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Common Name: Monarch Butterfly
Scientific Name: Danaus plexippus

(Information for this species page was gathered in part by Ray Zerjav for Biology 220W in Spring 2011 at Penn State New Kensington)

Female Monarch - Image credit Kenneth Dwain Harrelson, Wikimedia CommonsThe monarch butterfly (Danaus plexippus) is quite possibly the most widely recognized and widely admired insect in the United States. Its large, distinctive black and orange wings, its ability to migrate over thousands of miles to its over-wintering sites, and its life cycle that is intertwined with the ecology and distribution of the milkweed plant all make this species endlessly fascinating to a broad segment of our population.

Image credit - Kenneth Dwain Harrelson, Wikimedia Commons

The monarch’s spread wings are three and a half to four inches across. The wing membranes are orange and its venations are black. There is also a black border on the edges of the wings that is highlighted by white and orange spots. The male monarch is slightly larger than the female and is also typically more brightly colored. The male also has scent patches on its hind wings which are used to produce pheromones. The smaller, less intensely colored female has a greater amount of black pigmentation in its wing veins and borders. Monarchs that are participants in the long migration of the species are usually more deeply orange in color than the “local” individuals. Sometimes this deep orange coloration becomes so intense that it actually looks more red than orange.

Monarchs, like all insects, have six legs (three pair) that are attached to the middle body segment (the thorax). The first pair of legs in monarchs, though, is greatly reduced in size and are held tight against the thorax. These legs have abundant sensory structures and are used by the butterfly to “taste” potential plants to determine their suitability as nectar (“food”) sources. The remaining two pairs of legs are longer and are used for locomotion.

Life Cycle, Mating and Reproduction
The life cycle of the monarch can be examined from two different perspectives: the local cycle of an individual and the year-long cycle of the migrating population. The local cycle typically takes six to eight weeks from egg to senescing adult, while the migrating cycle may extend the life span of an individual to up to nine months.

The local cycle begins with the adult butterflies emerging from their cocoons (their “chrysalises”). These adult may live for two to five weeks depending primarily on temperature and other weather conditions and also on the availability of their food supply (flower nectar). Females release pheromones which attract males. Females that have not mated release more pheromones than previously mated females and, thus, attract more males. Males fly after the females and force them to the ground to mate. Only about one third of these mating attempts, though, actually result in the transfer of the male’s packet of sperm (the “spermatophore”). Further, males may hold the female in place on the ground for several hours in order to delay transferring their spermatophores until late in the afternoon or early in the evening so that another male of this obligatorily diurnally active species will not be able to immediately mate with that female.

Females will, though, eventually mate with many males, and both the females and their fertilized eggs and larvae benefit nutritionally from the proteins in the accumulated spermatophores. Females that have had the weeks of their adult life span in which to feed and grow are larger than newly emerged females. These older females produce larger eggs which, in turn, hatch into larger and more successful larvae (caterpillars). There is, then, a distinct female strategy to delay copulation (or at least fertilization) until their full growth has been attained.

Relationship with Milkweed
Females lay their eggs on the leaves of milkweed plants. There are many species of milkweed that are acceptable to this species, but the use of a milkweed is absolutely required. A female in the wild can lay a total of three hundred to four hundred eggs and will spread these eggs over many milkweed plants. The eggs hatch in three to five days depending on the temperature. The emerging larvae feed first on the egg capsule and then begin to eat the milkweed leaves. They molt five times during this larval life stage and increase their body mass more than two thousand times. Since the larvae are feeding exclusively on milkweed leaves they are accumulating the milkweed’s cardeolides (a cardiac glycoside that can cause the heart of a vertebrate to stop its contractions) in their body tissues. These cardeolides make the larvae (and, eventually, the adults, too!) poisonous to most vertebrates. Relatively few monarch caterpillars or adult butterflies, then, are consumed by vertebrate predators.

While avoided by vertebrates, the eggs and the larvae are under intense predation pressure from non-vertebrate species. More than ninety percent of the eggs and caterpillars will fail to survive. Eggs are eaten by ants, earwigs and snails, and larvae are eaten by beetles and other insects (like paper wasps) or killed by parasitoid wasps, bacteria, or fungi.

