MANAGING FUNGAL GNATS ON YOUR HOUSEPLANTS
Around this time of year, a steady stream of people come to garden centers all reporting the same problem. Small annoying flies keep appearing around the base of their beloved house plants and no matter what they do, they can't seem to stop the swarm from appearing. Fear not, I'm here to help, for you my friend have 'FUNGUS GNATS'!!!!!
The reason you see a peak in the population of fungus gnat during the late fall and winter period is due to several factors:
First, house plants that have enjoyed growing outdoors for the summer months, more often than not have already been colonized by fungus gnats. When the plants are brought into the home for the winter season, the population subsequently increases due to temperatures inside a home being optimal for continued reproduction.
Second, fungus gnats are attracted to moist conditions to lay their eggs and for the larvae to develop. The reduction of day length during the winter season causes the plant to slow its growth rate and reduce its water consumption, leaving the growing media wetter for longer. Coupled with this, if the growing media is old it tends to break down or degrade, holding more moisture for longer.
Finally, we tend to notice an increase in fungus gnats only because the colder weather keeps us inside where we notice the problem more. The chances are high that the plant already had fungus gnats over the summer but we didn't notice it until we started to spend more time inside with the plant.
Fungus gnats are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots and can be a problem in potted plants. Adult fungus gnats may emerge from houseplants indoors and become a nuisance.
Fungal Gnat Larvae
Adult fungus gnats don’t damage plants or bite people; their presence is primarily considered a nuisance. Larvae, however, when present in large numbers, can damage roots and stunt plant growth, particularly in seedlings and young plants. Significant root damage and even plant death have been observed in houseplants when high populations were associated with moist, organically-rich soil.
Once you have a fungus gnat infestation, using consistent management and prevention techniques is the key to ending it. Because adult fungus gnats are attracted to light, you first might notice these pests flying near windows indoors. However, in comparison with more active species such as the common housefly (Musca domestica), fungus gnats are relatively weak fliers and usually don’t move around much indoors. Fungus gnats often remain near potted plants and run across (or rest on) growing media, foliage, compost, and wet mulch piles.
Adults live about one week and lay up to 300 eggs in rich, moist soils. Within 4-6 days tiny larvae emerge and begin feeding on plant roots during their two week period. The pupal stage lasts 3-4 days before young adults leave the soil and begin the next generation. The entire life cycle from egg to adult may be completed in as little as 3-4 weeks depending on temperature. Because of their proclivity and relative short gestation, potted plants can host each stage — egg, larvae, pupae, adult — in multiple generations at once. Because of this remedies usually require repeated applications until there are no surviving eggs.
Fungus gnat damage will appear similar to that of any other root-related issue, such as root rot. Lower leaves may turn yellow and drop, and the plant’s growth may slow down or stop completely. In particularly bad cases, wilting of the entire plant could occur, followed by the death of the plant if roots are extremely damaged.
How to Prevent Fungus Gnats
Keep soil dry: Fungus gnats seek out moist soil, so allowing your houseplants to dry out a bit between waterings can slow down or stop an infestation. If pests are present, allow the soil to dry to a depth of one to two inches between waterings. This not only kills larvae and inhibits the development of eggs, it also makes the soil less attractive to egg-laying females.
Mosquito dunks (with beneficial bacteria): Mosquito dunks are used to keep mosquito larvae from populating fountains, animal troughs, fish ponds, and other small bodies of water. The product consists of a dry pellet containing a type of bacteria called Bacillus thuringiensis subspecies isrealensis. This beneficial bacteria infects and kills the larvae of flying insects, including mosquitoes, fruit flies, and fungus gnats.
To use mosquito dunks: Fill up a gallon jug (or watering can) with clean water and toss in a mosquito dunk. It’s a good idea to break up the dunk a bit before placing it in the water, or you can wait for it to soften before breaking it apart. Let the dunk soak in the water for as long as possible (at least overnight), then remove it from the water (the dunk can be reused) and use this water for fungus gnat–infested plants. The bacteria will have leeched into the water and will now infect and kill any larvae that come into contact with it in the soil. Repeat this process every time you water your plants for at least a few months.
Cover drainage holes: Though gnats typically remain near the tops of pots, they may find their way to the drainage holes on the underside of a pot and start laying eggs there, too. If this happens, cover the drainage holes with a piece of synthetic fabric to prevent the gnats from getting in or out of the hole, but to also let water pass through freely. Attach with tape or rubber bands.
Cover exposed soil with sand: Some folks report that covering their houseplant’s soil with a layer (at least 3/4 inch thick) of sand prevents fungus gnats from accessing the soil and laying eggs. This can be an effective deterrent if used in conjunction with other prevention methods listed above, especially covering drainage holes.
How to Get Rid of Fungus Gnats
Getting rid of fungus gnats is all about consistency. Catching the adults in gnat traps is fairly easy, but because the adult population comes in cycles, you need to make sure that your traps are refreshed regularly. Here are a few things we have found to help:
Sticky cards traps: These traps consist of a yellow note card covered in a sticky adhesive. They are most effective when cut into small squares and placed directly on top of the soil or attached to skewers just above the soil. Adult gnats will fly or crawl onto the card and become trapped in the glue. Fungus gnats are attracted to the color yellow, so use the yellow sticky cards rather than the blue ones. Both can be bought at most hardware or garden stores, as well as online.
Cider-vinegar traps: Simple and effective, cider-vinegar traps consist of a shallow container with a small amount of apple cider vinegar, water, and liquid dish soap.
To make a cider-vinegar trap: Find a shallow container—a tuna can is perfect—and fill it with equal parts water and apple cider vinegar. (The liquid should be at least 1/4-inch deep.) Put a few drops of liquid dish soap into the mixture and stir gently. Place the trap near the base of the affected plant or, ideally, inside the pot on top of the soil. Check it every few days to refresh with new vinegar and water.
Sometimes chemical control is deemed necessary and though caution is recommended there are products approved for use inside a home. The most effective are systemic insecticides that contain imidacloprid, available in granules and slow release spikes that also contain fertilizers. This will deal with the larval stage but not the adult flies. Several applications of pyrethroid based sprays should finish off what's left of your unwanted house guests.
Changing the way we take care of the plant will effect how the gnat can build up its population. Replacing the top half inch of compost with sterile coarse sand will prevent the adult flies from laying eggs in the soil. In addition, re-potting your plant yearly will reduce any broken down growing media that creates a food source and habitat for the larvae.