Northern Prairie Wildlife Research Center
The caterpillars of many moth and butterfly species may have restricted ranges of suitable host plants upon which they can feed. Such species are termed monophagous or host plant specialists. In such cases the caterpillar may feed only on one species, on only a few species, or on all species within one genus of plant. For instance, any of the species of Drepanulatrix only feed on certain species of Ceanothus.
On the other hand, many caterpillars are generalists and are termed polyphagous. That is, the caterpillar can feed upon any plant among a wide range of plant species and still develop into an adult in the usual period of time. Generalist feeders often are able to live on plant species belonging to a wide array of families. For instance, the caterpillars of Neoalcis californiaria, Hesperumia sulphuraria, and Aseptis binotata can develop on 15 to 23 plant species belonging to 10-12 plant families. Although the caterpillars may be polyphagous they may exhibit preferences for certain plant species. Few caterpillars, such as Neoalcis californiaria and Anomogyna mustelina, are capable of feeding on both coniferous foliage and leaves of flowering plants.
Within a given environment caterpillars can be found in a variety of habitats and microhabitats. In general, they may be aquatic or terrestrial. Caterpillars can be found in fruits, roots and stems as borers or miners; in leaves as miners; on the surface of leaves as skeletonizers or chewers; in galls; or in the nests of other insects, such as ants and bees.
Most caterpillars feed and develop as solitary individuals; however, the caterpillars of a few species aggregate, some of which construct nests. For instance, the caterpillars of Lophocampa argentata aggregate on branches of Douglas-fir but do not construct nests. The caterpillars of Hyphantria cunea and Malacosoma disstria occur in large colonies living in silk nests spun across twigs and branches of trees.
Caterpillar growth rates are dependent upon temperature. Growth rates are slow at cold temperatures and up to a certain point are faster at warm temperatures. Caterpillars of species that occur in very cold climates may take more than 1 or 2 years to complete development because the short warm season limits feeding and growth.
Dependence of caterpillar development upon the nutritional quality of vegetation is strongly influenced by the amount of protein (nitrogen), water content, and allelochemicals. Most plants contain between 1% and 7% nitrogen by weight. Experimental manipulation of nitrogen in the diet of a caterpillar usually shows faster growth when nitrogen is provided at the higher end of the normal range. Likewise, caterpillar growth is enhanced when water content of their food is at the higher end of the normal range. Allelochemicals are plant-derived chemicals that may stimulate or deter feeding by caterpillars. Some of the better known allelochemicals are terpenes, alkaloids, and various proteins. These chemicals may also act as poisons to the caterpillar or in certain instances the caterpillar may store poisons and in turn become toxic to potential predators. Many of the poisonous caterpillars are aposematic (brightly colored). For instance, the brightly colored caterpillar of the monarch butterfly, Danaus plexippus, is poisonous to most prospective predators due to the storage of plant derived cardiac glycosides.
Caterpillars are not without defense mechanisms against such natural enemies. Physical and physiological protective features include stinging hairs (Hemileuca eglanterina), camouflage (Semiothisa new species), hiding in rolled leaves, storage of allelochemicals (bad tasting and poisonous), glands that emit repellant chemicals, and an ability to encapsulate foreign bodies. Behavioral protective features include flashing bright colors to startle predators, spitting, and feigning death.
In addition to natural enemies extreme mortality may be caused by chemical pesticides. Here also the caterpillar has certain defense mechanisms, namely, detoxication enzymes which breakdown pesticides (and plant allelochemicals) to nontoxic elements.