Bloomberg newswires said rubber was still up 2.5 percent in 2012.
September futures on the Futures Exchange fallen 0.7 percent to (3,906 dollars) a ton.
Thai rubber on a free-on-board basis fell 1.7 percent to 115.25 baht ($3.67) a kilogram on May 15, the report said, quoting data from Rubber Research Institute of Thailand.
Sri Lanka's rubber is still sharply higher from January.
Grade one sheet rubber which was auctioned at 359.25 rupees (3.18 dollars) a kilogram on January 03, fetched 480 rupees (3.68 US dollars) on April 26.