By Francis Markus BBC, Shanghai

The northern Chinese province of Hebei, the country's largest exporter of frog meat, says it has banned the trade in order to combat the effects on the environment.

Officials say that widespread killing of wild frogs has dramatically increased the number of locusts and other pests.

Frogs - or field chickens, when they appear on Chinese restaurant menus - are a popular dish in many parts of the country.

Europe and the United States also import significant numbers.

In fact, official figures say the province's exports of frogs' legs have reached 2,000 tons per year - and there has been no shortage of demand, as "mad cow disease" has sent Europeans off in search of non-beef sources of protein.

China has been using ducks to reduce locust numbers

The problem in Hebei, though, is that the economics of breeding frogs for export are precarious.

And the rampant killing of wild frogs has fuelled a dramatic increase in the numbers of locusts and other pests ravaging the farmlands.

In recent years, Hebei has banned the sale of frogs in local markets, to try to clamp down on the trade.

There are similar restrictions in a number of other areas, such as Shanghai.

But local residents say that, despite the ban, frog-sellers still appear in the markets.

They sell their bags full of the amphibians to eager frog consumers, and then vanish before they can be caught by local officials.