Gnats are tiny, non-biting insects, often confused with fruit flies or mosquitoes, that can quickly become an outdoor nuisance. The most common type encountered around gardens and patios is the fungus gnat, a small, dark fly that flies erratically near plants. These pests are primarily attracted to areas of excess moisture and decaying organic material, which provide the ideal habitat for their larvae. Controlling an outdoor gnat population requires a dual approach: eliminating the breeding grounds and actively repelling the flying adults.
Eliminating Outdoor Breeding Sources
The foundation of long-term gnat control is making the outdoor environment inhospitable for their larvae, which feed on fungi and organic matter in damp soil. Fungus gnat females lay their eggs in the top layer of moist soil. Larvae cannot survive if the soil is allowed to dry out, making moisture control the most effective preventative measure.
For container plants and garden beds, consistently allow the top one to two inches of soil to become fully dry before watering again. This interrupts the gnat life cycle by desiccating the eggs and larvae found in the top layer of the growing medium. Ensure all potted plants have proper drainage holes and promptly empty any water that collects in the saucer beneath them, as standing water provides a breeding reservoir.
Focus on removing other sources of stagnant moisture and decay around the home’s perimeter. Clear clogged gutters, fix leaky outdoor faucets or hoses, and remove piles of damp leaf litter or grass clippings. Overly thick layers of mulch should be raked back or reduced, especially in shady areas, as they retain moisture and promote fungal growth. Promptly disposing of fallen, overripe fruit also removes a significant attractant for small flies.
Active Repellent Strategies
Once breeding sites are managed, use active repellents to deter remaining adult gnats from congregating in gathering areas like decks and patios. Commercial repellents offer immediate, short-term protection for individuals. Personal application sprays containing picaridin or oil of lemon eucalyptus (OLE) are effective against gnats and can be applied directly to exposed skin and clothing. Picaridin is a synthetic compound, while OLE is a plant-derived alternative that offers protection for several hours.
For area protection, various products utilize scents that gnats find repulsive. Specialized outdoor foggers can pretreat an area up to 900 square feet, providing several hours of protection for outdoor events. Citronella candles and torches use their strong aroma to help reduce the presence of gnats in a confined space.
Essential oils offer a strong, non-toxic deterrent for those preferring natural options. Peppermint, lavender, and cedarwood oils are known to repel gnats due to their intense aromas. These oils can be used in an outdoor diffuser or diluted with water and a dispersant like alcohol to create a surface spray. This spray can be applied to non-plant surfaces, such as patio furniture and railings, to create a localized scent barrier.
Trapping and Population Control
Targeting the existing gnat population involves methods designed to kill both flying adults and developing larvae. A simple and effective way to trap adult gnats is by using a homemade apple cider vinegar trap. The acetic acid in the vinegar mimics the scent of their preferred food sources, drawing the gnats toward the liquid. Adding a few drops of liquid dish soap reduces the surface tension, causing any gnat that lands on the liquid to sink and drown.
Yellow sticky traps, placed near potted plants or areas of high gnat activity, are highly effective at capturing adult gnats. The bright yellow color is visually attractive to the flies, and the adhesive surface ensures they are quickly removed. Removing the adults prevents them from laying up to 200 eggs each and continuing the infestation cycle.
For direct larval control in soil, a biological insecticide containing Bacillus thuringiensis israelensis (Bti) is a targeted solution. Bti is a naturally occurring bacterium that is deadly to fungus gnat larvae but harmless to humans, pets, and beneficial insects. When applied by watering the soil, the larvae ingest the bacteria, which releases a toxin that disrupts their digestive system, eliminating them before they mature into flying adults. For immediate, localized relief in outdoor seating areas, placing an oscillating fan nearby can disrupt gnat flight patterns. Gnats are weak fliers and cannot navigate effectively against a steady stream of air, creating a temporary no-fly zone.