The insect_,_ a cockroach_,_ crawled on my food.
Yes it’s an appositive modifier. I’d tree “a cockroach” as a modifier child of “insect”.
Not all modifiers are or feel the same. New Oxford Dictionary has:
modifier:
• Grammar a word, especially an adjective or noun used attributively, that restricts or adds to the sense of a head noun (e.g. good and family in a good family house).
adjective:
a word or phrase naming an attribute, added to or grammatically related to a noun to modify or describe it.
If you like, you can check other dictionaries, wikipedia, and grammar explainers for more on this topic.