black root rot, Thielaviopsis basicola.
The adults are also capable of spreading foliar diseases such as Botrytis by carrying the conidia on their bodies.
While controlling fungus gnat adults and larvae is relatively straightforward, we cannot overstate the importance of sound irrigation practices and proper drainage to aid in their prevention and control. If fungus gnats are a recurring problem, poor cultural habits may be the root cause.
What to look for
Signs of fungus gnats include stunted plant growth, adult gnats hovering near the soil surface, larvae in the soil.
Life Cycle of Fungus Gnats
The fungus gnat life cycle has 4 stages:
Egg - Eggs are laid by adult females into the cracks and crevice of most soil, not directly on the soil surface
Larva - There are 4 larval stages, characterized by a black ‘head capsule’. The larvae feed on root hairs, the stems of unrooted cuttings, and organic material. They may be located near the top of the container, or the bottom.
Pupa -The pupal stage does not feed.
Adult -Adult fungus gnats emerge after pupation, and are poor flyers.
- The life cycle takes between approximately 18 to 28 days, depending on temperature.
- Adults highly prefer wet soils to dry.
- Females can lay between 75 and 200 eggs in her lifetime.
- Adults are can easily be distinguished by their bead-like antennae, the the ‘Y’ shaped vein in their wings.
- Yellow sticky cards, or potato slices monitored every 48 hours, can be used to assess adult or larvae populations, respectively.
I see gnats, now what?
There are a few options for control of fungus gnats. Some growers prefer the use of Nematodes such as Stenernema feltiae, while others choose to use Stratiolaelaps scimitis. Both of these pair well with Dalotia coriaria to gain the upper hand on fungus gnats. Apply heavier amounts up front for 2-3 weeks in a row has been the most effective to overpower a cycling fungus gnat populations
I don't see any gnats, but I'd like to prevent problems...
The same predators used to clean up a fungus gnat population can work very well as preventatives. The rate of application would be less, and you would only need to apply every 2-4 weeks to keep the population of beneficials at high levels.
* It is best to introduce soil predators anytime you bring in new soil or transplant/pot up to maintain root zone protection!
Stratiolaelaps scimitis(formerly H. Miles)
Stratiolaelaps scimitus (formerly called Hypoaspis miles) is a soil dwelling, generalist mite that feeds on fungus gnat larvae, pupating thrips, pathogenic nematodes, immature root aphids, and smallinsects in your substrate. Preventative applications will protect against many of the most common pests that feed on your roots or have a life stage in the soil. They can also be released onto the floors and corners of room and greenhouses.
Dalotia (Atheta) is a soil dwelling rove beetle. It is an effective predator of fungus gnat larvae, shore fly eggs, pupating thrips, as well as other small, soft bodied arthropods in and around your rooting system.
Dalotia are capable of flight and adapt well to various growth media including rock wool and coconut fiber. Results have been impressive when used in conjunction with Stratiolaelaps to help prevent root aphids.
Steinernema feltiae nematodes are highly effective and normally produce fast results, especially at higher application rates. Unfortunately, many growers do not have enough containers to justify their use or they are unable to properly store them for follow-up applications. Their host range is limited to fungus gnat and western flower thrips larvae.