The Monarch butterfly (Danaus plexippus) is internationally recognized for its phenomenal migration. The migration of these small insects, half a gram in weight, covers a large part of North America and is a marvelous and very complex phenomenon. It is one of the longest and largest insect migrations in the world. The migratory generation (the individuals themselves) makes the return journey and, unlike the migrations of birds, turtles and whales, the individual Monarch butterflies themselves have never previously been in their hibernation sites.
Like all butterflies, the Monarch has a brief life cycle that includes an impressive metamorphosis. For the first stage, the females place their small white eggs on the leaves of milkweeds (Asclepias spp.). Each female lays around 400 eggs, of two millimeters in size, individually (not in groups) on the underside of the leaves. After four to eight days, small caterpillars with white, black and yellow stripes hatch and feed voraciously on the leaves of the milkweed.
After nine to fifteen days, the caterpillar looks for a hidden site among the shrubs and, once hanging by the head, splits its skin and the new skin underneath hardens into a green chrysalis. Within the chrysalis, one of the most spectacular transformations in the animal world takes place. The brain, heart and digestive apparatus of the caterpillar change, as muscles, compound eyes, three pairs of legs and two pairs of wings develop. After two weeks, the chrysalis becomes transparent, splits and a new adult emerges.
This life cycle lasts approximately one month with the actual time depending on environmental temperatures. However, by mid August in the latitudes of Canada and the United States, the angle of sunlight lowers, temperatures drop and the days shorten. The generation born in this period and influenced by these environmental changes is different to all of the previous generations. It will delay its reproduction and live for up to 9 months, giving it enough time to journey to the south (2 months), spend 5 months in Mexico or California before returning to the north. This generation is known as "Methuselah" because of its longevity.
In Mexico, the Monarch butterfly is found under the risk category Subject to Special Protection by the Official Mexican Norm 059.
The Monarch butterfly belongs to the family of brush-footed butterflies (Nymphalidae). These butterflies are medium sized and can be recognized by the fact that one of the pairs of feet is shorter and looks like brushes. Under observation, we can clearly see two pairs of feet but the third pair is more difficult to see.
The habitat of the Monarch butterfly changes radically over its annual cycle. In Canada and the United States, it frequents pastures, open zones, gardens, and forests to a lesser degree. In Mexico, its most important habitat is the forest and much less frequently the deserts and scrublands.
The milkweeds (Asclepias spp.) are the main food of Monarch butterfly caterpillars. These plants produce toxic substances (cardiac glycosides) that reduce the voracity of their predators. However, Monarch caterpillars assimilate these substances and store them in their skin, in turn making themselves toxic and thus protected from a great number of predators. The adult butterflies also feed on the nectar of the milkweeds.
There are around 130 species of milkweeds (Asclepias spp.), of which around 70 grow in Canada and the United States and the rest in Mexico and Central America. Milkweeds are opportunistic plants; they grow in perturbed zones, along highways and in cultivated fields and are considered weeds that compete with the agricultural crops.
The common milkweed (Asclepias syriaca) appears to be one of the main sources of food for the caterpillars in North America, especially for those of the “Methuselah” generation. In Mexico, one of the most important species is the milkweed (Asclepias curassavica).
The Monarch butterfly visits a wide variety of native meliferous plants (producers of nectar) such as the Mexican marigold (Tagetes erecta), golden marigold (Tagetes tenuifolia), Texas tarragon (Tagetes lucida), and wild sage (Lantana camara).
Despite the fact that both caterpillars and adults of the Monarch butterfly are toxic, there are some predators that can eat them. Scott´s oriole (Icterus parisorum) and the black-eyed oriole (Icterus abeillei) feed selectively on the thorax muscles and abdominal fat without eating the toxic skin. For its part, the black-headed grosbeak (Pheucticus melanocephalus) is less selective, and eats the whole butterfly. Black-eared mice (Peromyscus melanotis) that live below the colonies can also feed on dead Monarch butterflies.
One of the main parasites of the Monarch butterfly is a protozoan (Ophryocystis elektroscirrha). This parasite, widely distributed in North America, affects the survival of the butterflies.
During its caterpillar stage, the Monarch butterfly is an important herbivore that feeds voraciously on milkweed (Asclepias sp.). During the adult stage, the butterfly feeds on nectar and thus transforms from an herbivore to an important pollinator. Millions of Monarch butterflies live in North America and, through their migration, transport the pollen of plants with flowers, thus promoting the genetic diversity of the plants. Finally, despite the fact that both caterpillars and adults are toxic, some predators and parasites are unaffected.