Garden Allies: Hover Flies

Meet the Hover Flies

If you have ever paused near a patch of cosmos on a sunny summer afternoon, you are certainly familiar with the unique flight of the hover fly, as it zigzags about the blossoms, occasionally stopping in mid-air, its shimmering wings barely visible. There is no mistaking it. This ability to hang suspended in mid-air explains its common moniker, hover fly. Unlike other winged insects, flies in the order Diptera (meaning two-winged) have only one pair of flying wings. The second pair of wings is reduced to two little knobs, the halteres; these function like miniscule gyroscopes, allowing the flies to quickly change direction—a useful ability when being chased by predators or a biped armed with a swatter. Not that the conscientious gardener would want to swat most syrphids! With few exceptions, hover flies are among our most abundant and helpful garden allies.

Even for those familiar with common garden hover flies, a closer look will reveal a surprising diversity of species. Many mimic bees, with coloring that advertises danger. The careful observer will note, however, that the hover fly has shorter antennae, a stout waist, and only one pair of wings (the bee has two pairs). Other common species lack the bee’s familiar markings or shape. Sphaerophoria cylindrica is slender, and, as the name implies, cylindrical; except for its markings, it really does not resemble a bee at all. Large hover fly has a wide, somewhat flattened abdomen, not at all like the plump shape of bees. Most hover flies have glossy bodies, but a few species are fuzzy and resemble bumble bees. A defining characteristic of the Syrphidae is a “spurious” wing vein—a fold in the middle of the wing membrane that is best seen with a hand lens.

Adult hover flies feed on nectar and pollen and, occasionally, on the honeydew exuded by aphids. They need nectar to fuel their high-energy flight, and females need pollen to produce their eggs. Most hover flies, and the majority of the common garden species, have aphid-feeding larvae, while a few prey on leaf-beetle larvae and eggs, or on other insects. Resembling small slugs or caterpillars, the larvae are easily mistaken for pests. Lacking eyes, hover fly larvae were long thought to blindly detect their prey by raising the front half of their body and swinging it from side to side until they bumped into a tasty morsel. Evidence now shows that at least some species detect their prey through chemical odors.

Hover flies are one of the most useful natural enemies of plant pests; some species have been estimated to eat up to 1,200 aphids during the larval stage. In one study, the diminutive chevroned hover fly was found to exert from seventy to one hundred percent control of aphids. After capturing its victim, the hover fly larva holds it in the air, sucking it dry of juices; nearby aphids or beetle larvae appear to pay no attention to the demise of their unfortunate neighbor. Not all hover flies prey on pests. The larvae of some species dwell in the nests of social insects, such as bees, wasps, and ants, where they scavenge refuse. Some are pests of roots, stems, and bulbs (narcissus fly larvae), but, for the most part, these are of no concern to the gardener, who is unlikely to suffer any significant damage. Those that frequent decaying organic matter aid in decomposition.

The British have a particularly well-studied insect fauna. Jennifer Owen, author of The Ecology of a Garden, recorded ninety-one of the 256 recognized species of hover flies in the British Isles over a period of fourteen years in her suburban garden. Each year brought changes in the composition of species in her garden, and she notes that the complex habitat and diversity of flowering plants found in most gardens create an ideal environment for attracting hover flies. Because many species pupate (and overwinter) in leaf litter, a permanent mulch will encourage persistent populations. Hover flies are also sensitive to wind; if you have a windy site, provide some shelter near flowerbeds. Predatory hover flies seek out pest colonies in which to lay their eggs; females of these species are bound to find a multitude of egg-laying sites in gardens!

Planning a long season of bloom is the best way to attract hover flies. The most obvious flowers may only harbor certain species, so, although hover flies may be abundant at flowers such as cosmos, do not overlook other plants. Because hover flies require pollen to produce eggs, plants such as willows, grasses, and sedges that provide pollen early in the spring may be especially valuable for syrphids that can control early outbreaks of aphids. My favorite plants for attracting hover flies (and many other beneficial insects) are those that reseed without much thought on my part. Alyssum tops that list, because it is virtually never out of flower and is attractive to many of the smaller species of hover flies. Favored lunch stops are calendula, cosmos, cornflowers (Centaurea), Agrostemma, meadowfoam (Limnanthes), and umbellifers such as Angelica and Queen Anne’s lace (Daucus carota). Butterfly bush (Buddleja) and perennials such as Iberis, yarrow (Achillea), and goldenrod (Solidago) have a place in any garden planted to attract insects. Perhaps the best flowers for late season are the fall asters, and I have recently found hover flies on a warm January day on the flowers of Bidens. The helpful hover flies love a garden full of flowers—and so do I!

In a Nutshell

Popular Names: 
Hover flies, flower flies

Scientific Name: 
Order: Diptera (flies), Family: Syrphidae

Common Beneficial Species: 
Aphid-eating syrphids include: chevroned hover fly (Allograpta obliqua), large hover fly (Scavea pyrastri), western hover fly (Syrphus opinator), Metasyrphus meadii, Sphaerophoria cylindrica, Paragus tibialis, Toxomerus occidentalis.

Distribution: 
Over 300 species known on the West Coast; about 6,000 worldwide.

Life Cycle: 
Holometabolous (a complete metamorphosis from egg to larva, pupa, and adult).

Appearance: 
Eggs: oblong, white to gray, laid singly near aphids or other prey. Larvae: like small caterpillars or slugs; often green with longitudinal white stripe; may also be brown, yellow, or almost transparent. Pupae: small brown cases found on leaves near aphid colonies, or in leaf litter. Adults: 4-35 mm (.14-.8″); usually marked with yellow or white bars or spots on a dark background, with short antennae and stout waists; often resemble bees or wasps.

Life Span: 
Different syrphids may have one or multiple generations per year; this may also vary with geographic location. Many species have a long flight season, and some overwinter as adults.

Diet: 
Larvae of beneficial garden species are mostly aphid-eaters, but a few eat other insects such as leafbeetle larvae, whiteflies, and moth caterpillars. Many syrphids are facultative feeders on aphids, meaning that, in the absence of aphids, they are able to complete their development by eating pollen or rotting vegetation. Some specialize on root aphids. Others feed on decomposing organic matter. Larvae of a few species feed on bulbs, plant stems, and sap. Adults feed primarily on nectar and pollen.

Favorite Plants: 
Flowers in the daisy (Asteraceae) and umbel (Apiaceae) families. Syrphids also feed on nectar and pollen in a wide range of other plant families, including rose (Rosaceae), buckthorn (Rhamnaceae), borage (Boraginaceae), and willow (Salicaceae).

Benefits: 
Larvae of many species are voracious predators of pest insects. Adults pollinate flowers.

Problems: 
A few syrphids are pests; larvae of the narcissus fly (Merodon species), for example, feed on bulbs.

Interesting facts: Males of most syrphids can be distinguished from females by differences in the eyes, which meet at the top of the head in males but are widely separated in females. A well-known bee mimic is drone fly (Eristalis tenax), introduced from the Old World. Eristalis is the source of the myth that honey bees were spontaneously generated from animal carcasses. Some species have aquatic larvae, known as rat-tail maggots, and can live in highly polluted environments; a long breathing tube (the “rat tail”) extends to the water’s surface.

Sources: 
Garden habitats that attract hover flies provide food, egg laying, and overwintering sites.

More information: 
British Hoverflies, second edition (AE Stubbs & S Falk, BENHS, 2002) is primarily a key for the enthusiast, but offers beautiful color plates of the insects.