insects suffer the same constraints. They can only tolerate a narrow group of chemicals to which they have been repeatedly exposed over thousands of generations. All other chemicals taste bad to insect herbivores and signal that the insect is on the wrong host. Specialists do not have the option of eating any plant in the neighborhood. Nor do they have the option of quickly adapting to the alien plants that have replaced their native hosts. Evolution simply does not work that fast. If an insect’s hosts are not present, it won’t be either.
