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Caterpillars of Pacific Northwest
Forests and Woodlands

Ecology of Caterpillars


With rare exception caterpillars are herbivorous, that is, they feed on plants. Most typically caterpillars feed on foliage, but also on roots, within branches and woody stems, in seeds, and on flowers. Few species of caterpillars are predaceous, feeding on animals.

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.


The growth rate of a caterpillar depends upon at least three factors: genetics, temperature, and nutritional quality of the food. The genetic basis for growth can be observed in the developmental time of caterpillars from different females. The average time for development among the caterpillars of a certain female can differ from that of another female of the same species. This trait can be inherited.

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.

Daily Patterns in Behavior

Caterpillars may switch between active and inactive periods based on the time of day. Diurnal caterpillars are active during daylight and nocturnal caterpillars are active during the night. The caterpillars of some species move among leaves and among plants in a daily pattern, often using branches, stems, and trunks as their pathways. Daily patterns in the behavior of caterpillars influence methods and opportunity for observation. While nocturnal habits of moths are well studied, few studies exist regarding observations of caterpillars throughout the night.

Seasonal Patterns in Occurrence

Species of butterflies and moths that have one generation per year (univoltine) will be in the caterpillar stage at a particular time, such as spring, summer, or fall. Typically, a univoltine species may be found in the caterpillar stage for a period of 8-10 weeks. Caterpillars of species that exhibit multiple generations per year (multivoltine) may be found for an extended period of time throughout spring, summer, and fall. Caterpillars from various generations in multivoltine species may be found over a period of 20-24 weeks.


A caterpillar may persist for 8 to 10 days or for 8 months or more. The short-lived caterpillars belong to small species that mature after a week or two of feeding. Many such species exhibit multiple generations per year. The long-lived caterpillars belong to species that eclose from eggs in fall, spend winter (overwinter) in a state of suspended development due to the cold, and then complete development during the spring of the next year. These species have only one generation per year. Only a few of the common species in the Pacific Northwest overwinter as a caterpillar. Some of these are the arctiids Gnophaela latipennis, Lophocampa argentata, and Pyrrharctia isabella; the geometrid Neoalcis californiaria; and the dioptid Phryganidia californica. A majority of the Lepidoptera in the Pacific Northwest overwinter as pupae.


Metamorphosis, the process of changing from a caterpillar into an adult occurs within the stage of life called a pupa. In butterflies the pupa is called a chrysalis. In moths the pupa may be covered in silk, which is called a cocoon, or the pupa is naked but perhaps encased in rolled leaves or in the soil. When a caterpillar has attained a critical size, it will change its behavior from feeding to searching for or creating a site to pupate. The pupal stage may last for 9 to 12 days, as in Ypsolopha cervella, or for more than 1 year, as in Coloradia pandora. In most species the pupa overwinters. Typically, overwintering pupae are in diapause, a state of arrested development. The adult will not mature and emerge from the pupa at the appropriate time unless the pupa is first exposed to a period of cold.

Natural Enemies

Caterpillars have many natural enemies. Predators devour caterpillars, often in great quantities. Some of the most important predators are rodents; reptiles; bats; birds; spiders; nematodes; and other insects like beetles, true bugs, and fly and wasp parasitoids. Also, many pathogens cause fatal diseases in caterpillars. Some of the most important pathogens are viruses, bacteria, protozoans, microsporidians, and fungi.

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.

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