The Montreal Protocol, the international treaty to protect the ozone layer, has been hailed as the most effective environmental agreement ever signed. Now, though, it’s efficacy could be jeopardized, because the Bush administration is calling for some exemptions to a part of the treaty that calls for a global ban on the pesticide methyl bromide by 2005. Administration officials say prohibiting use of the pesticide would cause economic harm for some users, such as farmers and golf-course owners, because there are no effective substitutes. The Bush administration has received 56 requests for exemptions, totaling 26 million pounds of methyl bromide; it has until tomorrow to determine how many of the exemptions it will request of the U.N. body that administers the treaty. Environmentalists fear that granting the exemptions would undermine the agreement and reduce the incentive to develop safer alternatives to methyl bromide.