Flower Flies
By Matthew Shepherd and Scott Hoffman Black with contributions from Carol Kearns *
Fly or bee? At first glance, this might appear to be a bee, but there are several features that make it easy to separate this flower fly (Syrphus sp.) from a bee: the eyes are huge, the antenna are short and stubby with a bristle half-way along; there’s nowhere to carry pollen; and there is only one pair of wings. Photo by Alex Wild, www.myrmecos.net.
This flower fly (Eristalis tenax) is such a convincing mimic of honey bees that it is called the drone fly. Photo by Alex Wild, www.myrmecos.net.
The mouth parts of flower flies differ between species. Some have tube-like structures that allow them to drink from deeper more tubular flowers. The mouth parts of others, like the one shown here (Chrysotoxum sp.), are basically an extendible sponge that can sop up nectar from open flowers. Photo by Alex Wild, www.myrmecos.net.
Flower flies (family Syrphidae) are among the most colorful and conspicuous insects found around flowers. Of the nearly 900 species in North America, most have yellow-and-black stripes and are excellent mimics of wasps or bees. Flies can’t sting, but sounding and looking like insects that can makes birds and other predators avoid them. Indeed, one species, Eristalis tenax, is such a good mimic of the male honey bee that it is known as the drone fly. (Like the honey bee it mimics, this is a European species now widespread in North America.)
For some species, this mimicry confers more than just protection while they are foraging. Flies in the genus Copestylum, for example, use their disguise to enter unnoticed the nests of bumble bees or social wasps, where they lay their eggs. Their larvae feed on dead bees and other detritus in the nests.
Flies are among the most frequent visitors to flowers and important pollinators of a wide range of plants. When approaching a plant, many flower flies exhibit a characteristic flight pattern: they hover (and can abruptly change their position). This habit led to them being called hover flies in Britain.
The mouth parts of flower flies vary between species, giving different species access to nectar in different shapes of flowers. The typical flower fly mouth is essentially an extendible sponge that can mop up either nectar or pollen. The species that have this can feed on open flowers with accessible nectar. Some species of flower flies are specialized to feed at tubular flowers, and have a modified mouth that can form a narrow tube. With this, these species can forage on plants such as violet (Viola), thistle (Cirsium) and hedgenettle (Stachys).
The dietary habits of flower fly larvae are quite different from the adults. Whereas the adults generally forage in the sunshine, many larvae live in murky places. The trash-eating larvae of species in genus Copestylum, were mentioned above. Larvae of Eristalis are commonly called rat-tailed maggots; they scavenge in stagnant water or wet carcasses, using a long siphon tube to the surface to breathe air. Mallota larvae live in rot holes in trees.
Flower flies are not only important as pollinators in farms and gardens, but they also help to control pests. About 40% of the world species belong to groups with larvae that eat aphids, scales, and other soft-bodied pests. Next time you are in the garden if you see flower flies hovering around your rose bush look closely and you may just see a grub-like flower fly larva rearing up to consume an aphid.
In the US few threats have been identified for flower flies although studies in Europe have shown that overall species diversity and the number of specialist species is much lower in areas of intensive human activity. There are none on U.S. Endangered Species Act lists, although this may simply be because there is a lack of information on these generally understudied pollinators. (In Britain, however, seven of the twenty-two flies for which Biodiversity Action Plans have been prepared are hoverflies.) Despite the fact that none are listed, it still makes sense to consider them in habitat conservation and pollinator planting projects. Providing flowers for adults is a useful thing to do, but ensuring there are appropriate egg-laying sites and places where larvae can live is a vital component of their habitat.
Resources
- Kearns, C. A. 2002. Flies and flowers: an enduring partnership. In Wings: Essays on Invertebrate Conservation. The Xerces Society.
- Kearns, C. A. 2001. North American dipteran pollinators: assessing their value and conservation status. Conservation Ecology 5(1): 5.
- The Diptera Site.
- To see images, check out Tom Murray’s Syrphid Flies gallery on PBase.
- For help with identifying flies (and other insects), check out the Kaufman Field Guide to Insects of North America, by Eric Eaton and Kenn Kaufman (2007, Houghton Mifflin).
- Pollinator of the Month: Syrphid Fly
* This profile was prepared by Matthew Shepherd and Scott Hoffman Black of The Xerces Society for Invertebrate Conservation (www.xerces.org), with contributions from Carol Kearns of the University of Colorado at Boulder.