An infestation of giant aphids clinging to the willows along the Whangaehu stream is ringing alarm bells for a Longbush resident, who scooped out more than 40 dead eels from the stream.

He says the eels' deaths are a result of huge amount of honeydew secreted by the invasive species of aphid, which has compromised the water quality.

Hamish Rogers has lived in the Longbush area in rural Carterton for 14 years and said he has never seen so much devastation along the stream which runs through his property.

He said low flows, warm weather, and the hundreds of litres of honeydew being dripped into the stream by "billions" of aphids in the area had caused low oxygen levels in the water, leaving eels "gasping for air" before dying.

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"We need to be addressing the problem big time because we've just lost a s**tload of eels here," Mr Rogers said.

He scooped a rotting eel out of the stream, one which he said "could be about 50 years old and would have lived for much longer too".

"The number of aphids that we had here at the height of the problem would be in the billions. It got so bad that you couldn't count one cluster the size of your hand, that's how dense they were.

"So over the past six weeks, there would be literally hundreds of litres of honeydew being poured into the creek. That has to have an effect on the water quality."

The giant willow aphid was first found in Auckland in 2013 and is one of the largest known aphids at up to 6mm in body length.

"Everyone needs to be aware of this issue, but I don't see a lot of interest in it," Mr Rogers said.

"Everywhere there are willow trees in Wairarpa, the aphids are there. People just think the sugar dew is being excreted from the willows, but it's actually passing through these bloody aphids."

Mr Rogers said the honeydew residue on the willows had attracted wasps to his property, having taken out 17 wasp nests over the past three weeks on his 3ha property.

"The willows are an abundant food source, but for all the wrong predators," he said.

"Even the bees aren't here because the wasps are now."

Mr Rogers said a recent shift to cooler weather had caused the aphid population to drop, then he pulled off a branch that was still infested with hundreds of aphids, some full size.

"People are quick to jump onto the dirty dairy bandwagon and blame any negative effect on our waterways on the farmers, but this problem is as bad, if not worse," he said.

"It's not due to intensification of farming or irrigation because there's none of that going on here.

"The issue is sitting here right in front of us, it's the bloody aphids in a poorly flowing warm temperature climate, in a creek that needs attention.

"The potential fallout of something like this on our waterways is pretty intense, especially since willows are planted for erosion control along almost every major waterway."

Greater Wellington Regional Council staff are investigating the Whangaehu stream site and confirmed that the low oxygen levels were due to "excessive honeydew falling from the willows overhanging the stream because of a massive infestation of invasive giant aphids".