(Beyond Pesticides, July 5, 2017) The Hoverfly is July’s pollinator of the month. Hoverflies, also known as flower flies and syrphid flies, are members of the “true fly” order Diptera, family Syrphidae. There are roughly 200 genera and 6,000 species of hoverflies throughout the world.
Hoverflies are commonly seen in in flowering landscapes across the globe. According to the United States Forest Service (USFS), there are almost 900 species of the family in North America. They live in a range of habitats, including decaying wood, still and moving freshwater, on plants, and sometimes even in other insects’ nests. They are not often found in desert regions, and no known species have been discovered in Antarctica.
Diet and Pollination
The diet of hoverflies varies widely among species. In general, they are vital pollinators for a range of common flowering plants. Most adult flower flies have generalized mouthparts structured to sap up nectar and harvest pollen from open flowers; others use a long, beak-like proboscis to imbibe nectar from tube-shaped flowers; and some reportedly feed on the honeydew secreted from aphids. Because certain syrphid flies will feed on human perspiration, they are often mistaken for sweat bees. Unlike sweat bees, however, flower flies cannot sting.
In addition to being crucial wild pollinators, larvae of many flower flies, such as the common Allograpta obliqua, are voracious predators of garden and agricultural pests — particularly aphids. Approximately 40% of larval stage hoverfly species prey on soft-bodied pests. Larvae of these species use their piercing mouthparts to suck aphids dry. The larvae of other hoverflies are not predatory, but instead, feed on fungi or plant material. Some syrphid flies lay their eggs in the nests of ants or bees. Hoverfly larvae inhabiting ant nests consume eggs and larvae of the ants. On the other hand, Volucella species larvae eat dead bees and other organic matter in bumblebee nests.
Flower flies provide an excellent example of Batesian mimicry. Although hoverflies do not have stingers, a majority of North American species are black and yellow, resembling some bees or wasps, thus mimicking the warning signals of more dangerous insects. In addition to deterring predators, this camouflage permits certain flower fly species to lay their eggs in other creatures’ nests.
Hoverflies and other Diptera species can be distinguished from bees and wasps by the number of their wings. Flies have one pair of wings, while bees have two. As their name implies, their wings are used to hover over flowers, providing hoverflies an ability to change direction and location quickly. They are among the small number of insects who can fly backwards.
Syrphid fly larvae are much less charismatic than adults: they grow out of dull-colored, oval-shaped eggs, and are born legless and blind. Hoverflies undergo complete metamorphosis, meaning that they will pupate for one to four weeks before emerging in their adult form.
Syrphid flies are important pollinators, and many are critical to the cross-pollination of certain plant species. Accounts of pollinators in Colorado’s prairie found that 44% of flowering plants species investigated were pollinated by 16 hoverfly species. While a few species specialize in pollinating certain flowers, most flower flies are generalist pollinators, though they do show a notable preference for white and yellow flowers. Certain hoverfly species are laboratory reared and placed in greenhouses for pepper pollination, or for the production of seeds for seed banks.
Flower fly larvae play an ecological role in pest control and nutrient cycling. Of those that feed on aphids, high larval populations can reduce aphid numbers by 70 to 100%, according to the University of Florida. The larvae of some hoverfly species have been shown to consume an average of more than 30 aphids per day. Although ladybugs and lacewing larvae are more recognizable pest predators in gardens and crop fields, the unassuming larvae of the flower fly may deserve a good share of the credit. For instance, preliminary research finds syrphid flies able to maintain adequate control of aphids in California lettuce crops. Flower fly larvae will also feed on mealybugs, scales, and caterpillars. Syrphid larvae who live in other habitats often assist in the decomposition of organic wastes.
Threats to Existence
Research conducted in Europe finds that hoverfly diversity differs significantly between areas of high and low human activity. Flower fly populations in natural, undisturbed areas are more diverse, with a higher number of species specializing in the pollination of specific flower species. In areas with high levels of human activity, researchers discovered that more generalist hoverflies tend to dominate the landscape. USFS notes that, although no syrphid flies are currently listed under the Endangered Species Act, this may be a result of a dearth of information about these pollinators.
The University of Washington indicates that flower flies are “highly susceptible” to insecticides, and recommends pest managers forgo insecticidal control of aphids if hoverfly larvae have been spotted feeding on them. There is also evidence that pesticide use may deter flower flies from moving into an area in the first place. A 2013 study indicates that syrphid flies may avoid feeding in areas where field-realistic levels of bee-toxic neonicotinoid class insecticides are present.
How to Protect the Species
People can protect hoverflies through some simple actions. Planting a diversity of flowers in and around one’s garden and yard is a surefire way to promote flower fly populations. Flowers in the carrot family, such as dill, Queen Anne’s Lace, fennel, and parsnip are favorites of many species. Hoverflies will also frequent daisies, asters, borage, and buckthorn, as well as perennials such as goldenrod. Try to cultivate a variety of flowers that bloom throughout the year to ensure that forage is always available.
Avoid the use of pesticides that can harm these valuable insects. Neonicotinoids, once applied, will make their way into a plant’s vascular system, and express themselves in pollen and nectar, putting flower flies at risk. Other insecticides, like synthetic pyrethroids, are acutely toxic to many non-target pollinators, and may leave harmful residue on plants and their flowers once applied. Avoid herbicide use as well, which can destroy important habitat and nesting grounds for flower flies. Fostering biodiversity in your yard and garden will ensure a strong population of syrphid flies and other natural predators that will eliminate the need for insecticide use in the first place.
For more information on pollinators and the impact of pesticides on their health, habitat, and the ecosystem in which they live, please visit Beyond Pesticides’ Bee Protective program and be sure to click on What You Can Do.
Photo Source: Matt Cole Photography
BugGuide, N.D. Syrphidae. http://bugguide.net/index.php?q=search&keys=syrphidae
Encyclopedia of Life, N.D. Syrphidae: Flower Flies and Syrphid Flies. http://eol.org/pages/9017/details
Brenner, Kelly, 2015. The Metropolitan Field Guide. Urban Species Profile: Hover Flies.http://www.metrofieldguide.com/urban-species-profile-hover-flies/
Lavoipierre Frederique. Pacific Horticulture, 2017. Garden Allies: Hover Flies.https://www.pacifichorticulture.org/articles/hover-flies/
Ssymank, et al, 2008. Biodiversity. Pollinating Flies (Diptera): A major contribution to plant diversity and agricultural production. Vol 9 (1 & 2). https://repository.si.edu/bitstream/handle/10088/9619/FCT_115.pdf
University of Illinois Extension, 2014. In the Backyard: Sweat Bees and Syrphid Flies.http://web.extension.illinois.edu/lms/eb107/entry_8792/
University of Florida, 2014. Featured Creatures. http://entnemdept.ufl.edu/creatures/beneficial/hover_fly.htm
Shepherd, Matthew, and Scott Hoffman Black, N.D. USDA/Forest Service. Flower Flies. https://www.fs.fed.us/wildflowers/pollinators/pollinator-of-the-month/flower_flies.shtml
Warner, Geraldine, 1993. Washington State University. Syrphid flies (hover flies, flower flies).http://jenny.tfrec.wsu.edu/opm/displaySpecies.php?pn=730
Wikipedia. List of Syrphidae Genera. https://en.wikipedia.org/wiki/List_of_Syrphidae_genera [for a hint of the diversity of the family]