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Macromoths of Northwest Forests and Woodlands

Life Cycle of Lepidoptera


The typical life cycle of a macromoth follows a sequence involving four life stages: the adult, egg, caterpillar, and pupa. This type of life cycle is termed holometabolous because it consists of complete development, including metamorphosis during the pupal stage, where the immature goes through a major transformation into an adult.

ADULT

A moth is the sexually mature adult life stage and serves three main functions in the life cycle: mating, dispersal, and oviposition. Many moths feed on nectar or a liquid sugar source, which primarily serves as fuel for flight. Some species of macromoths do not have functional mouthparts and cannot feed, therefore they are relatively short-lived and in turn will exhibit a short flight period.

Dispersal and flight activity.  Typically moths possess two pairs of wings, a pair of forewings and a pair of hindwings. The forewings are attached to the second thoracic segment, the mesothorax, while the hindwings are attached to the third thoracic segment, the metathorax. Moths capable of flight, which is the primary means for local and long distance movement, may beat their wings up to 60 or 70 times per second. However, not all moths have wings, and not all moths with wings can fly. Individuals that do not have wings, as well as those that have wings but are flightless, do not have flight muscles. Typically the female is subject to the loss of flight muscles, which is accompanied by a higher capacity for egg production. The male of the species has fully developed wings and is flight capable. Examples of species with wingless females are the lymantriids Orgyia antiqua and Orgyia pseudotsugata, and the geometrids Erannis tiliaria, Operopthera bruceata, and Operopthera danbyi.

The period for flight may be characteristic for a species and must be assessed in two types of time periods, the daily rhythm and the seasonal pattern. The majority of macromoths are night-flying species while a minority of species fly during the day. Some of the day-flying macromoths exhibit highly contrasting colors on their wings as exemplified by many of the arctiids such as Gnophaela vermiculata, Tyria jacobaeae, Leptarctia californiae, and Platyprepia virginalis; the male of the lymantriid Orgyia antiqua; the geometrids Rheumaptera subhastata and Mesoleuca gratulata; the noctuids Alypia langtoni and Schinia walsinghami; the saturniids Hemileuca eglanterina and Saturnia mendocino; and the sphingids Hemaris diffinis and Proserpinus clarkiae. The behavior of being either a night or day flying moth is characteristic to most species, however, a few of the night flying species, such as Hyles lineata, may be seen on the wing during the day.

The time of season and the length of time for the flight period of a species may also exhibit a diagnostic pattern. Most species occur at certain times of the year and may be present for a period of three to six weeks. For instance, the arctiid Lophocampa argentata will be in flight during the last few days of July and the first three weeks of August with a peak in flight around the end of the first week in August. Similarly, the males of the geometrids Operopthera bruceata and Operopthera danbyi are only in flight from the middle of November to the last week of December. On the other hand a few species may have individuals in flight throughout much of the year. For instance, the geometrids Orthonama centrostrigaria and Sabulodes aegrotata fly from the last week of January continously through the spring, summer, and fall until the last week in November.

Mating and oviposition.  Typically mating occurs soon after emergence from the pupa. The search for a mate is facilitated by volatile chemicals called pheromones. These chemicals are usually emitted by a virgin female and act as a sex attractant. Males detect the pheromone molecules with their antennae and fly upwind to locate the source of the chemicals, the female. The act of mating may take many hours, however, once the mating pair separates the female may begin laying fertile eggs. Pheromones are often species specific and help to reproductively isolate closely related species that occur in the same area.


EGG

Females may lay eggs singly or in clusters, depending on the species. Some species, such as Orgyia antiqua, will deposit eggs on the silk surrounding the pupal skin. Other species, such as Euxoa saris, scatter eggs on the soil surface. Most species attach their eggs to the vegetation that will serve as the caterpillar host plant. For instance, Phyllodesma americana will attach a single egg to the leaf of various flowering trees that will then serve as food for the caterpillar. Egg production in species of macromoths may range from a low number of less than 100 eggs per female to a high number exceeding 1,000 eggs per female.


CATERPILLAR

The caterpillar is the actively feeding immature stage of moths and butterflies and is perhaps less obvious at first glance but can be abundant on certain plants at certain times of the year. 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 foliage as miners, on the surface of foliage as skeletonizers or chewers, in galls, or in the nests of other insects, such as ants and bees. Only the larval stage of Lepidoptera is called a caterpillar. Caterpillars initially develop in the egg and then emerge through the eggshell that they sometimes eat. The caterpillar increases in size each time it sheds its skin, a process called molting. The individual caterpillar is termed an instar between molts. Typically, a caterpillar passes through five instars as it eats and grows. In certain species a caterpillar that will become an adult female may develop through an additional instar and thus grow bigger than the male. Even into the last instar it is usually difficult to distinguish between the sexes.

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 californicum occur in large colonies living in silk nests spun across twigs and branches of trees.

Caterpillar growth rates are strongly influenced by temperature and nutritional quality of host plants. Growth rates are slow at cold temperatures and up to a certain point faster at warm temperatures. 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. Also, growth is enhanced when water content of the 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, phenolics, 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, that is they are brightly colored, with the colors acting as a warning signal to would be predators. For instance, the brightly colored caterpillar of the cinnabar moth, Tyria jacobaeae, is poisonous to most prospective predators due to the storage of plant derived alkaloids.

The caterpillar life stage of many of the common species found in forests and woodlands of the Pacific Northwest are presented in a similar format as this book in Miller (1995). Also, the caterpillars of many species found in forests and woodlands of the eastern United States are presented in Wagner et al. (1997).


PUPA

Metamorphosis, the process of changing from a caterpillar into an adult occurs within the 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 may be naked but perhaps encased in rolled foliage 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 2 to 3 weeks, as in Cosmia calami, or for more than 1 year, as in Coloradia pandora. In many species the pupa overwinters. Typically, overwintering pupae are in diapause, a state of development when the eventual emergence of the adult is in an arrested condition. 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.

Overwintering.  A majority of the species of macromoths in the Pacific Northwest overwinter in the pupal stage or in the egg stage. However, some species of macromoths will overwinter in the adult stage such as the noctuid Xylem cineritia and the geometrid Triphosa haesitata. Only a few of the common species in the Pacific Northwest overwinter as a caterpillar. Some of these are the arctiids Gnophaela vermiculata, Lophocampa argentata, and Pyrrharctia isabella; the geometrid Neoalcis californiaria; and the dioptid Phryganidia californica.

Natural Enemies.  Lepidoptera have many natural enemies. Predators of many types devour Lepidoptera, 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 parasitoids. Also, many pathogens cause fatal diseases in Lepidoptera. Some of the most important pathogens are viruses, bacteria, protozoa, microsporidia, and fungi.

Lepidoptera are equipped with defense mechanisms against such natural enemies. Physical and physiological protective features include stinging hairs in the caterpillar of Hemileuca eglanterina, camouflage, or crypsis well illustrated by the white, gray, and black tones in the forewing and hindwings of adults such as Semiothisa and Itame. Behavioral protective features include flashing bright colors or eyespots to startle predators as seen in the hindwings of the noctuid Catocala ophelia, the sphingid Paonias excaecatus, and the saturniid Antheraea polyphemus.


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