Blaming pesticide manufacturers for the deaths of thousands of lobsters last year in Long Island Sound, Connecticut and New York lobstermen on Monday are planning to file suit against the companies for $75 million in damages. The lobstermen say the pesticides applied by the companies to fight the West Nile virus last fall killed the lobsters and led to the loss of millions of dollars in profits. In the past, the pesticide companies have said the amounts of chemicals used were far too small to harm the lobsters. Meanwhile, a toxicologist at the U.S. EPA yesterday testified before an independent scientific advisory panel that malathion, one of the pesticides sprayed to fight the West Nile virus, is perhaps a greater cancer threat than his agency has contended.