You simply spray a liquid form of Bt on the leaves of a plant, and when the caterpillars come to dine the next time, they ingest the Bt which proliferates in their intestines and paralyzes their digestive systems. The caterpillars quickly starve to death. Thrips are brown or tan, about one-eighth of an inch long. They're very narrow, appearing the shape of a tiny alligator. They're hard to see with the naked eye, but their damage is very clear.
Thrips chew the top surface of leaves, leaving a striped, tan dead layer of leaf behind.
The damage can appear as 'white spots' on leaves, or a lacy or skeletal appearance on the leaves. Insecticidal Soap will kill thrips very effectively. Repeat about every three days to kill young thrips hatching from existing eggs on the plants. These are most common on indoor plants, and they can do serious damage. They are very hard to see with the naked eye.