Syrphid Flies: Mimics, Pollinators, and Predators

**Three species of syrphid flies on sunflowers. *Eristalis dimidiata* (upper right), *Eristalis transversa* (lower right), and maybe a *Syrphus* sp. (upper left).**

This was an unusual sight. Three species of syrphids, each on a separate wild sunflower head, are getting a meal of pollen and nectar. Two of the species I could identify are Eristalis dimidiata (upper right) and Eristalis transversa (lower right). The third one in the upper left didn’t show enough details. I could only place it as possibly a Syrphus sp.

A swarm of bees?

I first got interested in syrphid flies about two decades ago when I was doing an inventory of plant species in a fen. Suddenly, I became aware of what sounded like a swarm of bees in a large patch of nodding bur-marigold and asters. To my relief, they were not bees but hundreds of syrphid flies nectaring at the flowers. Over the years, I’ve gradually learned more about these bee-like insects and their importance.

*Bidens cernua* (Nodding Bur-Marigold) in early September, flourishing in a restored fen.

When to see them

Late summer is one of the best times to observe syrphid flies. You can see them on sunflowers, coneflowers, goldenrods, joe-pye-weed, bur-marigolds, and asters, all members of the aster family. Buzzing loudly, they go from flower to flower like bees, and with the bees, in search of pollen and nectar.

A wet meadow in late August carpeted with joe-pye-weed (Eupatorium maculatum) and smooth goldenrod (*Solidago gigantea*). Perfect foraging habitat for syrphids and other pollinators.

Syrphid flies can also be seen in spring and early summer. But to see them, you may need to go into the woods. Some of these forest-dwelling species can be attracted to sugary baits painted on tree trunks or boards attached to posts. They can also be found on woodland wildflowers like Canada mayflower.

Bee and wasp mimics

Many syrphids have body patterns and body shapes that resemble those of bees and wasps. This mimicry (Batesian mimicry) is a form of camouflage to deter potential predators. The resemblance to wasps and bees is striking.

Physocephala furcillata, Eumenes crucifera, and Doros aequalis resemble potter’s wasps, and Ocyptamus fascipennis, an ichneumon. Sericomyia chrysotoxoides looks like a yellow jacket wasp, and Eristalis flavipes could be mistaken for a bumblebee.

**Sericomyia chrysotoxoides (Oblique-banded Pond Fly), a yellow jacket wasp mimic. The larvae live in mud rich in organic matter, filtering out microorganisms.**

Syrphids are pollinators

Like bees, syrphids are pollinating insects. But it is not just aster family plants they seek out. I have seen syrphids on mustard (Brassica nigra), amaranth (Amaranthus cruentus), milkweed (Asclepias syriaca), purple prairie clover (Dalea purpurea), grass-of-parnassus (Parnassia sp.), and cinquefoil (Potentilla recta).

*Physocephala furcillata* on milkweed (*Asclepias syriaca*) flowers. This species is a wasp mimic incorporating features of *Eumenes* potter wasps.

Not just pollinators but predators and recyclers

Syrphids do more than visit flowers. The larvae of many species are important predators of aphids and other soft-bodied crop pest insects. Some larvae may eat 400 or more aphids in their lifetime.

Other species larvae (rat-tail maggots) live in mucky habitats, eating microorganisms in the detritus and so contribute to nutrient recycling.

Encouraging syrphids

Syrphid flies, like the bees and wasps they often mimic, are important parts of the pollinator fauna. They don’t sting or bite, but their appearances can give you pause.

Planting nectar-rich domesticated plants like buckwheat, sunflowers, coriander, and dill, and wildflowers. Even small patches will help them. From there, they can launch forays into gardens and fields, pollinating crop plants. Predatory species will lay eggs that hatch into maggots that eat pest insects like aphids, scale insects, and spider mites.

Three insects added to my checklist this morning

**Big-leaf aster (*Eurybia macrophylla*) in bloom out in the woods.**

It is the first day of October, but it doesn’t feel like it. Our temperatures are in the high 70s, and the warm weather will continue at least until Sunday. So I decided to look for any flowers that might still be blooming on this unseasonably warm autumn day. Although there is not a profusion of flowers this far into autumn, some still can be found.

Two that are still in full bloom are showy goldenrod (Solidago speciosa) and trumpet weed (Eupatorium fistulosa). These two plants are always pollinator magnets, and today they were loaded with bumblebees, syrphid flies, and wasps.

While taking photos, I saw three insects that I had not recorded from here before: Pararchytas decisus (a tachinid fly), Sericomyia militaris (a syrphid fly), and Eumenes crucifera (a potter wasp).

Warm autumn days like this one never fail to surprise. I wonder what I will find later today and tomorrow?

Flower flies

Syrphid fly on flower — ***Helophilus fasciatus on Lindley’s aster (Symphyotrichum ciliolatum) looking for pollen and nectar.***

On a warm late summer day if you walk into a field of flowers particularly yellow flowers like goldenrods you might think you have stepped into a swarm of bees or wasps. There is intense buzzing and many small yellow and black insects on the flowers and flying all around you. But despite all appearances these are not bees or wasps at all. They are flies. Syrphid flies, also known as flower flies and hover flies, to be more exact, and there are many species that mimic bees and wasps, a good strategy if you are trying to fend off predators.

Syrphid flies are true flies in the Order Diptera (“two wings”) and Family Syrphidae. Adult syrphid flies feed on nectar, pollen, and sugary secretions from aphids. You can use their love of sweets to attract them with mixture of sugar or honey and water sprayed on surfaces like tree trunks in open areas. When moving about on flowers syrphid flies assist in the pollination of many plant species including crops we grow.

Eristalis transversa on sawtooth sunflower (Helianthus grosseratus). Small hairs on the body of the fly are dusted with pollen. It will probably eat most of the pollen but enough will be spread across the flower head to ensure pollination.

The larvae have more varied diets depending on the species. Some syrphid fly larvae feed on decaying plant matter, damp wood (perhaps for the bacteria and fungus which are more nutritious?), and subterranean parts of plants. Others prey upon aphids, scale insects, thrips, and similar sap-sucking insects and can be beneficial in crop fields and gardens. Some species lurk in ant nests where they disguise themselves with chemical secretions and eat ant larvae.

Syrphid larvae in the tribes Eristalini and Sericomyiin live in muddy stagnant water, even cesspools, where they feed on detritus. A long breathing tube from the anal segment pokes above the water to access the air. This long breathing tube has earned them the name “rat-tailed maggots”.

I recently bought “Field Guide to the Flower Flies of Eastern North America” and have found it very helpful in figuring out species of syrphids that live in my area. The book is well illustrated and includes descriptions and range maps. In the descriptions, the authors have included flowers that species typically visit if known. I have been able with this book to identify three syrphids (Eupeodes americanus, Sericomyia chrysotoxooides, Toxomerus marginatus) that had been in my “unknown” files for several years. And I’ve identified three others I photographed last week (Eristalis dimidiata, Eristalis transversa, Sphaerophoria philanthus).

Below is a small gallery of some of the syrphid flies I’ve seen here over the years.