Keeping track of where genetically modified crops end up is proving to be more difficult than U.S. regulators had anticipated. Exhibit A is the ongoing controversy over food products found to illegally contain StarLink corn, a modified variety not approved for human consumption. In addition to recent recalls of two brands of taco shells, the largest U.S. manufacturer of tortilla products, Mission Foods, announced on Friday that it will voluntarily recall all its snack chips, tortillas, and taco shells made with yellow corn because the products may contain StarLink corn. Government and industry officials are trying to find and buy up all the StarLink crop to make sure it doesn’t get into more food products, but they are having a hard time. Some farmers growing the crop hadn’t signed the required contracts that obligated them to take certain steps to keep StarLink out of the food supply, and some didn’t follow recommendations that they plant buffer zones around the corn to keep it from contaminating other crops. One food company executive, speaking under the condition of anonymity, said, “The whole system has been self-policing by the seed industry. And obviously it hasn’t worked.”