The monarch (Danaus plexippus) is a milkweed butterfly (subfamily Danainae), in the family Nymphalidae. It is perhaps the best known of all butterflies. Since the 19th century, it is also found in New Zealand, and has been known in Australia since 1871. In Australia it is also known as the wanderer butterfly. In Europe it is resident in the Canary Islands and Madeira, and is found as a migrant in Russia, Azores, Sweden and Spain. Its wings feature an easily recognizable orange and black pattern, with a wingspan of . (The viceroy butterfly has a similar size, color, and pattern, but can be distinguished by an extra black stripe across the hindwing.) Female monarchs have darker veins on their wings, and the males have a spot in the center of each hindwing from which pheromones are released. Males are also slightly larger.
The monarch is closely related to two very similar species which formed the Danaus (Danaus) subgenus before 2005. The first is the Jamaican monarch (D. cleophile) from Jamaica and Hispaniola. The second is the southern monarch (D. erippus), of South America south of the Amazon river. The southern monarch is almost indistinguishable from the monarch as an adult, the pupae are somewhat different, and is often considered a subspecies of the monarch proper. But analysis of morphological, mtDNA 12S rRNA, cytochrome c oxidase subunit I, nuclear DNA 18S rRNA and EF1 subunit α sequence data by Smith et al. (2005) indicates that it is better considered a distinct species. The separation of the monarch and southern monarch is comparatively recent. In all likelihood, the ancestors of the southern monarch separated from the monarch's population some 2 mya, at the end of the Pliocene. At the time sea levels were higher and the entire Amazonas lowland was a vast expanse of brackish swamp that offered hardly any butterfly habitat.
Following the review of Smith et al. (2005), two sub-species of the monarch are recognized:
The male has a black patch of sex-scales on the hindwings, and the black veins on its wing are narrower than the female’s. The male is also slightly larger.
The eggs are creamy white and later turn pale yellow. They are elongate and subconical, with approximately 23 longitudinal ridges and many fine traverse lines. A single egg weighs about , and measures about high and wide.
Monarchs are especially noted for their lengthy annual migration. In North America they make massive southward migrations starting in August until the first frost. A northward migration takes place in the spring. (In Australia, they make limited migrations in cooler areas, but the blue tiger butterfly is better known in Australia for its lengthy migration. ) Female monarchs deposit eggs for the next generation during these migrations. By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve in the Mexican states of Michoacán and México. The western population overwinter in various sites in central coastal and southern California, United States, notably in Pacific Grove and Santa Cruz.
The length of these journeys exceeds the normal lifespan of most monarchs, which is less than two months for butterflies born in early summer. The last generation of the summer enters into a non-reproductive phase known as diapause and may live seven months or more. During diapause, butterflies fly to one of many overwintering sites. The generation that overwinters generally does not reproduce until it leaves the overwintering site sometime in February and March. It is thought that the overwinter population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. It is the second, third and fourth generations that return to their northern locations in the United States and Canada in the spring. How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research; the flight patterns appear to be inherited, based on a combination of circadian rhythm and the position of the sun in the sky.
Monarch butterflies are one of the few insects capable of making transatlantic crossings. They are becoming more common in Bermuda due to increased usage of milkweed as an ornamental plant in flower gardens. Monarch butterflies born in Bermuda remain year round due to the island's mild climate.
A few monarchs turn up in the far southwest of Great Britain in years when the wind conditions are right, and have been sighted as far east as Long Bennington. Monarchs can also be found in New Zealand. On the islands of Hawaii no migrations have been noted.
Monarch butterflies are poisonous or distasteful to birds because of milkweed poison stored by the caterpillar stage; their bright colors are warning colors. During hibernation monarch butterflies sometimes suffer losses because hungry birds pick through them looking for the butterflies with the least amount of the poison onboard, but in the process killing those that they reject.
The mating period for the overwinter population occurs in the spring, just prior to migration from the overwintering sites. The courtship is fairly simple and less dependent on chemical pheromones in comparison with other species in its genus. Courtship is composed of two distinct stages, the aerial phase and the ground phase. During the aerial phase, the male pursues, nudges, and eventually takes down the female. Copulation occurs during the ground phase and involves the transfer of a spermatophore from the male to the female. Along with sperm, the spermatophore is thought to provide the female with energy resources that aids her in carrying out reproduction and remigration. The overwinter population returns only as far north as they need to go to find the early milkweed growth; in the case of the eastern butterflies that is commonly southern Texas. The life cycle of a monarch includes a change of form called complete metamorphosis. The monarch goes through four radically different stages:
Monarchs can live a life of two to eight weeks in a garden having their host Asclepias plants and sufficient flowers for nectar. This is especially true if the flower garden happens to be surrounded by native forest that seems to be lacking in flowers.
Monarchs are foul-tasting and poisonous due to the presence of cardenolide aglycones in their bodies, which the caterpillars ingest as they feed on milkweed. Both forms advertise their unpalatability with bright colors and areas of high contrast on the skin or wings. This phenomenon is known as aposematism.
Many people like to attract monarchs by growing a butterfly garden with a specific milkweed species. Others enjoy raising them for pleasure or for educational purposes. For migrating flocks, sanctuaries have been created at favorite wintering locations. Even tourism revenue is thus generated.
Many schools also enjoy growing, and attending to monarch butterflies, starting with the caterpillar form. When the butterflies reach adulthood they are then released into the wild.
On Oahu, a white morph of the monarch has emerged. This is because of the introduction, in 1965 and 1966, of two bulbul species, Pycnonotus cafer and Pycnonotus jacosus. They are now the most common insectivore birds, and probably the only ones preying on insects as big as the monarch. Monarchs in Hawaii are known to have low cardiac glycoside levels, but the birds may also be tolerant for the chemical. The two species hunt the larvae and some pupae from the branches and underside of leaves in milkweed bushes. The bulbuls also eat resting and ovipositing adults, but rarely flying ones. Because of its colour the white morph has a higher survival rate than the orange one. This is either because of apostatic selection (i.e. the birds have learnt the orange monarchs can be eaten, because of camouflage; the white morph matches the white pubescence of milkweed, or the patches of light shining through foliage, or because the white morph does not fit the bird's search image of a typical monarch, and is thus avoided.
Another problem in North America is the black swallow-wort plant. Monarchs lay their eggs on these plants, thinking it is similar to milkweed. Once the eggs hatch, the caterpillars are poisoned by the toxicity of this invasive plant.
The bacteria Micrococcus flacidifex danai also infects the larvae and causes “black death”. As usual, just before pupation the larvae migrate to a horizontal surface. They die a few hours later, attached only by one pair of prolegs, thorax and abdomen hanging limp. The body turns black shortly after. The bacteria Pseudomonas aeruginosa has no invasive powers, but causes secondary infections in weakened insects. It is a common cause of death in laboratory reared insects.
The protozoan Ophryocystis elektroscirrha is another parasite of the monarch. It infects the subcutaneous tissues and propagates by spores formed during the pupal stage. The spores are found over all of the body of infected butterflies, with the greatest number on the abdomen. These spores are passed, from female to caterpillar, when spores rub off during egg-laying and are then ingested by caterpillars. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have a shortened lifespan but probably occur a low frequencies in nature. This is not the case in laboratory or commercial rearing, where after a few generations all individuals can be infected.