It’s difficult to control fungus gnats when they invade houseplants and greenhouses. Organic gardeners can kill fungus gnats without using chemicals.
Fungus gnats are more than annoying pests that buzz around the gardener’s face during plant care activities. These insects can damage plants with their feeding activities, and they serve as a vector for disease.
Identify Fungus Gnats
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Fungus gnats resemble mosquitoes in that they are tiny, winged black insects that have a slow, erratic flight pattern. Gardeners may first notice adult fungus gnats when they water their houseplants or greenhouse seedlings, as the gnats fly about when disturbed.
Fungus gnats thrive in warm, moist environments, but they can appear mysteriously at any time in indoor environments, even in winter.
This is because fungus gnats often gain entry into the household or greenhouse by hitching a ride as eggs in a bag of potting soil or larvae in a newly purchased plant.
Life Cycle of Fungus Gnats
Fungus gnats are attracted to fungal growth, and therefore the winged adults lay their eggs in environments rich in organic matter and moisture, which support the growth of fungi.
The eggs hatch into larvae in less than a week, and the larvae spend two weeks feeding on roots and stems. After maturation, the larvae spend several days morphing into adults in a pupa stage, after which they emerge from the soil.
Fungus Gnat Damage on Plants
Although clouds of adult fungus gnats swarming around the kitchen windowsill are a nuisance, fungus gnats do their worst damage in the larval stage.
The larvae eat young roots of seedlings, and they enter stems inserted into soil during plant propagation efforts. Fungus gnats cause the most damage to succulent stems, so gardeners may notice coleus, African violets, and geranium exhibiting leaf drop and yellowing foliage.
In addition to damaging plants directly, fungus gnats transmit diseases in the feeding process. Fungus gnats carry common plant pathogens like Verticillium and Fusarium from one plant to another.
Organic Controls for Fungus Gnats
The presence of all stages of the fungus gnat life cycle at once in affected plants makes control challenging. Organic gardeners must first discourage gnat infestations by maintaining a clean environment in the greenhouse or houseplant nursery.
Inspect floor drains for standing water, and clean any spilled compost or potting soil. Avoid purchasing torn bags of planting medium that could harbor fungus gnats.
Gardeners who notice adult fungus gnats can attack the insect at different parts of the life cycle. Gnatrol is a toxic bacterium that poisons the larvae when they consume it.
Beneficial nematodes sold as NemaShield or Nemasys kill gnats by entering the larvae and releasing toxic bacteria into the insects. Finally, gardeners can control the pesky adults with insect soap or ultra-fine horticultural oil.