Herbicidal warfare is the use of substances primarily designed to destroy the plant-based ecosystem of an area. Although herbicidal warfare use chemical substances, its main purpose is to disrupt agricultural food production and/or to destroy plants which provide cover or concealment to the enemy, not to asphyxiate or poison humans and/or destroy human-made structures. Therefore, herbicidal warfare cannot be classified as "chemical warfare" and does not fall into the category of weapons of mass destruction (WMDs). Herbicidal warfare has been forbidden by the Environmental Modification Convention since 1978, which bans "any technique for changing the composition or structure of the Earth's biota".