Maize samples (Zea mays) waiting to be tested at the International Maize and Wheat Improvement centre's breeding laboratories outside Mexico City. Farmers in Mexico are being encouraged to continue cultivating strains of local maize, rather than switching to commercial varieties. The local varieties act as genetic insurance. As pests adapt to attack the commercial strains most widely used, pest resistance characteristics from the local strains can be bred into the commercial strains to protect them. Without this insurance the world could face a major food shortage in the future. Maize is the world's most important crop, and is a staple for hundreds of millions in the Americas, Africa and Europe.