Fungus gnats are annoying pests that have plant-damaging larvae. Though they are harmless to humans and animals, fungus gnats can collect in swarms in outdoor herb plant beds or move into indoor herb gardens and land all over the house. Fungus gnats can make herb plants become sickly, slow growers. Reducing the number of fungus gnats is easy to do with attention to proper herb plant care and watering techniques.
Fungus gnats are small fly-like bugs that look like tiny mosquitoes. A fungus gnat problem indoors will start with annoying gnats everywhere, particularly on damp, sunny windowsills. Fungus gnats do not bite humans, but their larvae damage plants by chewing on the roots. Fungus gnat larvae are white and translucent, so they are difficult to see. An herb plant with a fungus gnat infestation may look sickly and have few leaves toward the bottom of the plant.
Fungus gnats are around one 10th of an inch long. Fungus gnat larvae can be up to 1/4 inch long.
Fungus gnats thrive in over-watered soil in indoor herb gardens. Fungus gnats on herb plants usually live for seven to 10 days as adults. Once an adult fungus gnat has laid an egg in an herb garden, the eggs take around four days to hatch, then approximately two weeks to develop into an adult fungus gnat.
The fungus gnat is often mistaken for moth flies, and infestations of the two can often occur at the same time. Moth flies, like fungus gnats, are tiny flies that don't bite. While fungus gnats can perform their entire life cycle indoors, moth flies are unable to reproduce indoors. In addition to the larvae attacking plant roots, moth flies also chew on plant leaves.
Since moist conditions like an over-watered herb garden can contribute to a fungus gnat problem, reducing watering and allowing the plant beds or pots to dry out between waterings can be the easiest way to reduce a fungus gnat problem.
When drying out the soil doesn't work, fungus gnats can be controlled using electrocution traps or yellow sticky traps. Fungus gnats are attracted to light, particularly to yellow light. To make homemade fungus gnat sticky traps, spread a thin layer of petroleum jelly onto a square of yellow construction paper and place it on the ground, jelly side up. Once the yellow square is flat on the ground, drizzle a small amount of vegetable oil into the card. Avoid getting oil in the soil. The petroleum jelly will seal the construction paper, keeping the oil on the surface to act as a deadly trap for unsuspecting fungus gnats.