Entomology is the study of insects. A biologist who specializes in this branch of biological science is known as an entomologist.
Enterprise had at least two Entomologists on board. When studying the rogue planet Dakala, T'Pol recommended the first survey team contain "the entomologists" and that they would move on from there. (ENT: "Rogue Planet")
Jake Sisko tutored Mardah in entomology in 2370. Nog thought that she was studying "bugs" to learn how to be a chef. (DS9: "Sanctuary", "Playing God")