Wiktionary
oligophagy
n. feeding on few specific types of food (e.g. a single plant genus)
Wikipedia
Oligophagy
Oligophagy refers to the eating of only a few specific foods. The term is usually associated with insect dietary behaviour. Organisms may exhibit narrow or specific oligophagy where the diet is restricted to a very few foods or broad oligophagy where the organism feeds on a wide variety of specific foods but none other.
Polyphagy, on the contrary, refers to eating a broad spectrum of foods. In the insect world it refers usually to insects that feed on plants belonging to different families.