Growth of Herbivorous Caterpillars in Relation to Feeding Specialization and to the Growth Form of Their Food Plants

Larvae of 9 species of swallowtail butterflies (Papilionidae), 10 species of bombycoid moths (Saturniidae and Bombycidae), and of the southern armyworm (Noctuidae: Spodoptera eridania) were reared under standardized conditions on mature leaves of many of their typical food plants. Growth rates, feeding rates and efficiencies of food and nitrogen utilization of larvae in their penultimate and final instars were measured by standard techniques. The study revealed a clear relationship between larval growth and growth form of the food plants quantified as leaf water content. Larvae grew faster and more efficiently on herbaceous plants than on the foliage of shrubs and trees. These differences were greater than could be accounted for by variation in the degree of feeding specialization among the insect species tested. The particular leaf characteristics responsible for the relationship between plant growth form and larval growth are not known, but they probably include leaf water content, nitrogen content, toughness, and fiber content. Since trees are likely to be more apparent to enemies than are most herbaceous plants, our results are consistent with recent suggestions that the mature foliage of apparent plants is generally poorer food for herbivores than is the foliage of unapparent plants. However, the extent to which the low food value of mature tree leaves is actually a consequence of the selective action of herbivores remains an open question.