The question of whether flies consume aphids is common for anyone managing a garden or farm. The answer is yes, but it depends entirely on the specific fly and its stage of life. Adult flies are not the ones actively hunting pests; their offspring are highly specialized predators. This difference in diet and lifestyle between the young and the adult is characteristic of this beneficial insect group.
Identifying the Predatory Species
The primary aphid predators belong to the family Syrphidae, commonly known as hoverflies or flower flies. They are often mistaken for bees or wasps due to their distinctive yellow and black striped bodies (Batesian mimicry). Unlike stinging insects, hoverflies possess only a single pair of wings, identifying them as true flies.
The adult hoverfly feeds mainly on nectar and pollen, making them valuable pollinators. They also consume honeydew, the sugary waste excreted by aphids, which helps them locate sites for laying eggs. The female deposits her eggs, often singly, directly onto leaves near an established aphid colony.
The Larval Hunting Mechanism
Predation occurs during the young stage of the hoverfly, known as the larva. These larvae are slug-like, legless, and typically range from translucent green to brown, blending in with the foliage. Lacking legs, the larvae move by inching and wriggling across the plant surface, relying on chemical cues to find prey.
Once an aphid is encountered, the larva quickly seizes it using specialized mouth hooks. It pierces the aphid’s soft body and sucks out the internal contents, leaving behind only the empty exoskeleton. This highly efficient feeding method makes them effective biological control agents. A single larva can consume 100 to over 600 aphids during its brief developmental period.
Attracting Beneficial Flies to Gardens
To encourage these natural pest controllers, gardeners should provide food sources for the adult flies. Since adults feed on nectar and pollen, a continuous supply of flowering plants is necessary throughout the growing season. Flies prefer flowers with open structures and white or yellow coloration, as these are easily accessible.
Specific plants known to attract adult hoverflies include herbs like dill, fennel, and coriander, and annuals such as sweet alyssum. Planting a diverse mix ensures food is available from early spring until late fall, supporting multiple generations. Avoiding broad-spectrum insecticides is also important, as these chemicals can indiscriminately kill the beneficial larvae.