Edible Gardening Series: Question of the Week – fungus gnats
By Carol Wyatt-Evens and Sarah Bostick
Gardening in Florida can be incredibly rewarding and incredibly frustrating, at the same time. If you are new to the region, you soon learn that gardening in the Sunshine State can quickly become a full-time job. While our subtropical climate is perfect for growing an abundance of different vegetables, fruits, and herbs, it also can present some overwhelming challenges.
We can help!
UF/IFAS Extension Sarasota County agents and staff have created an online edible gardening resource center. The website features short videos from our 25-episode “Edible Gardening Series” webinars, along with blog posts and resources list for episodes. Get help on an array of topics that befuddle many gardeners.
Each blog post in the Question of the Week blog series is associated with a specific EGS episode, but they are written for everyone to enjoy!
Question of the Week:
What are the little black flying insects around my plants? How do I get rid of them?
Frustrating Fungus Gnats
A common insect that infests house, landscape, and garden plants is the dark-winged fungus gnat (Bradysia spp). The fungus gnat is a small, black, delicate-looking fly that people often mistake for a mosquito, shore fly, or drain fly.
The adult fly is much smaller than an adult mosquito. The fully grown adult is only 1/8 inch long, but its long, thin legs and see-thru wings make it a mosquito look-a-like.
The adults stick close to the plant and can be found flying around the plant during the day. They can be found resting on the plant foliage or even on top of the moist soil in the evening. Like many insects, fungus gnats are attracted to light. They can become quite a nuisance as they fly around table lamps or outside lights that are left on during the evening hours.
Although a nuisance, the adults do not feed, bite, or damage plants. It is the larval or immature stage of the insect that has the potential to inflict damage to plants if there is a heavy infestation.
Fungus Gnat Life Cycle
Fungus gnats lay their eggs on the surface of the plant soil. The eggs take approximately 3 days to hatch where the larvae then feed on fungi and damp, decaying organic matter in the soil. Since fungus gnats are flies, their larval stage is a maggot that looks like a small, translucent worm with a shiny black head moving around the soil. If the soil is particularly moist, they will leave slime trails along the surface of the soil.
If there is a large number of larvae, they can move to feed on the roots, causing plant damage and possibly even plant death.
It takes an average of 17 days for a fungus gnat to go from egg to reproductive adult. That timeline can be shorter depending on the temperature- the warmer it is, the quicker they develop. Once they reach the adult stage, the fungus gnat has a relatively short life of about 7 days.
Fungus Gnat Control
Fungus gnats are fairly easy to control in most planted areas including gardens, contain gardens, and houseplants through the use of integrated pest management practices.
- Watch the Water: Since fungus gnats require moist soil to complete their life cycle, the key to eliminating the insect is to reduce excess moisture and avoid overwatering. If you have a fungus gnat problem or prefer to NOT have a fungus gnat problem, allowing the top two inches of the soil to completely dry out between waterings will have a big impact on reducing fungus gnat larvae.
- Get rid of dead stuff: Removing the dead leaves and debris from the planter will reduce the amount of available food for the larvae.
- Monitor (for adults): Use yellow sticky cards to trap adult fungus gnats. Attach the sticky cards to skewers or popsicle sticks and place them around the perimeter of the plant so the cards rest just above the soil line.
- Monitor (for larvae): Place chunks of raw potato on the surface of the soil with the cut side down to attract the larvae. Throw out the old pieces once they are covered with larvae and replace them with fresh chunks.
- Lights Out: Turn off lights in the evening when not in use or use ‘bug bulbs’ (they usually come in shades of yellow, pink, or orange).
Bacillus thuringiensis, subspecies israelensis (Bti): Bti is a biological control agent, a bacteria, that targets the larval stages of fungus gnats, mosquitoes, and black flies. The bacteria are applied to the soil and ingested by the larvae as they feed on the organic material. The bacteria is toxic to the larvae and will kill the larvae within days of ingesting the bacteria.
A convenient way to apply Bti is through a product called Mosquito Bits®. Mosquito Bits® are pieces of ground-up corn cob impregnated with the Bti bacteria and can be purchased at any big box store or online. The bits are sprinkled on the surface of the soil and as the plant is watered, the bacteria are washed into the soil where the fungus gnat larvae are feeding. Be sure to read the label to determine the amount of product needed per plant as well as application timing.
Conventional chemical applications are usually not needed to control fungus gnats in the home. Breaking the lifecycle is key to eliminating the pest insect and is easily accomplished by simply:
- Decreasing the amount and frequency of water applied to the plant;
- Passive trapping of the adults using sticky cards and
- Applying Bti to the soil surface when fungus gnats are present
Following these three steps will help to eliminate a fungus gnat infestation.
Factsheets and resources for gardening in Florida
Fungus gnat biology and control:
- Natural Products for Managing Landscape and Garden Pests in Florida
- Pests in Gardens and Landscapes – Fungus Gnats (UC IPM)