Yiu Vor, chairman of the Hong Kong Entomological Society, said they had been puzzled by the loss of the butterflies. 'We then realised the host plant, Passiflora moluccana, which is a vine of restricted distribution, had been cut,' he said.

Larvae of red lacewing need to feed on the plant, which has almost all been burned and cut now in the valley. Instead, aloe vera had been grown, and the red lacewing are nowhere to be seen.

More than 1,000 square metres of land have been affected, and the Entomological Society says the butterfly population has been halved.

The society has repeatedly reported the problem to the Agriculture, Fisheries and Conservation Department and the Conservancy Association, a non-governmental organisation.

The association also contacted the department over the matter, but in reply received only a letter confirming the site had been destroyed by illegal planting and that a protective fence had been put up. However, when the Post visited the site, it was easily accessible with no obvious fencing.

Peter Li Siu-man, the association's campaign manger, said: 'The valley is identified by the government as a site with scientific value, but how come there is no one taking care of the place? Aloe vera is not a local plant. It is planted by someone else. It is a pity the red lacewing cannot be found now.'

The Lung Kwu Tan valley, despite having been identified as a site with scientific value, is not a designated area for conservation, and thus not protected by the Town Planning Ordinance.

The department says it is now considering identifying the location as a 'site of special scientific interest'.

After sites have been given this designation, government departments are supposed to take into account the conservation factor when developments in or near these sites are proposed.

However, Li feared the site might have been completely destroyed before it could be made a site of special scientific interest.

'The area is neither included in a country park nor subject to any statutory planning control. Although part of this area is government land, no one is monitoring what has happened,' he said.

For environmentalists, the mess in Lung Kwu Tan valley is a case of deja vu. A spate of eco-disasters in some rural areas brought about by illegal dumping and land filling have hit headlines recently.

In July in Ho Sheung Heung village, Sheung Shui, construction waste was found to have been dumped on farmland. Several plots were covered in concrete and plastic.

Last December in Nam Sang Wai, near the Mai Po Nature Reserve in Yuen Long, construction waste was found to have been dumped on fishponds. But the Environmental Protection Department said it did not receive any complaints until after it was alerted by media reports.

And earlier last year, there were media reports about illegal dumping of construction waste inside Tai Lam and Shing Mun country parks.

Environmental Protection Department figures showed the number of complaints about illegal dumping of construction waste increased from 1,546 in 2006 to 2,732 last year. And for the first seven months of this year, there were already 1,807 complaints.

Hong Kong is not short of laws against trashing a rural area, but authorities have first to be alerted to the problem, which often takes place in remote areas. And there is a need to thread through bureaucracy, as at least six departments might have roles to play - environmental protection, lands, planning, buildings, food and hygiene, and drainage.

Dumping on government land is prohibited. But not all such activities on private sites are illegal, as long as they are compatible with the site's zoning. There is also a Waste Disposal Ordinance, which requires that waste be properly disposed of at designated facilities.

The Environmental Protection Department can also hold waste producers accountable. But tracking the origins of waste, and those responsible, can sometimes be an impossible mission.

Ng Cho-nam, a member of the Town Planning Board, called for a holistic approach to be taken towards conservation.

'There is still much land in the New Territories that falls outside the outline zoning plans. The town planning should protect areas with conservation value from a long-term perspective instead of pointing at specific sites only,' Ng said.

Some critics have also pointed to a 'loophole' in the town-planning laws. For example, at Tung Tsz in Tai Po, which has been zoned as a green belt, planning officers have no power to take action because the area is not covered by a development permission plan - a requirement for enforcement against land filling under the Town Planning Ordinance.

The Lands Department has no role either, as the old leases impose no restrictions on dumping.

Some lawmakers have proposed extending town-planning controls to disallow all unauthorised land filling, to areas not covered by development permission plans. But the government disagreed, saying it could not afford the 'enormous manpower' that might be involved.