Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden.
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Dark-winged fungus gnats are grayish-black, delicate-looking, and slender with comparatively long legs and antennae, like tiny mosquitoes on steroids.
Fungus gnats have segmented antennae longer than their heads and thin legs, and their long antennae can distinguish them from shore flies with shorter, bristle-like antennae. Fungus gnats (Bradysia) have a distinct Y-shaped pattern on their forewings.
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How to Identify Fungus Gnats
Fungus gnat adults typically measure between a sixteenth to an eighth of an inch (2 mm) long, while some species can grow up to half an inch, though these are not the ones you need to worry about.
The larger species, known as winter gnats (Trichoceridae), resemble oversized gangly mosquitoes. They’re more common in the UK.
Adult fungus gnats are drawn to light, so you may initially see them flying close to windows indoors. However, fungus gnats are relatively poor flyers and typically don’t move around indoors, in contrast to more active species like the common housefly (Musca domestica).
Fungus gnats can frequently be seen around potted plants or standing on growing mediums. CO2 attracts them, so they often fly to your face – a considerable irritant.
In potting soil or damp organic waste, females lay their small eggs. Larvae have an elongated, whitish-to-clear body and a shiny, black head.
They consume compost, root hairs, fungi, leaf mold, grass clippings, and organic mulch. If the environment is particularly damp and there are lots of fungus gnats, larvae may produce slime trails on the surface of the medium that resemble those of little snails or slugs.
Fungus gnats are common and can be found anywhere that the right conditions exist for their development. These insects are common in nurseries or greenhouses where there’s moist organic soils and plants are overwatered.
Identifying Fungus Gnats in Their Different Stages
Fungus gnats are holometabolous insects and undergo complete metamorphosis in each of the four stages: egg, larvae, pupa, and adult.
Identifying Fungus Gnat Eggs
The tiny yellowish-white eggs are so small you may need a microscope to see them individually. A single egg is between four-hundredths of an inch wide (0.1 mm) and eight-hundredths of an inch (0.2 mm) long.
The tiny eggs can be found in damp organic media and hatch when soil temperatures are around 75 to 77 °F (23 – 25 °C) after about three days.
A female fungus gnat may lay between 100 and 300 eggs in clusters of 20 to 30. Eggs are laid on the soil or in little crevices if she finds any.
Identifying Fungus Gnat Larvae
The insect larva goes through various molting stages, during which it repeatedly sheds its old cuticles to grow new ones.
Fungus gnat larvae are campodeiform with a whitish-to-clear body and a shiny, black head. They consume compost, root hairs, fungi, leaf mold, and organic mulch.
Fungus gnat larvae live in damp, decaying organic material—they can commonly be found in outdoor mulch beds, compost piles, rotting logs, wet soil, and indoor plants overwatered.
If the environment is particularly damp and there are lots of fungus gnats, larvae may produce slime trails on the surface of the medium that resemble those of little snails or slugs.
They can cluster in a group and move above the ground, resembling a small snake or a really long worm. Mature larvae are about a quarter of an inch (5.5 mm) long.
Identifying Fungus Gnat Pupae
The pupa spends most of this period sleeping and not doing much. The pupa’s mature form is formed by the tissues of the larva collapsing and reforming.
The pupa goes through this process, molts, and then emerges as an adult with wings. When an insect is in the pupa stage, its wings take on their final shape and emerge as an adult.
Initially white, fungus gnat pupae become dark shortly before the adult emerges after about a week.
Identifying Fungus Gnat Adults
These are not very hard to miss, and I have described them above. What’s important to note is that adult gnats are harmless to plants and humans and only live for a week.
Adult fungus gnats have poor flight abilities and may fly in unpredictable, brief patterns. They are frequently observed close to an infected houseplant indoors. Adult flies, however, can travel short distances and often gather along window frames.
Identifying Fungus Gnat Damage
Adult fungus gnats don’t cause a lot of direct harm to plants, but be aware of the risks fungus gnat larvae present. Damaged roots that impede plant growth, particularly in young plants, result from fungus gnat larvae infestations.
High numbers of fungus gnat adults and larvae are linked to potting soils that are kept too moist. They also thrive in lawns that are kept wet.
Damaged plants begin to wilt and lose their vitality, but you will generally see the adults before you notice any plant damage. Fungus gnats are also prime spreaders of pathogens, so the damage isn’t isolated to their activities.
Summarizing How To Identify Fungus Gnats
I have provided a comprehensive overview of identifying fungus gnats at their different metamorphic development stages. It is becoming more apparent how damaging these seemingly harmless creatures can be.
To completely eradicate fungus gnats, stop the eggs from hatching to prevent the larvae from developing. Failure to proactively fight these insects in their early stage means you’ll keep dealing with the nuisance of adult fungus gnats.
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