The end stage caterpillar then forms a cocoon (“chrysalis”) within which the tissues and organs of the larvae dissolve and are reformed into the structures of the butterfly. This metamorphosis takes between nine and fifteen days. The emergence of the butterfly from the chrysalis then starts the cycle all over again.

Migration
In the migrating life cycle there are great differences in life span and timing of reproduction especially in the Fall migrating forms. This migrating life form does not mate when it emerges from its chrysalis, instead it begins its long flight toward its frequently far distant over-wintering habitats. In these particular habitats (described below) the migrating life form enters a hibernating condition called “diapause” which can last many weeks or even months. Emergence from this diapause state then triggers mating and the beginning of the return migration back to the Spring and Summer ranges. These migrating monarchs may live up to nine months but spend much of this time period in the inactive, diapause state.

The two migrating populations of monarchs in North America are separated by the Rocky Mountains. The larger area east of the Rockies supports a much larger population of monarchs. All of these butterflies overwinter in the coniferous forests in the mountains of the Mexican states of Michoacán and Mexico. For the monarchs that reach the northeast states of the United States and the southeast provinces of Canada, this migration to and from this very specific overwintering site in Mexico covers several thousand miles. The monarchs that live in the smaller area west of the Rockies, on the other hand, overwinter in coastal sites in Southern and Central California. Their migratory route only measures hundreds of miles at the most. In both overwintering sites, however, the numbers of monarchs covering the trees and shrubs while waiting out the winter months in their diapause states can be truly staggering!

Following the eastern population through their cycle blends together the local and the migratory aspects of the monarch’s life cycle. Between February and March the monarchs who have spent possibly four or five months in their diapause state, re-awaken, mate, and then begin their flight north. They fly as far north as Texas and Oklahoma and out across the southern states. With luck, they have timed their arrival in these areas with the emergence of the new, Spring crop of milkweed. The overwintering migrants then lay their eggs on the milkweed and die. The next generation then undergoes a local life cycle and the adult butterflies mate on emergence and then continue their fight northward in late March and early April. This cohort of adults then gets further north into the Midwest and mid-Atlantic states. This cohort again has ideally timed their northernmost arrival to the emergence of the new crop of milkweed. This first, post-migrant generation then lays their eggs on the milkweed and dies. The second post-migrant generation then undergoes a local life cycle sequence and the emerging adults in June or July head into the northern most states and southern Canada. Again, they lay their eggs near the end of their brief lives and die. The next generation (the third, post-migrant generation) can have two different types of individuals. One type continues on its collective northern flight while the other type turns to the south and gets a head start on the Fall return migration. The northward flying cohort lays its eggs on the northern edge of the milkweed plants while the southward flying cohort lays its eggs on the southern mass of milkweed. Out of these eggs are hatched the larvae that metamorphose into the adults that will be the long-lived migratory life forms that will then attempt to fly all the way back to the coniferous forests in the mountains of Mexico.

The migrating monarchs stop at nectar sites to drink and re-fuel. They follow a variety of cues to stay on their course including polarized light patterns, UV light patterns and the Earth’s geomagnetic fields. They also utilize weather fronts and prevailing winds to give them a flight boost and save a great deal of wear and tear on their delicate wings.

Threats
There are a number of serious threats that are impacting the national population of monarch butterflies. The loss of patches of milkweed along their migration pattern due to human manipulation of the environment greatly reduces the ability of the monarchs to lay their eggs at the ends of their migratory legs. The loss of patches of suitable, nectar producing flowering plants along the migration routes greatly reduces the ability of the migrating butterflies to find re-fueling stations along their lengthy migratory pathways. And, finally, the severe reduction of their overwintering habitats in both Mexico and California greatly reduces the ability of this species to find suitable cover within which they can survive the harsh conditions of the winter months.

The monarch butterfly is the state insect of seven U.S states. In 1990 it was proposed to be the national insect of the United States, but the legislation did not make it all the way through Congress. This is an iconic species of our North American ecosystems, and is in need of help to make it through the Twenty-first Century.


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