The question of whether flies are harmful to plants is not a simple yes or no, as the term “fly” is a broad classification encompassing thousands of insect species. Belonging to the order Diptera, this group includes insects with vastly different diets and life cycles. Some species are serious agricultural pests, while others are harmless or even beneficial to plant health. Understanding the specific identity of the insect is the first step in assessing its impact on your garden or houseplants.
Common Flies That Damage Plants
The most frequently encountered flies that pose a threat to plants are the fungus gnat and the whitefly, each causing damage through distinct mechanisms. Fungus gnats (family Sciaridae) are primarily problematic in their larval stage, which lives concealed within the moist soil of potted plants or garden beds. The tiny, legless larvae feed on organic matter, fungi, and the fine root hairs of plants.
High populations of fungus gnat larvae consume delicate root tissue, impairing the plant’s ability to absorb water and nutrients. This results in symptoms like sudden wilting, stunted growth, and yellowing foliage. This damage is particularly devastating to seedlings and young cuttings. The adult gnats, which are weak fliers often seen resting on the soil surface, perpetuate the cycle by laying eggs in the damp growing medium.
Whiteflies (family Aleyrodidae) are closely related to aphids and scale insects, and they are significant pests of garden vegetables and greenhouse plants. Both the adult and nymph stages possess piercing-sucking mouthparts that they insert into the plant’s phloem to feed on sap. This feeding weakens the plant, causing leaves to turn yellow, dry out, and drop prematurely.
A secondary problem arises from the whiteflies’ excretion of a sugary waste product called honeydew. This sticky residue coats the leaves and serves as a growth medium for black sooty mold, which blocks sunlight. The sooty mold interferes with photosynthesis, reducing the plant’s vigor and overall health. Certain species of whiteflies are also capable of transmitting plant viruses, which can lead to crop losses.
Flies That Are Harmless Nuisances
Not every small, dark fly hovering near a plant is a cause for alarm, as many are harmless nuisance species often confused with the destructive fungus gnat. Shore flies (family Ephydridae) are frequently mistaken for pests because they thrive in moist, algae-rich environments like greenhouses. However, their larvae feed exclusively on algae and decaying organic matter rather than live plant roots.
Adult shore flies are stronger fliers, appearing more robust than fungus gnats, and often feature short antennae and distinct light-colored spots on their dark wings. While they do not damage the plant directly, their presence and the dark fecal droppings they leave on leaves can be an aesthetic issue for ornamental plants. Common fruit flies (Drosophila species) are another common indoor fly, but their life cycle is centered around fermentation and decaying fruit. They are rarely a threat to healthy plant tissue, only seeking out ripe or rotting produce.
Flies That Benefit Plant Health
Flies play beneficial ecological roles, often serving as natural pest control agents and important pollinators. Hoverflies (family Syrphidae), also known as flower flies, are a prime example, with adults acting as generalist pollinators second only to bees in some ecosystems. These adults are often mistaken for bees or wasps due to their black and yellow striped bodies, but they are harmless and feed on nectar and pollen.
The benefit of the hoverfly comes from its larval stage, which is a predator of soft-bodied garden pests. A single hoverfly larva can consume hundreds of aphids during its development, making them effective biological control agents against common nuisances like aphids, thrips, and scales. Another beneficial group is the Tachinid flies (family Tachinidae), which are parasitoids of many destructive insects.
Tachinid flies lay their eggs on or near the bodies of host insects, such as caterpillars, beetle larvae, and true bugs. Once the egg hatches, the fly larva burrows inside the host, feeding internally and eventually killing it before emerging to pupate. This parasitic lifestyle makes them invaluable natural enemies that help regulate the populations of garden pests.
Targeted Control Methods
Controlling pest flies requires methods tailored to the specific life stage that causes the damage. For fungus gnats, the most effective strategy is to target the root-feeding larvae and disrupt the moist conditions they require. Allowing the top inch of soil to dry out completely between waterings eliminates the favorable egg-laying sites for the female gnats.
A biological control agent containing Bacillus thuringiensis israelensis (BTI), a naturally occurring soil bacterium, can be applied as a soil drench. The BTI is ingested by the larvae, where it releases a toxin that destroys their gut lining, providing targeted control without harming other organisms. Yellow sticky traps placed horizontally on the soil surface will capture the low-flying adults, reducing the number of eggs laid.
Whitefly management requires focusing on both the adults and the immobile nymph stages, which cluster on the undersides of leaves. Yellow sticky traps placed vertically among the foliage can capture the flying adults, which are strongly attracted to yellow. For active infestations, insecticidal soaps or neem oil should be applied with thorough coverage, especially targeting the leaf undersides where the pests congregate. These treatments work by smothering the pests and disrupting their life cycle, but they require repeated applications to catch newly hatched nymphs.