Eating only a few specific kinds of food - used especially for an insect subsisting on a few usually related plants.

Feeding on few kinds of food (adjective); also nouns oligophage and oligophagy.

feeding upon a limited variety of food; feeding upon only a few species of plants, usually within a single family

Animals or parasites that restrict feeding to a few host species, often they are a group of closely related species.

A species which uses several closely related hostplants.

(specialised) feeder on a few species of prey organisms.