One year after the Super Bowl season was marred by a ban on Mexican avocado shipments, another threat has emerged: An environmental complaint that avocado growers are destroying forests that provide critical habitat for monarch butterflies and other creatures. The complaint, filed with the trilateral Commission for Environmental Cooperation, part of the U.S.-Mexico-Canada free trade accord, accuses the Mexican government of failing to enforce its own laws on deforestation, water conservation and land use.