The puss caterpillar is the most poisonous caterpillar in the United States and is found mostly in southern states. It feeds on shade trees such as elm, oak, and sycamore. If you were stung, you may feel an immediate, intense pain that often comes in waves, swelling, itching rash of red blotches and raised ridges, restlessness and anxiety, according to WedMD. Vomiting, cramps and fever can also be symptoms. Some people experience only an itching or burning sensation.