You really shouldn't treat the plant until you know what the pest is, that's a waste of chemicals and hard on the plant, too. Try going out and looking at night with a flashlight. The critters often show up much better in the dark.
If it is caterpillars, then a spray of Bt solution will take care of it very quickly. Look carefully for large tomato or tobacco hornworms, they are the size of your finger, and will de-foliate a plant in very short order, they're so voracious.
Even though they're large, they can be hard to spot because they're green and have camouflage.
