Fungus gnats are members of the insect superfamily Sciaroidea. Winged, dark-colored, and relatively short-lived, some of these gnats are considered to be pests as they may inflict serious damage on seedlings and vulnerable houseplants. While they may be beneficial as pollinators or as prey for larger, predatory insects, their presence can be troublesome as their adult forms may be vectors of plant diseases.
Infestations of fungus gnats, which lead to the rapid production of eggs and their subsequent larvae, can also compromise the fragile root systems of plants that are grown in fungus-rich soil. Thus, many horticulturists and gardeners prefer to keep these insects away at all costs. Preventive maintenance is key to keeping their numbers to a minimum.
Warm, moist conditions may lead to the production of more generations of fungus gnats within a single year. The consequences of their presence are usually most apparent in indoor setups, where their natural predators may be absent and ventilation may be poor. Fortunately, there are a few plants that can be grown to help keep their spread and reproductive success at bay. Grow these within the sunlit corners of your home or close to its key entry points.
1) Lavender (Lavandula spp.)
Though the scent and appearance of lavender plants are remarkably appealing to us humans, they are known for repelling a wide range of troublesome insects. Fungus gnats are just one of many groups of pests that will do their best to stay clear of dense lavender stands. The strong odor, coupled with the bitter taste of the grayish-green foliage, indicates the presence of potentially toxic phytochemicals.
To use a variety of lavender species for keeping gnats away from the home and garden, cultivate them in dedicated beds and along the edges of fully sunlit, outdoor structures. Tough and drought-resistant, these plants favor well-draining substrates and are known for attracting many beneficial insects and pollinators. For them to aid in keeping pests away, however, they need to be well-maintained. Keep the shoots pruned and tidy, and place cuttings around the home to spread their scent.
If lavender plants are not compatible with your garden’s substrate conditions and your local climate, you may invest in its extracted oils instead. Lavender oil can be mixed into a water-based solution and sprayed directly onto the foliage and substrate of plants that are targeted by fungus gnats. After some time, the adult forms of gnats should begin to avoid the strong scent and opt to lay their eggs elsewhere.
2) Cape sundew (Drosera capensis)
One of the most fascinating plants to grow indoors, the cape sundew is a carnivorous species with a steady appetite for fungus gnats and flies! Also, as it is one of the most commonly grown ornamentals in its genus, it has been introduced into regions far outside of its native range. Some of its eye-catching cultivars, though small, have received the RHS Award of Garden Merit.
The cape sundew kills fungus gnats by trapping their adult forms on their mucilaginous leaf tentacles. These tentacles are brightly colored, and the mucus borne on their tips resembles the appearance of morning dew. Once an unsuspecting gnat sticks to the mucus, the leaf can roll in on itself to securely entrap its prey. This motion also allows more of its digestive glands to come into contact with the victim.
The cape sundew doesn’t exactly “repel” fungus gnats in the strictest sense, as it does the complete opposite. Over time, however, this plant can significantly aid in reducing the indoor populations of these pests. It will consistently eliminate any egg-laying adults that dare to come close to its chemically attractive secretions. Keep it close to a sunny windowsill and promote its mucus production by maintaining moderate-high ambient humidity levels.
3) Dewy pine (Drosophyllum lusitanicum)
The sole member of its genus, D. lusitanicum is another carnivorous plant that functions much like the cape sundew as a natural predator for fungus gnats. This perennial species has short, erect leaves which arise in a rosette formation around its crown. Its leaves are heavily covered in mucus-producing glands that are borne on the tips of short tentacles. Unlike the motile foliage of sundews, however, the leaves of the dewy pine cannot roll in on themselves.
Instead, the insects are simply ensnared by the sticky mucus. As they struggle and inadvertently come into contact with more mucus glands, they become more permanently trapped on the surface of the leaves. Over time, the victimized gnats and flies become tired and eventually die of exhaustion. The plant secretes enzymes to rapidly break down its victims and release their nutrients, which are absorbed by the leaves.
This fungus gnat predator is a fine choice for gardens and homes that are located in drier regions with mild climate conditions. It favors well-draining and porous substrates as it is naturally found in forests and heaths with gravelly or silicon-rich substrates. To keep its mucus-production levels at optimal rates, it should be exposed to full sun.
4) Butterworts (Pinguicula spp.)
Winged pests are often tricky to manage as they can rapidly flit about from plant to plant, avoiding those with strong scents and targeting others with more vulnerable features. This is why using carnivorous plants to control their populations is a great way to prevent their spread. As adult fungus gnats get trapped by the predatory plant, they become permanently removed before they can unleash their eggs on another herb.
Pinguicula species, commonly known as butterworts due to the glistening appearance of their leaves, are flowering carnivorous plants that feed on flies and gnats. They are often used as a natural form of pest control in commercial, indoor gardens. When their winged prey land on their colorful, sticky leaves, they become trapped and digested by enzymes. The leaves of some species are able to bend their margins inward to securely trap their victims.
The glandular hairs of the leaves, each topped with a droplet of mucus, imitate the appearance of tiny water droplets. Not particularly selective about the type of prey, these passive insectivores are the perfect options for humid and moist locations. Note that some temperate species undergo a dormant, non-carnivorous phase through winter.
5) Citronella grass (Cymbopogon nardus)
As fungus gnat larvae tend to infest fungus-rich substrates and may damage the roots of vulnerable plants, they may be controlled by targeting the soil. While adding dry, porous substrates to topsoil may aid in keeping them away, incorporating gnat-toxic oils would be a better means of eliminating their populations. Citronella oil, sourced from C. nardus, is one of the best options as a natural gnat repellent.
