Almost all stick insects eat the leaves of bramble/blackberry Many such as the Indian or Laboratory stick insecthe Thorn Legged stick insect (Eurycantha calcarata), feed on bracken and other ferns.
It is important to make sure that your stick insects always have plenty of fresh food, and it is often wise to take from sites away from major road ways to avoid the poisoning effects of the traffic fumes. If this is unavoidable then the plant material should be washed before being offered to the stick insects.