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

Consuming a small range of types of food items.

(app. polyphagous) Having a narrow range of hosts.