Workers who apply certain pesticides to farm fields are twice as likely to contract melanoma, a deadly form of skin cancer, according to a new scientific study. The researchers identified six pesticides that, with repeated exposure, doubled the risk of skin cancer among farmers and other workers who applied them to crops. The findings add to evidence suggesting that frequent use of pesticides could raise the risk of melanoma. Rates of the disease have tripled in the United States in the last 30 years, with sun exposure identified as the major cause. Four of the chemicals - maneb, mancozeb, methyl-parathion and carbaryl - are used in the United States on a variety of crops, including nuts, vegetables and fruits. Two others, benomyl and ethyl-parathion, were voluntarily cancelled by their manufacturers in 2008.