Caterpillars (larvae) of Lepidoptera species (i.e. of butterflies and moths) are mostly (though not exclusively) herbivores, often oligophagous, i.e. feeding on a narrow variety of plant species (mostly on their leaves, but sometimes on fruits or other parts).
The dependency on specific food plants (also known as host plants) is among the key factors restricting the distribution of Lepidoptera species, although food plants of certain species may vary across different geographical populations (races). It also makes some of them important pests in agriculture or forestry. As such studies require meticulous observation and experimentation, for many Lepidoptera species food plants have not yet been identified, and even for the most common European and North American species there is often some minor disagreement over the exact selection of plants suitable to them. Adult females normally lay their eggs on or near specific food plants (which often have to be abundant enough). Larvae can't feed and end up starved to death if provided with unsuitable plants on purpose, but the choice of plant species suitable for different Lepidoptera in experimentation is often wider than the choice of plants on which their eggs are laid in nature. Closely related Lepidoptera tend to have similar food plant preferences. Many caterpillars sequester the toxins from their food plants and use them as a defense against predators. Though it is common for Lepidoptera to prefer a certain plant genus or family, some species feed on a narrow selection of unrelated taxa. The choice is unrelated to nectar plant preferences of adult Lepidoptera, which are much less strict.