Laidback Gardening with Robert Kourik
Syrphid Flies: The Unknown Aphid Buster
The larvae of syrphid flies (of the family Syrphidae, also called hoverflies) are some of the most voracious predators of aphids. Compared to ladybugs, however, they’re little known, and their larvae, easily mistaken for slugs, are often mistakenly killed. Fortunately there are plenty of them—one common genus of hoverfly includes over 300 species on the American West Coast alone. The syrphid fly lives up to its alternate name— it can hover in the air in front of flowers like a helicopter, something bees can’t do. Syrphid flies are also often mistaken for bees or wasps because of their various types of horizontal yellow-and-black striping, and thus, like their larvae, mistakenly killed.
If they survive, syrphid fly larvae eat lots of aphids. A single larva may eat much more than its body weight in aphids in its life, making them much better predators of aphids than ladybugs. The adults fly from flower to flower to gather the nectar and pollen they need to survive, and should be treasured as pollinators as well.
Because the larvae look like tiny slugs, they’re often mistaken for pests and squished or sprayed—the poor little guys get it on all fronts. All sprays, organic or chemical, will not only kill syrphid larvae but also their favorite food—aphids, not to mention all other insects, whether beneficial or not. Check to see if a plant you are concerned about has such a low level of aphids that it’s not a problem, and leave the plant alone, as nature maintains its natural balance. The good guys can’t survive without some of the bad guys to eat. If you’ve ever paused near a patch of pollenladen flowers on a sunny summer afternoon, there’s no mistaking the unique flight of the hoverfly as it zigzags about the blossoms, occasionally stopping in midair, its shimmering wings barely visible. Unlike most other winged insects, flies in the order Diptera (meaning two-winged) have only one pair of flying wings. The second pair of wings, known as “halters,” consists of two little knobs that function as miniscule gyroscopes, allowing the flies to quickly change direction—a useful ability when being chased by predators or by a biped armed with a swatter. (Not that any conscientious gardener should want to swat syrphids! With few exceptions, hoverflies are among our most abundant and helpful garden allies.)
In an article by the respected entomologist Frederique Lavoipierre, originally published in Pacific Horticulture Magazine, 2007, Vol. 68, No. 2.), we read: “Adult hoverflies 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 hoverflies, 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, hoverfly 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.
Hoverflies are among 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 hoverfly was found to exert from 70- to 100-percent control of aphids. After capturing its victim, the hoverfly 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. The complex habitat and diversity of flowering plants found in most gardens create an ideal environment for attracting hoverflies. Because many species pupate and overwinter in leaf litter, a permanent mulch will encourage persistent populations. Hoverflies are also sensitive to wind; if your garden occupies a windy site, provide some shelter near flowerbeds. Predatory hoverflies 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.
In yet another study, phacelia ( Phacelia tanacetifolia, a flowering plant in the borage family also known as lacy phacelia, blue tansy, or purple tansy) attracted both bees and hoverflies. The favorite flowers of syrphid flies are in the daisy ( Asteraceae) and umbel ( Apiaceae) families. The flies also feed on nectar and pollen in a wide range of other plant families, including roses ( Rosaceae), buckthorns ( Rhamnaceae), borages ( Boraginaceae), and willows ( Salicaceae).
In a thesis by Elsa Laubertie, it was found that phacelia (Phacelia tanacetifolia), followed by buckwheat ( Fagopyrum esculentum), and then coriander ( Coriandrum sativum), in order of attractiveness, had the best potential to attract feeding female syrphids. The hoverflies that had fed on phacelia and buckwheat pollen were found up to 17.5 meters (57 feet) from the actual floral areas, so companion planting the plants right next to each other isn’t required.
Robert Kourik is the author of 18 books on gardening, including Lazy-Ass Gardening and Understanding Roots. For a full list, visit www.robertkourik.com. This column is an excerpt from Kourik’s forthcoming book,
Sustainable Food Gardens, Myths and Solutions.