As we watch the flow of bees and insects pollinating our vegetables and flowers, we may be struck by bee-like insects that  are slightly different.  These insects may be flower flies, also called hover flies. Most of us know that bees are important pollinators of our food crops.  Yet flower flies are also an important pollinator, particularly those in  the family Syrphidae.

There are about 6,300 species of flower flies, comprising 4 subfamilies.  They live in all climates, from the tropics to the arctic and have a variety of habits. Some species feed on roots, others on ant larvae,  others on fungi and decomposing plant matter, a few feed on tree sap as borers, while  still others are feeders of bacteria and insects in ponds.

Syrphids perform many functions that are of value to humans. Not only are they responsible for about 30% of the pollination of native plants and crops, the larvae of many syrphids prey on soft-bodied insects, such as aphids and scale.  When aphids infect our plants, it is a delight to have syrphids nearby!

Mimicry

Most adult syrphids are mimics- they resemble either bees or wasps in some way.   Full mimicry is uncommon, but there is often enough mimicry to confuse predators. With a little patience and observation it is easy to differentiate between bees and syrphids.  Here are some things to look for:

• Flower flies, like all flies,  have two wings. Bees and wasps have 4 wings.
• Flower flies have large, segmented eyes on the side of their heads.  Bees and wasps have dark eyes with a solid, shiny  appearance. In bees and wasps, the  eyes are located more on the top of the head, rather than the side.
• Wasps and bees have long antennae, whereas the antennae of flower flies are short and club-like.
• Bees often have pollen sacs on their back legs where they store pollen. Flower flies have no pollen sacs.
• The heads of flower flies are larger than those of bees or wasps.

Although flower flies have evolved to mimic creatures that sting, they are harmless- they  do not possess a stinger.

Life cycle

Flower flies undergo complete metamorphosis, from egg to larva to pupa to adult.  The larvae undergo 3 molts before entering the pupal stage, which lasts about 14 days. They overwinter as mature adults or mature larvae. In southern Ontario, they emerge in late March and are most active between late May to the end of June.  The adults prefer moderate temperatures and are most active in temperature ranges between 15-25 degrees C.  Like all flies, flower flies are not able to store water on their bodies and will dessicate if it gets too hot.

About 1/3 of syrphid species prey on soft bodied insects such as aphids and scale. Females will lay about 400 eggs, often near aphid colonies. The larvae have almost transparent bodies, and are about 5-12 mm long. They   are voracious feeders, consuming about 500 aphids in their larval stage.  They pierce the soft-bodied insects with a mouth hook and suck out the fluids. Larvae are most active at night, while adults are most active during the day. The photo below shows a flower fly larva near a colony of aphids.

What’s in a name?

Flower flies are often called hover flies.  Many syrphids are amazing fliers and are able to fly like hummingbirds- backwards, forwards, sideways, as well as being able to hover almost motionless for minutes at a time. 

Pollination

In Ontario, flower flies are important pollinators of fruit crops, with 25 species having been identified in apple orchards.  Like most pollinators, flower flies are accidental pollinators- the adults are searching for the nectar, not pollen. In their foraging for nectar, they transfer pollen from the stigma to the carpel.

In your garden, you are most likely to attract flower flies if you plant native species- they have a preference for flat topped white flowers.

Looking for more information?

Check out the following resources for more information on these remarkable insects.

http://www.omafra.gov.on.ca/IPM/english/apples/beneficials/hover-flies.html#advancedhttps://

cwf-fcf.org/en/resources/encyclopedias/fauna/insects/hover-fly-sp.html

Skevington, Jeffrey H., Locke, Michelle M. Field Guide to the Flower Flies of Northeastern North America. Princeton University Press, 2019.

Submitted by Sabine Behnk