Question

insert two dashes where needed.
a fungus has been reducing the bean yield of coffee trees in central america, so the united states the worlds largest consumer of coffee has funded a research program to investigate the outbreak.

Explanation:

The phrase "the world's largest consumer of coffee" is an appositive that provides additional information about "the United States". To set off this appositive, we need to insert two dashes (em dashes) around it. So we insert a dash after "States" and a dash before "has".

Answer:

A fungus has been reducing the bean yield of coffee trees in Central America, so the United States—the world's largest consumer of coffee—has funded a research program to investigate the outbreak.