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.

Another insect added to the checklist: a gall-forming fly

It might be getting colder outdoors, but there are still insects to be found. While out walking a few days ago, I came across this odd growth on a Dryopteris cristata (crested wood fern) frond.

This tightly coiled knot on the frond is caused by a galling insect that specializes in ferns. The species responsible for the gall is a fly (Diptera, Anthomyiidae) named Chirosia betuleti.

The fly’s larvae form galls on several fern species: Athyrium filix–femina (lady’s fern), Dryopteris carthusiana (spinulose wood fern), Dryopteris cristata (crested wood fern), Dryopteris filix–mas (male fern), Matteuccia struthiopteris (ostrich fern), and Pteridium aquilinum (bracken fern).

Another fern with a gall

A search for other galls the next day located one empty gall on the frond of a Dryopteris carthusiana plant in a tamarack swamp. I looked for Athyrium filix–femina and Matteuccia struthiopteris, but their fronds had already withered away now that it is fall. I’m sure I’ve seen these odd growths on them before. None of the Pteridium aquilinum plants had galls, and I don’t recall ever seeing them on the fronds before. There’s always next year.

Gall formation

The larvae of Chirosia betuleti feed on the trichomes (hair-like growths) along the midrib of the newly emerging frond, and then mine along the midrib, causing it to coil into a mophead shape. The related Chirosia grossicauda forms galls on bracken fern. Another insect to watch for next year.

The Anthomyiidae

Chirosia betuleti is a fly in the family Anthomyiidae (root maggot flies). Most species in this family feed on plants as larvae, feeding on roots, seeds, or mining leaves.

Some feed on dung, decaying plant matter, or mushrooms. Other species are endoparasitoids of grasshoppers, and some are kleptoparasites of Hymenoptera.

Adults feed on nectar and pollen and may be pollinators. Most species resemble small houseflies.

Range

Chirosia betuleti is reported on iNaturalist from the western coast of North America from Alaska to California, and inland to Saskatchewan, Canada, and Idaho, and Montana, around the Great Lakes and New England, then east to the Maritime Provinces of Canada. The range then follows the Appalachian Mountains south to the Great Smoky Mountains. There are two isolated occurrences in South Carolina and Florida.

Parasites of Chirosia betuleti

Larvae of Chirosia betuleti are parasitized by wasps in the genus Aphaereta (Braconidae) and the wasp genera Dimmockia and Elachertus (Eulophidae).

Missing the colors of mushrooms in the fall

This has been an exceptionally warm and dry summer and fall, affecting everything in the forests, fields, and wetlands. Fall colors have been rather drab this year. Wetlands are drying up. Many trees began dropping their leaves in August.

September and October are times when the forests are full of colorful fungi. I always look forward to going out and looking for mushrooms. But this year mushrooms of any kind are far and few between. A few puffballs have popped up, and some Amanitas tried to grow before succumbing to the heat.

This slideshow showcases some of the colorful fungi that should be here, but for lack of rain, are not.