Flies belong to the insect order Diptera, meaning they possess only two functional wings, and whether they eat leaves depends entirely on their life stage. The vast majority of adult flies do not consume solid plant tissue like leaves because their mouthparts are not designed for chewing. However, the immature, non-winged larval stage of certain fly species are herbivores, feeding directly on the internal structure of leaves, stems, or roots. This difference between the adult and the larva dictates the specific diet and interaction with plant life.
The Typical Adult Fly Diet
Mature, winged flies sustain themselves on a diet composed exclusively of liquids. Their feeding apparatus is specialized for this liquid diet, typically involving a proboscis adapted for either sponging or piercing. The sponging-type mouthparts, common in species like the house fly, cannot bite or chew solid material.
Instead of consuming leaves, these flies use digestive fluids to liquefy solid substances, which are then sponged up for ingestion. Common food sources include sugary plant secretions, flower nectar, decaying organic matter, and bodily fluids. Flies with piercing mouthparts, such as mosquitoes, are built to penetrate surfaces to access blood or plant sap.
Even when adult flies interact with plant surfaces, they are usually seeking moisture, sugar, or a site to lay eggs, not solid leaf matter. They lack the mandibles necessary to process the tough, fibrous cellulose that makes up a leaf.
When Fly Larvae Consume Plant Tissue
The larval stage, often called a maggot, contains the exceptions to the “no leaf-eating” rule, as these immature forms are dedicated herbivores. These legless larvae possess mouthparts that allow them to physically consume plant tissue, in stark contrast to their parents. Specific families of flies have larvae that specialize in feeding on different parts of a plant.
One prominent example is the leaf miner, which are the larvae of small flies in families like Agromyzidae. The female adult fly lays eggs directly on or into the leaf surface, and upon hatching, the maggots tunnel into the leaf’s interior, feeding on the soft mesophyll cells between the upper and lower epidermal layers. This internal consumption creates the characteristic serpentine or blotchy trails known as mines, directly destroying the photosynthetic tissue.
Root maggots, such as the onion maggot and cabbage maggot, demonstrate another form of herbivory, targeting subterranean plant structures. These larvae feed on the roots, basal stems, and bulbs of host plants. The maggots use hooked mouthparts to rasp and tunnel into the plant tissue, causing significant damage that can lead to wilting and plant death, especially in seedlings.
The larval stage is structurally adapted to burrow and feed within the moist, protected environment of the plant or soil. This feeding provides the energy and nutrients the maggot needs before it pupates and transforms into a non-herbivorous adult fly.
Other Ways Flies Affect Plant Health
Beyond direct consumption by larvae, flies interact with plants in ways that both harm and benefit their overall health. One significant negative interaction is the transmission of plant pathogens. Adult flies can mechanically vector bacteria and fungal spores, carrying them on their feet and bodies from decaying matter to healthy plant tissue.
When a fly lands on a rotting piece of fruit or feces and then walks across a healthy plant, it can introduce disease-causing microorganisms. The fly’s own feeding punctures or egg-laying wounds can also create entry points for these pathogens, leading to secondary infections like soft rot.
Fungus gnats, another group of small flies, are often pests whose larvae primarily feed on soil fungus and decaying organic material. However, when these preferred food sources are depleted, the larvae will turn to consuming delicate plant root hairs and roots, stunting the growth of seedlings and potted plants. This root damage inhibits the plant’s ability to absorb water and nutrients, resulting in indirect harm to the plant’s foliage.
On a beneficial note, many types of flies, including certain fungus gnats and hoverflies (Syrphidae), act as effective pollinators. As they move between flowers to feed on nectar, they inadvertently transfer pollen, playing a part in the reproduction of various plant species. This pollination service is a positive contribution to plant health and biodiversity.