The island is about 15 minutes by ferry from Marina South.

Most of the visitors go to the shrine after stopping at the popular Da Bo Gong temple.

But they also add work and cost to those who look after the island.

Said the shrine's caretaker, Mr Ishak S, 46: 'When there's no more space, we paint over all the numbers, but then they keep on writing.'

He added that once a week, workers hired by Sentosa Leisure Group, which manages the island, check the trees for the plastic bags and throw them away.

He said: 'We don't want water to get stuck inside and cause mosquito breeding.'

A Sentosa Leisure Group spokesman said the estimated cost for clean-up of Kusu Island is $5,470 per month.

She said: 'We understand that Kusu Island appeals to many as a place of worship and some devotees would leave 'markings' behind for added assurance and peace of mind.

'As Kusu Island is also a place of interest for locals and tourists, we hope visitors and devotees will help to keep the island clean so that the others can also enjoy the island.'

Devotee Neo Teck San, 54, confessed he is one of the many who have written four-digit numbers near the shrine over the years.

Said Mr Neo, who is in the car tyre business: 'There's one tree there that's like a wishing tree. In the past, people who wanted to have babies would make a wish, tie a rock to a plastic bag and throw it onto the tree.

'I don't know how it started, but people who wanted to strike 4-D later also started to write on rocks and throw their plastic bags onto the tree.'

He recalled that when he last went to the shrine about two years ago, there were some youths collecting donations there.

After he gave them $2, they chanted over him for additional 'luck'.

But not everyone believes in vandalising the rocks and trees.

One such couple is Mr Sam Tan, 45, a businessman, and his wife Madam Lena Chao, a housewife in her 30s.

Earlier this month, the couple visited Kusu Island to pray at the Chinese temple, then made a brief stop at the shrine.

Madam Chao prayed for lucky numbers and didn't strike 4-D - but her husband did.

He won about $500 after the number he bought - their unit number - was one of the starter prizes.

The couple said they didn't write their numbers on the rocks and trees.

Madam Chao said in Mandarin: 'I think praying is sufficient, no need to write like this.'

But she didn't disapprove of the other devotees' actions.

'It's up to each person's belief, no harm trying - you won't lose anything,' she said.

Others are against it.

Said Taoist priest Master Chung Kwang Tong (Wei Yi), 23: 'This is a folk belief, that if you throw the stone high up on the tree, the deities will be able to see it more clearly.

'But from a religious point of view, we do not encourage devotees to do this. In Taoism, we talk about caring for nature and preserving the environment, so any act of vandalism is prohibited.'

Last year, some 76,200 visitors went to Kusu Island for their annual pilgrimage during the ninth lunar month.