The Swallow, the Milkwood and the Kelp Fly

Wander along the southern Cape coast at this time of year and you’ll quickly become alert to the pungent, summertime scent of the Milkwood tree. This is not everyone’s cup of tea, however, with words such as ‘ghastly’, ‘foetid’ and ‘smelly socks’ being bandied about by locals. I’m not sure if I should be admitting it, but I don’t find the powerful Milkwood fragrance at all unpleasant. It is so sharp, that if you inhale deeply, it seems to burn the back of your nostrils – with a distinctive salty zing! I guess that mustard or menthol might be the closest comparison, but the Milkwood is not as fresh, and perhaps it is a little rancid.

The other thing that I have noticed about the flowering Milkwood trees, is that they attract large numbers of Barn Swallows. Every so often, a particular tree is surrounded by dozens (if not a hundreds) of swallows, as well as martins and swifts which sweep above and around the shiny, evergreen foliage.

It doesn’t take much to deduce that these aerial insectivores are capturing the insects that are being lured to the tree’s tiny flowers. But what kind of insects are drawn to such a strong, overpowering perfume? To find out, I whisked a small net through the foliage of a flowering Milkwood tree. The net was immediately abuzz with tiny flies – about a third of the size of the common House Fly. Close inspection (did you know that reversing your binoculars turns them into magnifying glasses?) revealed these flies to have extremely hairy middle legs. Pollen from the anthers of the waxy little flowers will stick to these hairs and be transported to the ovaries. I can find no published information on the pollinators of Sideroxylon inerme, but these flies are almost certainly Coelopa ursina – the so-called Kelp Fly (not Trichopoda, as originally thought, and erroneously captioned below).

Why are some trees surrounded by feeding swallows and others not? My theory is that the rapidly moving birds create enough air turbulence to disturb the flies and send them into the air where they can be scooped up. A gentle shake of a flowering stem bears this out, as clouds of little flies take to the air. One swallow darting around a Milkwood tree full of flies might struggle to find sufficient food, but a flock of swallows that keeps sending swarms of flies into the air provides enough prey for all the participants.

From what I have seen, Barn Swallows are more abundant here in the southern Cape than in the eastern Lowveld (where I lived for the past 20 years). But perhaps this is only an impression caused by the large number of flowering Milkwood trees in our neighbourhood. Who would have thought that the migratory Barn Swallows that must soon make the 10,000 km journey back to Europe are fuelled – in no small way – by the kelp flies lured by the smelly Milkwood flowers?!

The Milkwood tree (Sideroxylon inerme) is adapted for life by the sea – its glossy, evergreen foliage being able to withstand the salty air and strong winds. It is able to grow on sand dunes but does better along seeps and watercourses. In general, the more sheltered from the wind, and the closer its roots are to water, the greater the Milkwood’s stature. A height of 15 metres seems to be the record, but it is usually wider than it is tall. Trees that grow on sea facing dunes are ‘pruned’ by the wind and salt, shaped more like a hedge than a tree. Like other members of the family Sapotaceae, the stems and leaves contact a white latex – thus the name, milkwood! Once the little flies have performed their pollination role, the stems become crowded with inky-black berries – relished by mousebirds, bulbuls and white-eyes.

Vermont, Hermanus, Southern Cape, South Africa. February 2015