First, look at the damage to determine if it's old or new damage. New damage will appear "fresh" and be green around the edge of the wound. Old damage will show browning around the area that has been chewed. If you have old damage, it's most likely too late to provide treatment as the bugs could have moved on to bigger and yummier plants. If it's new damage, first look for the bugs that could be doing the damage. This is where a bit of basic entomology comes in handy so you can properly identify what you're seeing. Try looking images up online (make sure to utilize a reliable resource such as a university or extension service). The TAMU entomology website hosts a variety of resources here:
If you can't find what you've got, you can always contact me for help.
With insect pests, it's usually better to try managing them early when they are still small. Why? Well, for one, they're small which is often easier to kill. Secondly, they are not as mobile as in the adult stage. Remember, adult insects have wings and can fly away, immatures have to hop, walk or crawl.
So for chewing insects, what are your options for control? Cover the plants with row cover to exclude the insects or hand pick the insects (wear leather gloves in case they try to bite you) from the plant and drop them in a bucket of soapy water. If you have a lot of insects or a lot of plants, you can try using a shop vac to suck the insects off the plants. Other options would be using products with active ingredients such as spinosad (get good coverage on foliage since insects have to consume this for it to work!), neem, d-limonene, pyrethroids (cyfluthrin, bifenthrin, deltamethrin, etc.) or carbaryl. Remember, when you choose pesticides, find ones that have the location you are treating on the label and follow all label instructions!