Insects caught up in the southerly wind change over Canterbury drifted north above the plains this afternoon, as seen by the MetService's Rakaia weather radar.

You couldn't see them from the ground. But somewhere up there a mini-plague of insects was sweeping across Canterbury.

They would have gone undetected but for MetService's weather radar unit at Rakaia spotting their presence on Thursday afternoon.

It seems flies and other insects were on the wing above South and mid-Canterbury, enjoying the warmth and buoyant air over the plains, when an approaching southerly wind change sucked them up and transported them north.

On the radar images it looked like an area of tiny yellow pinpricks, behind what appeared to be a trail or ribbon of smoke.

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Looking up, there was nothing to see. But if you squinted, was there possibly, maybe, a small hint of something unusual way up in the blue?

Unlikely, given the insects' size and altitude, MetService meteorologist Claire Flynn said.

The insect phenomenon was seen from time-to-time by forecasters but this was a "particularly nice example".

In warm conditions, insects flew higher than usual, allowing them to be picked up by the radar beam.

"This does not mean that the insects are in a swarm – just that the radar sees each insect as a big, fat raindrop," she said.

"The insects are being pushed along with the southerly winds. The sharp line at the leading edge of the yellow echoes is a convergence line where northwesterly winds are converging with southerly winds.

STACY SQUIRES/STUFF The insects were up here, somewhere, admiring the nor'west arch from close up.

"Along the convergence line, air gets pushed upwards, and so too do the insects. They only have little wings – and once they are caught up in these winds, they struggle to fly out again."

It was difficult to estimate the height they had been lifted to, but it was probably several thousand metres, she said.

"They could end up a long way from home."