Learn Spanish (Latin America): Por mucho que uses el pesticida, esta plaga destruirá la cosecha. - No matter how much you use the pesticide, this pest will destroy the harvest.

In concessive clauses like por mucho que, you must use the subjunctive when the action is hypothetical or happening in the future. Because the main clause is in the future tense (destruirá), the pesticide use is viewed as an ongoing or future possibility, so the present subjunctive uses is required.

Could I say por más que uses instead of por mucho que uses?

Yes, they are completely interchangeable here. Both translate to 'no matter how much you use.' Por mucho que places a slight emphasis on the volume or quantity of the pesticide, but the overall meaning is exactly the same.

Why is it el pesticida with a masculine article if the word ends in -a?

Chemical substances that end in the suffix -cida (meaning 'killer') are generally masculine in Spanish. You will see this same pattern with words like el insecticida (insecticide) and el herbicida (herbicide).

Since the outcome is in the future, could I use a future form and say por mucho que usarás?

No, Spanish does not use the future tense directly after por mucho que or por más que. Just like with time conjunctions (such as cuando), you must use the present subjunctive (uses) to talk about future actions in this type of clause.