Sandflies, ugh! Many a picnic or tramp in the high country has been spoiled by turning up in shorts and getting besieged by sandflies.

And there’s been many a conversation at those picnics on the question of what sandflies eat when there’s no humans around.

As well as humans, sandflies attack other vertebrates such as birds, bats and marine mammals, such as seals. Penguins are highly sort-after for a meal – it seems their feathers are not enough of a barrier to the sandflies’ mouthparts.

In New Zealand, there are 13 species of sandflies, but it is only the females of two species that bite.

They need the extra protein and nutrients found in blood to make their eggs.

Sandflies breed along streams or rivers, laying eggs at water level.

The larvae hatch and collect food from the water using a net inside their mouths to filter the water. They catch microscopic bits of vegetation, algae and bacteria.

Fortunately sandflies can’t see in the dark, so biting stops as the sun goes down but starts up again in the morning. There is often a peak of morning and evening activity – the morning peak may come from young sandflies that have just hatched and the evening peak is from females needing food after laying eggs earlier in the day.

The yellow-eyed penguin and other birds are also prey for sandflies. Photo: David Hallett

Female sandflies are attracted to their prey by the extra carbon dioxide in the air from exhalations, the sight of exposed skin, warm temperatures and the smell of sweat.

They must have incredibly good eye sight and finely-tuned scent detectors.

Dark coloured clothing also seems to be more attractive than pale, and they prefer humid, damp and sheltered places, and still conditions, as they aren’t very strong fliers.

The bites itch because histamines are made by our bodies as an immune response to the saliva the sandfly releases when it bites. This causes swelling and histamines can also make some nerve cells fire repeatedly, which we perceive as itchy.

So at the next picnic, spare a thought for the hungry sandfly – she’s just after a feed so she can lay her eggs.

Take a long-sleeved shirt, long pants and socks!