GREAT BARRINGTON, Mass. — On a recent sunny morning at the Big Y grocery here, Cynthia LaPier parked her cart in the cereal aisle. With a glance over her shoulder and a quick check of the ingredients, she plastered several boxes with hand-designed stickers from a roll in her purse. “Warning,” they read. “May Contain GMO’s (Genetically Modified Organisms).”

For more than a decade, almost all processed foods in the United States — cereals, snack foods, salad dressings — have contained ingredients from plants whose DNA was manipulated in a laboratory. Regulators and many scientists say these pose no danger. But as Americans ask more pointed questions about what they are eating, popular suspicions about the health and environmental effects of biotechnology are fueling a movement to require that food from genetically modified crops be labeled, if not eliminated.

Labeling bills have been proposed in more than a dozen states over the last year, and an appeal to the Food and Drug Administration last fall to mandate labels nationally drew more than a million signatures. There is an iPhone app: ShopNoGMO.

The most closely watched labeling effort is a proposed ballot initiative in California that cleared a crucial hurdle this month, setting the stage for a probable November vote that could influence not just food packaging but the future of American agriculture.