The cocktail of chemicals being used in agricultural insecticides is killing too many insects, a vital part of the food chain, says the head of an outdoor recreation advocacy group.

Bill Benfield, of Martinborough, said the falling number of insects in places like the Ruamahanga River was a warning that chemicals were crippling the ecosystem.

He is co-chairman of the Council of Outdoor Recreation Association of New Zealand, a group whose membership comprises of recreational fishing, hunting and boating groups.

"It's just total environmental destruction going on," he said.

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The chemical diazinon, a replacement for DDT which was used to combat grass grubs and insects on crops was being chucked around the country in large quantities, Mr Benfield said.

"It's like DDT, it kills everything. These chemicals can have side effects which manifest themselves in adversely affecting insect and vertebrate populations."

These days there was almost no insect life at all within the Ruamahanga River, he said.

A decline in insect population was a sign of a struggling ecosystem.

"Insects are the basis of the food chain and the land. They feed the birds."

Mr Benfield said the decline in native birds such as kea and kiwi was too often blamed on animals such as stoats and rats.

People needed to look at the use of chemicals like diazinon and 1080.

"Basically we are blaming nature for our own destruction and proceeding to do more destruction in the name of aiding nature. We are crapping in our own nests."

NZ Federation of Freshwater Anglers spokesman Ken Sims said trout anglers were concerned at the disappearance of mayfly and caddis sedge hatches on rivers and the subsequent loss of the trouts' evening rise.

"We strongly suspect the ecosystem is under attack from man and his shortsighted use of chemicals in various forms," Mr Sims said.

Trout anglers had also noted declines in freshwater eels, koura, frogs, invertebrates and other aquatic life, Mr Sims said.

"It's all symptomatic of an ailing environment."