Fly Pollination

 Flies don't get much respect, we think of them mainly as pests. However, they are important pollinators.

Even as pollinators, although flies are common and numerous, they are often considered poor pollinators. Pollination is not as simple as an insect visiting a flower and getting pollen on it. To pollinate, the pollen has to come off the insect onto the stigma of a receptive flower. While many plants self-pollinate, others require that the pollen moves to a flower of a genetically different member of the same species. That is where flies fail. They feed at flowers, yes, but may fly to something else that interests them, or a flower of a different species, where the pollen rubs off, wasted. (Compare a honeybee flying iamong the penstemons, stopping at flower after flower.)

fly on a salsify (Tragopogon) flower

Self-pollinated flowers, or clusters of flowers such as the flower heads of sunflowers or carrots, can and do take advantage of pollination by flies. As the flies walk across or between flowers looking for food, they move pollen from one flower to the next. I have lots of photos of flies standing on flowers. Presumably some of them pollinated the flowers. 

The catch on the above viewpoints is that flies are incredibly widespread and diverse. The stereotypic house fly (Musca domestica) is just one of more than 110,000 species of flies, the second-largest group of insects (after beetles). At least 71 of the 108 (or 110 or 130) fly families have species that regularly pollinate. Among the pollinators, some are specialized pollinators in a tight mutualism with a plant species. Others are generalists that nevertheless are the major pollinators of some plants. They are the primary pollinators of cacao (source of chocolate, Theobroma cacao), and important polliators of crops from onions and carrots to apples and tea. 

Flies that pollinate can look like house flies but often they do not. 

This fly is a bee mimic. It is not a bee, because it has fly-like compound eyes and only one pair of wings (bees have two pairs).  Maybe a drone fly (genus Eristalis, family Syrphidae, but there are 200 genera of hoverflies and lots of them bee mimics. They feed at flowers as if they were bees and so pollinate) (link)

bee-mimicking fly on a bachelor button (Centaurea cyanus) flower

Here, hovering on the penstemon is another fly, a beefly (Bombylidae). These are specialist flower-feeders and good pollinators. 

beefly visiting penstemon flower

And another flower-visiting fly that isn't an ordinary housefly, again probably a hoverfly. 

fly visiting goldenrod (Solidago) flower

Some flies are attracted to rotting meat, carrion flies for example, and flowers have adapted to that, creating rancid smells to attract them. Pretty much all of the pollinators of red trillium (Trillium erectum) and eastern skunk cabbage ( Symplocarpus foetidus) are flies. 

red trillium Trillium erectum 
smell the flower, if you get a chance

A variety of waterlilies attract flies (and/or other insects) with putrid odors and trap them in their flowers. The flowers close so that, overnight, the flies get a decent meal of nectar or oil but are dusted with pollen at the same time. In the morning, the flower reopens and the flies hurry away to the next flower that smells enchantingly of decay.

Fly behavior is very temperature- and humidity-sensitive and so fly visits to flowers vary greatly with the weather. 

Some species of fly can visit flowers at temperatures too cold for bees. In the Arctic and at high elevations, this is an important service and in many tundra or near-tundra communities, flies are by far the most common and most important pollinators. 

At the other climate extreme, the complexity of interactions in hot humid tropical environments means some plants have specialized on flies as their pollinators and others are mostly pollinated by flies even if they are also visited by other insects. 

fly on a chokecherry flower

 

Because dozens of different groups of flies visit flowers, all over the world, the topic of fly pollination is huge. Three papers covering the subject (listed in references) show both the diversity and distribution of fly pollination. They also highlight that there is more unknown about fly pollination than is known. 

Thus, when you see a fly on a flower, relax of your prejudice against flies and consider whether it is an important pollinating insect. And, when you see a bee, look closely in case it is really a bee-mimicking fly. 

Comments and corrections welcome.

References

Inouye, D. W. ,  B. M.H. Larson, A. Seymank, and P. G. Kevan. 2015. Flies and Flowers III: Ecology of foraging and pollination. Journal of Pollination Ecology. 16: 115-133.

Larson, B.M. H., P.G. Kevan and D. W. Inouye. 2001.Flies and flowers: taxonomic diversity of anthophiles and pollinators. The Canadian Entomologist. 133: 439-465.

Mlynarek, J. J. 2022. Diptera pollinators. Current Biology Magazine. 32: R295-R310.  link (Accessed 8/10/25)

Woodcock, T.S. B. M. H. Larson, P. G. Kevan, D. W. Inouye and K. Lunau. 2014. Flies and Flowers II. Floral attractants and rewards. Journal of Pollination Ecology. 12(8): 63-94.A Wandering Botanist

Kath Keeler

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