If you get bitten by tiny black flies it is likely that you have been the victim of what New Zealanders call sandflies (namu in Māori). Sandflies, like mosquitoes and other flies, are members of the order Diptera, and belong to the family Simuliidae. Similar species found elsewhere are called blackflies.
There are 13 species in New Zealand, all belonging to the genus Austrosimulium. Only two species bite: the New Zealand blackfly (Austrosimulium australense), and the West Coast blackfly (A. ungulatum). At only 2–3 millimetres in length, they look the same to the naked eye.
Sandflies are found wherever there is flowing water and bush. They are often found at beaches, and at the edges of lakes or swamps. The New Zealand blackfly occurs in the North Island and around the coasts of the South Island. The West Coast blackfly is confined to the South Island, where it is a nuisance.
The West Coast and Fiordland are infamous for their sandflies. The terminus of the Milford Track, where trampers board the ferry to Milford Sound, is called Sandfly Point.
While surveying Doubtful Sound in the summer of 1851, Captain John Lort Stokes of the Acheron was tempted but resisted putting the names Venom Point, Sandfly Bay and Bloodsuckers Sound on the map, after encounters with the biting insects. Road builders on the Milford Road and Haast Pass suffered clouds of them. While surveying road routes near Haast in the 1930s, Alan Dawber played a game with his mates: ‘We used to compete with each other by baring our forearm to the sandflies, then when the first one made its presence felt, we would start killing them off one by one. I think the record was 64 before wiping the stinging mass clear.’ 1
Sandflies breed in fast-flowing streams or rivers. Eggs are laid on rocks or plants around or below water level. Larvae hatch and collect food from the current, using foldable ‘nets’ that surround their mouths. These expand to catch passing organic particles, algae, and bacteria. The larvae pupate and spend around 12 days in this form, before emerging as flies at the water’s surface.
The length of the life cycle varies, depending on the time of year, but averages around six to seven weeks.
Worth writing home about
The first instance of the word sandfly (rather than blackfly) for the New Zealand species is in the journal of James Cook. He came across the insects at Fiordland’s Dusky Sound, possibly at a sandy beach, in May 1773. His journal reads:
‘The most mischievous animal here is the small black sandfly which are exceeding numerous … wherever they light they cause a swelling and such intolerable itching that it is not possible to refrain from scratching and at last ends in ulcers like the small Pox.’ 2
Only females bite
After mating, the female searches for a meal of blood – needed to produce eggs. (Little is known about the male, who is a vegetarian.) Females attack vertebrates such as penguins and other birds, bats, seals, domestic animals and humans. They pierce the skin, creating a drop of blood that they suck up.
When do they bite?
Sandflies cannot see at night, so they seldom bite in the dark, and generally remain outdoors. Peaks in biting often occur when light intensity increases in the morning and decreases at dusk. The morning peak comes from young sandflies that have recently emerged from pupae, and the higher evening peak is often the result of sandflies taking blood after laying eggs earlier in the day.
Sandflies are most active in dull, overcast and humid conditions, when they may bite at a similar rate throughout the day.