JEMPOL, Malaysia—On a sun-scorched hillside, bulldozers clear land to plant more of the trees that yield palm oil, which is used in everything from Oreos to Pop-Tarts to Old Spice deodorant. Demand for palm oil has surged in recent years, making it the most-consumed vegetable oil in the world.

Nearby, an army of migrant workers toils in the heat, using long poles with attached sickles to saw off fruit bunches high in the trees, then hoisting the heavy clumps of fruit onto trucks headed to mills that extract oil.

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