Pesky little gnats flying around your plants? Are there tiny, narrow slime trails crisscrossing your plant's soil? If so, fungus gnats have taken up residence in your potted plants. Fungus gnats are tiny greyish black flies about 1/8 inch long. They feed on decaying, organic material and breed in the soil of potted plants and outdoor in compost piles. Fungus gnats live very short lives--about four weeks. An adult female lays around 200 eggs before dying. Fungus gnat larvae feed on small feeder roots of plants making them susceptible to root rot. Larvae infestation signs are sudden wilting, yellowing leaves, and poor growth.
Over-watering potted plants creates a fertile feeding ground for these gnats. Allow the top inch of plant soil to dry completely before watering again. Lowering the soil moisture will kill the larvae. Good drainage aids in destroying moist habitats.
Lay yellow sticky traps across the soil surface to trap adult gnats. Adult flies are attracted to the yellow colour. Replace traps as they become covered with gnats, insects or debris. Sticky traps are available at hardware stores. Traps must be combined with other control methods to stop the gnat cycle.
Drench the soil with beneficial nematodes. Nematodes enter the larvae and release a toxin that kills the host. Nematodes are so small they cannot be seen without a magnifying glass. Nematodes are sold through catalogues and online as ScanMask and Nemasys.
Wet the soil thoroughly with Gnatrol. Gnatrol is a bacterial treatment which paralyses the larvae gut which stops feeding and ultimately kills the larvae. Gnatrol stays active for two days so repeat applications are required. Gnatrol is sold through catalogues and online.
Sprinkle predatory mites, hypoaspis mites, over the soil or mix them into the potting soil before planting. These tiny mites live in the top layer of soil and attack gnat larvae and even scavenge on dead insects. Adult mites kill an average of five eggs and larvae per day and can survive a month without food. Hypoaspis mites are sold online.
Chemical sprays are ineffective for fungus gnats. Sprays kill the adults, but rarely affect the larvae. Removal of larvae is only accomplished by insecticide drenches. Very few insecticide spray and drenches are safe for use in the home. As a last resort, move plants outside to spray and drench with insecticide. Once the spray has dried thoroughly and the odour is gone, move the plants back inside.
Watch for infestations in August. When the weather is hot and dry, the gnats move inside to find moist soils. Repair broken window screens to ensure the gnats stay outside.