Hover flies, also known as flower flies or syrphid flies, belong to the insect family Syrphidae, encompassing over 6,000 species globally. These insects are often mistaken for bees or wasps because their black-and-yellow banded bodies exhibit a form of protective mimicry. A defining characteristic of hover flies is the shift in diet between life stages, which allows them to occupy highly specialized ecological niches.
The Adult Diet: Fueling Flight and Reproduction
The adult stage of the hover fly is strictly non-predatory, focusing its diet entirely on plant-derived liquids and solids. Their primary energy source comes from nectar, which provides the simple sugars necessary to power their distinctive, high-energy, hovering flight behavior. Because of their need for accessible fuel, adult hover flies are frequently found visiting flowers with open blooms where nectar is easily reached.
Pollen consumption is equally important for the adult female, as the proteins and lipids within pollen are required for the maturation and production of viable eggs. Most adult species possess generalized mouthparts that function for sponging or lapping up these liquid and semi-solid food sources from the floral surface. Some species, however, have evolved a longer, beak-like proboscis to access nectar from more tubular flowers.
In addition to floral resources, many adult hover flies are also known to feed on honeydew, a sugary waste product secreted by sap-sucking insects like aphids. This sugary substance provides an alternative source of quick energy, and by consuming it, the flies help prevent the growth of sooty mold on plant leaves. The adult diet establishes the fly as a herbivore or detritivore, completely contrasting with the carnivorous habits of many of their offspring.
The Larval Diet: Specialized Roles
The larval stage of the hover fly is where the most significant dietary specialization occurs, with species falling into one of three distinct feeding groups. Larvae of different species are highly adapted to their specific food source, making this life stage the main determinant of the fly’s ecological role.
Predatory Larvae
A substantial portion of hover fly species have larvae that are voracious predators of soft-bodied agricultural and garden pests. These larvae are blind, legless, and maggot-like, relying on chemical senses and touch to locate their prey. They are particularly effective against aphids, which is why the adult flies often lay their eggs directly onto aphid-infested plants.
Once a larva finds a victim, it seizes the prey with its hook-like mouthparts, pierces the body, and sucks out the internal contents, leaving behind only the shriveled exoskeleton. A single predatory larva can consume between 100 and 400 aphids during its development period, depending on the species and the size of the prey. Their diet is not limited to aphids, however, as they also consume other slow-moving pests like thrips, mealybugs, and scale insects.
Saprophagous Larvae
Another large group of hover fly larvae are saprophagous, acting as nature’s decomposers by feeding on decaying organic matter. These species consume rotting vegetation, animal dung, and liquefied organic waste found in stagnant water or sewage. This feeding habit is exemplified by the drone fly larva, Eristalis tenax, commonly known as the “rat-tailed maggot”.
The rat-tailed maggot lives submerged in oxygen-poor, highly organic liquids and possesses a telescopic breathing tube that can extend several times the length of its body. This specialized siphon allows the larva to breathe air from the water’s surface while remaining submerged to filter-feed on microorganisms and decaying materials below. Other saprophagous species feed on dead wood and compost, contributing significantly to the recycling of nutrients in various ecosystems.
Phytophagous Larvae
The smallest group of hover fly larvae are phytophagous, meaning they feed on live plant material. These species are sometimes considered minor pests in agriculture or horticulture, as they burrow into and consume plant tissue. Examples include the larvae of the large bulb fly (Merodon equestris), which tunnel into and feed on the inside of plant bulbs, such as daffodils.
Ecological Impact: Pollinators and Pest Controllers
The disparate diets of the adult and larval hover fly stages result in two major, interconnected ecological services: pollination and biological pest control. Adult feeding on nectar and pollen makes the hover fly a highly valuable pollinator for both wildflowers and commercial crops. They are widely considered to be the second most important group of insect pollinators globally after bees, visiting at least 72% of the world’s major food crops.
The secondary role of the adult as a pollinator is particularly significant in regions or seasons where bee populations are less active or absent. This pollination service is estimated to be worth hundreds of billions of dollars annually to global agriculture. The adults’ need for floral resources also directly supports the reproductive success of their predatory offspring by encouraging females to lay eggs in areas with abundant prey and pollen sources.
The predatory behavior of the larvae provides an extremely effective form of natural pest control. High populations of these larvae can suppress aphid numbers on plants, with documented reductions ranging from 70% to 100% in certain crops. This biological control mechanism offers a non-chemical solution for managing agricultural and garden pests like aphids and thrips. Furthermore, the saprophagous larvae contribute to essential nutrient cycling by breaking down organic waste, which cleans up environments and makes nutrients available to the soil.