EL TARRA, Colombia—The coca leaf used to make cocaine has made such a dramatic comeback in this country that plantings now cover more territory than they did before a multibillion-dollar U.S.-sponsored eradication campaign began 16 years ago, U.S. and Colombian officials said.

The explosion in the coca crop has come since 2013, as the U.S. was forced to reduce and ultimately end spraying from crop dusters because of health concerns over the use of a toxic defoliant and the Colombian government’s worries that the program was...