A coalition of commercial beekeepers and environmental groups is urging U.S. regulators to suspend the use of a pesticide they say may be contributing to a sharp decline in honeybee populations. In a petition to the U.S. Environmental Protection Agency, the group called for a ban on clothianidin, which belongs to a class of chemicals that disrupts the central nervous system of insects. Some researchers say the chemicals — known as neonicotinoids — make the insects more vulnerable to pathogens by weakening their immune systems and could be a factor in so-called “colony collapse disorder,” a mysterious phenomenon that has taken a heavy toll on U.S. honeybee populations since 2006. The EPA granted a conditional registration for clothianidin in 2003, contingent upon further field studies to confirm that the chemicals did not cause “unreasonable adverse effects” on pollinators. While clothianidin is toxic to honeybees, EPA officials say there is no proven link between the chemical and colony collapse disorder.