Hover flies are common in our garden except in the dry season and we are beginning to see them again now that the humidity and temperature are rising. Their common name comes from their distinctive, odd-looking habit of hovering in one spot, darting forwards or sideways, and then hovering again; it is behaviour which make mid-air photos feasible and that’s what you see above.
We see at least a dozen species around our garden (photos here, on flickr) and Graeme Cocks has identified twice that many around Townsville, out of about 6000 species worldwide. Most of them are bee or wasp mimics but appearances are, as so often, deceptive: they are all completely harmless.
My photo shows a Common Hover Fly (Simosyrphus grandicornis, Syrphidae) near Pentas leaves. As I watched, it landed on a seed-head and then took off to fly down into the base of a nearby flower-head and lay some eggs:
That made me wonder about the life cycle because I had always (vaguely, rashly) assumed that the larvae would feed on decaying matter just as the larvae (maggots) of the most familiar flies do. The Australian Museum’s fact sheet on the family told me, “Hover flies are also called flower flies because they are commonly seen during warmer months hovering among flowers, feeding and mating. They pollinate many plants and help keep aphids under control.” That made me look further. Wikipedia to the rescue:
Hoverflies are important pollinators of flowering plants in a variety of ecosystems worldwide. Syrphid flies are frequent flower visitors to a wide range of wild plants as well as agricultural crops and are often considered the second most important group of pollinators after wild bees. However, there has been relatively little research into fly pollinators compared with bee species.
Unlike adults, the maggots of hoverflies feed on a variety of foods; some are saprotrophs, eating decaying plant or animal matter, while others are insectivores, eating aphids, thrips, and other plant-sucking insects. This is beneficial to gardens, as aphids destroy crops, and hoverfly maggots are often used in biological control.
Our Common Hover Fly is one whose larvae are aphid predators, so laying eggs on Pentas makes perfect sense. Brisbane Insects describes a closely related species (their Ischiodon scutellaris is now Simosyrphus scutellaris, putting it in the same genus as mine) as doing the same on hibiscus plants, and provides photos of the larvae.
I’ve got to say, though, that I find the adult flies more attractive than the grubs. Here’s one sipping nectar: