The authorities in the Chinese city of Tianjin have released five million wasps in an attempt to control insects which have been damaging crops in the surrounding countryside.

The millimetre-long insects, known as Ichneumon Wasps, are stingless.

They were bred at an environmental research institute in the city, east of Beijing.

Researchers said the wasps were ideal for the task as they were much less harmful to the envornment than pesticides and their main diet was exactly the kind of pest which has been damaging crops in the city.

A researcher at the city's Institute of Horticulture and Green Issues says that Tianjin, one of China's largest ports, was particularly vulnerable to pests carried in the cargos of ships coming in from abroad.

Enemy number one

She said the worst culprit was a moth whose caterpillars had munched their way through large quantities of grain and other crops.

She said the institute had decided that conventional pesticides caused too much environmental damage, and the wasps had been chosen because they survived by eating the pupa of exactly the kind of moths which were causing all the harm.

The institute has now bred millions of the wasps and released them into Tianjin's rural suburbs.

Special mission

The Tianjin Evening News said they were expected to kill around 60% of the pests and despite lingering concerns about the ecological impact, the researchers stressed that the millimetre long wasps posed no threat to humans and would die off within a year.

Environmental experts say Chinese scientists are increasingly seeking ways of reducing the use of pesticides but they will be hoping China has learnt the lesson of its most famous attempt at pest control - the anti-sparrow campaign of the 1950s.