The antifungal properties of citronella can aid in keeping fungus densities in the substrate to a minimum, making it unappealing to the hungry larvae of fungus gnats. Moreover, the lemony scent of the plant’s oil-rich sap effectively repels adult flies, discouraging them from laying their eggs on the plant. To maximize the fungicidal and insecticidal nature of the oil, use it to make a plant-based pest repellent.
For indoor use, you may consider mixing citronella oil into a solution for use in a scent diffuser. Allow the vapors to diffuse through rooms with susceptible house plants. The smell should effectively keep most troublesome flies and gnats away, all while lifting the atmospheric mood in the home.
6) Lemon thyme (Thymus citroidorus)
In the same way that the scent of citronella grass can repel many troublesome gnats and flies, the citrus odor of lemon thyme should keep most pests away. As it acts as an aromatic repellent, it also serves as an attractant to some predatory insects and pollinators. Thus, this is an ideal member of the mint family to grow as a ground cover plant in pest-infested gardens.
To release the scent of lemon thyme, regularly prune its shoots. Propagate its cuttings to expand its coverage. Place its strongly-scented, crushed leaves around substrates that tend to be infested with the larvae of fungus gnats. To increase its effectiveness, you may extract its oils and incorporate them into a water-based oil and soap spray. This should help damage all life stages of gnats upon contact. It should also reduce the chances of re-infestation.
To ensure that your lemon thyme doesn’t attract its own host of opportunistic pests, which would perhaps overthrow its efforts to repel mosquitoes and flies, make sure it is planted in a well-draining substrate and exposed to ample sunlight. Individual plants should be placed at least 12 inches (30 cm) apart to prevent overcrowding. Keep the shoots neat by pruning them regularly.
7) Mexican marigold (Tagetes erecta)
Marigolds are widely known for their pest-repellent, medicinal, and decorative properties. These useful herbaceous plants are distinguished by shallow and fibrous roots, pinnate leaves made of up to 17 leaflets, and eye-catching inflorescences. The solitary blooms attract many natural predators, some of which will readily feed on adult fungus gnats and their larvae.
Apart from drawing beneficial insects to the garden, the Mexican marigold is equipped with an antioxidant-rich and aromatic essential oil. Giving its leaves a strong and musty aroma, the oil is especially effective as a repellent in its extracted form. Wounded leaves, from which the sap may seep out, should dissuade any scent-sensitive insects from coming too close.
Mexican marigolds can be planted around vegetable beds and more vulnerable ornamental plants. Their mature and well-maintained stands should help prevent infestations from occurring above the soil or underground. The stunning colors of their vivid blooms make them well worth some care and effort.
8) Chrysanthemum (Chrysanthemum spp.)
The pest-repellent properties of chrysanthemums are largely concentrated in their summer-to-autumn blooms. Their heavy floral heads contain compounds that are scientifically referred to as pyrethrins. Usually extracted in a powdered form, these potent phytochemicals are one of the most important natural insecticides as they directly attack the nervous systems of insects.
Unfortunately, pyrethrins are only present in blooms and are absent from the rest of the plant. This means that outside of the bloom period, chrysanthemums aren’t exactly known for repelling pests. If you cultivate these flowering perennials, you can harvest their blooms, pulverize them, and save their powder for use as an ingredient in your own homemade pesticidal sprays.
Pyrethrin-based sprays can quickly decimate populations of adult fungus gnats. However, as these gnats’ life cycle consists of four distinct morphological stages, its effects may need to be reinforced with the help of insecticides that can permeate through the substrate and attack the larvae. If larvae are not treated, then new adults will continuously emerge from infested substrates.
9) Venus flytrap (Dionaea muscipula)
The most popular carnivorous plant due to its appearance in pop culture and lore, the venus flytrap is an endlessly fascinating species with a knack for surprising all sorts of unsuspecting insects. Instead of using artificial sticky traps to control adult fungus gnat populations, the venus flytrap can be placed around greenhouses and homes to naturally eliminate them.
This species feeds on insects that land on its modified leaves, which take the form of a mouth-like trap. Once the victim touches one of its sensory hairs, a cascade of signals triggers the leaf flaps to snap together. When flies or gnats struggle to escape the trap, the flaps only shut more tightly and begin to release their digestive enzymes. These break down the victim until its nutrients are released and can be absorbed by the plant.
The venus flytrap is a smart option for indoor pest control as it favors humid conditions. It can be grown under artificial light or placed on a windowsill that receives bright, direct light for at least 6 hours a day. In insect-free, indoor locations, this species will need to be deliberately fed a few times a week. Of course, specimens that are kept outdoors should have no issues coming across natural prey.
10) Chamomile (Chamaemelum nobile)
Known for its use as a source of flavor for herbal teas, chamomile is rich in fragrant compounds. Its oil, which is a complex mixture of terpenes and polyphenols, is concentrated in its flowers. When vast fields of this species are at the peak of their bloom period, they exude their unmistakable scent. If you wish for your garden to smell like chamomile blooms, you can cultivate a lawn with up to 100 individual C. nobile plants per square meter. The scent emitted by the lawn should attract beneficial insects and repel troublesome ones.
Chamomile oil has fungicidal and insecticidal properties. Even in its tea form, it may be effective at repelling various winged pests – including fungus gnats. Freshly made chamomile tea can be sprinkled onto plants (and their substrates) that are affected by adult and larval fungus gnats. The oil should aid in significantly reducing the availability of fungi in the soil, which is what the larvae rely on to develop into their mature forms.
Once the life cycle of the fungus gnat is disrupted, the subsequent generation may struggle to expand their colonies and infest other plants. Repeated treatment targeting the larvae should reduce their chances of developing into egg-laying adult gnats. Consistently target problematic areas until there is a notable reduction in the occurrence of adult and larval gnats.