Mother Nature's Montrose Garden: Garden Pollinators: Syrphid flies (Hoverflies; Flower Flies)

The word ‘pollinator’ conjures up images of Honey Bees for many. But the world of pollinators is far more complex. Our Garden Pollinators series features some of the many interesting pollinators seen in Western Colorado gardens. For more on pollinators in general – and tips for attracting pollinators to your garden – see: https://mothernaturesmontrosegarden.blogspot.com/2020/06/life-friendly-gardening-planning-for.html

Today we’d like to focus on an important group of pollinators, the Syrphid flies. Syrphid flies, commonly known as Hover flies and Flower flies, are frequent visitors to Western gardens. They are most often seen hovering near flowers. But many gardeners don’t appreciate their significant role, not only as pollinators but also as beneficial predators of well-known garden pests.

The Hoverflies belong to a large family of true flies – the Family Syrphideae. This family contains nearly 6000 species and 200 genera; there are around 900 species in North America alone [1, 2]. Syrphid flies inhabit much of the planet, the one exception being the continent of Antarctica. Few species also live in the very driest deserts. Syrphids are an old group of insects. Fossil species date back to the Eocene epoch (about 56 to 33.9 million years ago).

Like the native bees, Western U.S. Syrphid Flies range in size from small (less than ¼ inch) to about the size of a Bumble bee (nearly 1 inch). For examples of some common species see: https://en.wikipedia.org/wiki/Hoverfly#/media/File:Syrphidae_poster.jpg

Many species superficially resemble either bees or wasps (protective mimicry, also known as ‘Batesian mimicry’). Some have bodies that are compact and dense (like a bee) with a rounded head; others have slender bodies more like wasps. Their coloration may also mimic that of bees and wasps, often yellow with dark stripes. However, Hover Flies differ from bees and wasps in several important ways: 1) a single pair of wings (bees have two pairs) held in a characteristic, swept back (‘bomber’) position when perched (see above) ; 2) no stinger; 3) very large, compound eyes (compare above); 4) short, segmented antennae (see above); 5) mouths with relatively short, non-specialized mouth parts

Adult Flower flies are attracted to flowers as a source of food. They need both nectar (for energy) and pollen (for reproduction). They are superb fliers, with the ability to hover and fly backwards, a rarity among insects. All of this requires lots of energy (nectar). Some Flower flies specialize in a limited range of flower species. Many, however, are generalists, visiting many types of flowers and spreading pollen (pollinating) as they go. Pollinator studies in Colorado’s prairie found that ‘44% of flowering plants species investigated were pollinated by 16 hoverfly species.’ [3] They are likely also important pollinators in Western gardens.

In general, Flower flies are attracted to plants with many small, open flowers, often those growing in dense clusters. The yellow and white flowers of plants in the Sunflower (Asteraceae), Carrot (Apiaceae or Umbelliferae) and Buckthorn (Rhamnaceae) families, as well as those of the Borage (Boraginaceae) and Rose (Rosaceae) families provide both nectar and pollen.[4] As plants from these families are often planted in gardens, it’s not surprising that Flower flies are common garden visitors.

Wind pollinated species, such as grasses, sedges, pines, willows, cottonwoods, aspen and alders can be important sources of pollen, particularly for forest-dwelling species. Pollen provides the protein needed for egg production. So female Syrphid flies can be seen collecting and eating pollen, in addition to sipping nectar.

The life cycle of Syrphid flies contains several stages, starting with the egg and ending with the adult. Syrphids undergo a complete metamorphosis from egg to larva, pupa, and adult (see below).

The larval stages look very different from the adult. Larvae are legless and blind, emerging from white-gray eggs onto the plants on which the eggs were laid. They can be distinguished from caterpillar larvae by their tapered head, lack of legs and opaque skin (can see internal organs). The larvae grow quickly, through several stages (instars). All this growth requires food, and this is where many Syrphid species are invaluable as beneficial insects.

Larvae of many species eat aphids, thrips and other soft-bodied insects, the banes of many farmers and home gardeners. And they eat a lot; it’s estimated that a single larva consumes approximately 1200 aphids in its life. No wonder some farmers now plant flowers specifically to attract Syrphids to their vegetable crops. Larvae are even raised and released in greenhouses to pollinate crops and control such pests!

We hope you’re now convinced that Flower flies can play an important role in garden ecosystems, providing both pollinator and insect control services. In fact, you may want to attract more Syrphids to your home garden. The trick is to plant the plants they favor, being sure that some are blooming from spring to fall.

Since Syrphids (and other beneficial insects) are killed by insecticides, you’ll want to use these chemicals only when absolutely necessary. Beneficial insects are particularly susceptible to insecticides that contain neonicotinoids and synthetic pyrethroids.[6] A garden with many beneficial insects and birds may require little or no use of such chemicals.

We hope you’ve enjoyed learning about the Hover flies. Go out in your garden and look for them. They are fascinating and welcome creatures!