Final answer:
The independent clause in the sentence is 'they can devour millions of pounds of plants in a single day.'
Step-by-step explanation:
The independent clause in the sentence is 'they can devour millions of pounds of plants in a single day.' An independent clause is a clause that can stand alone as a complete sentence, expressing a complete thought. In this case, the independent clause states a fact about what locusts can do when they congregate into huge swarms.
Learn more about Identifying dependent and independent clauses