They are picky because these insects are specialized to feed in particular ways. Part of the specialization seen in tree-eating insects is the result of plant defenses. Plants do not tamely submit to being gobbled up by insects. Different parts of a tree have different kinds of tough, inedible protective coverings. Tender tissues, such as young leaves, may have a protective coat of barbed or hook-shaped hairs.
Most tissues have chemical defenses, usually several kinds.
In movies, a special agent who is trying to retrieve a stolen document or invention is equipped with all kinds of fancy gadgets to bypass the elaborate security measures of the enemy stronghold and storage vault. Insects that feed on plants are like those secret agents with their fancy gadgets. Like real secret agents, insects are not always successful in penetrating the defenses and gaining the prize. A question often comes up as students begin to explore the diversity of insects that may feed on a single species of tree. This topic is considered at the end of the project, when the students list the habits of the animals they see on the tree, including predators.