Wild Side: Hoverflies

You can hardly miss them on the Island.

Toxomerus politus in the author’s Oak Bluffs yard. — Matt Pelikan

Despite the rainy weather that has afflicted us, our Oak Bluffs yard is bursting with flowers: catmint, clover, oxeye daisy, hawkweed, and coreopsis fill the Meadow Formerly Known as Our Lawn. Attracted by this floral jackpot, the yard is also humming with one of my favorite groups of insects: Syrphid flies.

Syrphids are also known by the common names “hoverflies” and “flowerflies,” both of which make a certain amount of sense. While other types of flies also hover or associate with flowers, these behaviors amount to a religion among syrphids. Adults can’t seem to get enough pollen or nectar from flowers, and the aeronautical prowess of this family is unbeatable: Their darting, swooping flight is often interrupted by prolonged hovering as a syrphid looks around or assesses an opportunity.

A diverse family, Syrphidae comprises some 6,000 known species worldwide (and no doubt thousands more that have not yet come to the obtuse attention of human observers). The Island surely supports a hundred or more species, though I’m far away from documenting that many. But the family is well represented here, often numerous, and usually easy to find.

Easy to find, that is, if you know what to look for. Syrphids run small, and their active flight can make them hard to spot. Still, if you exercise a little patience around nearly any type of flower in early summer, you can hardly miss this family.

The Island’s most abundant hoverfly genus features some of the smallest syrphids: Toxomerus, with at least three species occurring on the Vineyard, averages about a quarter-inch long. At this point in the season, they are everywhere: yards, gardens, meadows, even in sunny spots in woodland clearings. A pass through our yard earlier today turned up about 50 individuals in this genus, which is surely just a very partial count.

Like many other syrphids, Toxomerus is strikingly patterned in black and yellow. The effect is vaguely beelike, and this coloration in syrphids is a solid example of protective mimicry: By resembling stinging insects, syrphids are presumed to make would-be predators think twice before attacking. The flies, however, are defenseless, and utterly incapable of harming humans.

A size larger than Toxomerus, Sphaerophoria is another widespread bee-mimicking genus of hoverfly. One species, Sphaerophoria contigua, seems fairly common here, and not at all fussy about habitat: It’s a common sight in yards and gardens. Another member of the genus, Sphaerophoria philanthus, is at the other extreme: I’ve only found it at one Vineyard location (though it’s an accessible one: the parking area at the Land Bank’s Pecoy Point Preserve in Oak Bluffs). I have no idea why this species is so limited in its distribution, or why it likes Pecoy Point so well. But I’ve found it there, and only there, several years running.

Regular readers will know that I’m wary of “Is this species beneficial?” discussions. All species, in my view, have a right to live the lives they have evolved to live, and with exceptions for disease vectors and serious agricultural pests, I don’t give a hoot whether human beings approve or not. But in general, the habits of syrphid flies align neatly with human priorities. This is one of the most beneficial families to be found in the insect world.

For one thing, adults are versatile pollinators. While some species are fussy, most hoverflies happily visit any type of flower that is in bloom. And while syrphids are not as effective at transporting pollen as bees are, because they lack the extensive body hair of most bees, their active lifestyle means they visit large numbers of flowers in the course of a day. Even if they’re only transporting small quantities of pollen, the aggregate effect of very common flies visiting numerous flowers translates into significant pollination activity.

Better still, the larvae of many hoverfly species are carnivores, often specializing in aphids — among the most annoying of garden pests, from the human perspective. Little more than sightless blobs with pincers at the front end, larvae of these syrphids simply grope about until they encounter something that feels like an aphid. Then they bite it, suck its body juices out, toss the husk aside, and grope on for the next victim. Since adult female syrphids typically lay their eggs in aphid-rich locations, these very primitive tactics work just fine from the larval perspective. A single syrphid larvae may dispatch scores of aphids during its development.

This single column cannot do justice to Syrphidae. The family is active on the Vineyard from early April well into November, and the group includes a huge diversity of appearance, life history, and ecological associations. But if you’re curious, now is the point in the season to begin to appreciate hoverflies, since their beelike bodies and deft flight are common right outside your door.