Monarch Butterfly

Description

Monarchs are large butterflies with a wingspan of about 9 to 10 cm. They have orange wings with black borders, and two rows of white spots along the margins. Adults have a long, coiled proboscis which they extend and use like a straw to drink nectar and water. Monarch caterpillars have yellow, white and black stripes, with a pair of slender black appendages near each end of the body. They use a set of chewing mouthparts called mandibles for eating plant tissues.

Migration

Each spring, monarchs migrate north from Mexico and California across the United States to southern parts of Canada. This northward migration is slow and gradual, involving up to three successive generations of butterflies. Each generation travels partway to the northernmost destination before breeding. Once they arrive in Canada, monarchs reproduce at least one more time, and only the final generation, born in the late summer, travels the full distance back to the overwintering site.

Because monarchs breed and reproduce on milkweeds, each generation tracks the seasonal development of these plants. Monarchs orient their flight using an internal compass, which is calibrated by the position of the sun and possibly also the Earth’s magnetic poles. This innate sense of direction is essential because the adults that return south for the winter have never been to the overwintering site themselves.

All of North America’s monarchs overwinter in one of two locations: individuals born west of the Rocky Mountains overwinter in California, and those from central and eastern North America overwinter in central Mexico. The southward migration begins in late summer and is undertaken by a single generation of adults. These individuals travel approximately 3,000 to 4,000 km south to their overwintering sites, where they enter a dormant state until the following spring.

Diet, Reproduction and Development

Monarch caterpillars are milkweed specialists, meaning they eat only the tissues of plants in the milkweed family (Asclepiadaceae). In the spring, females lay their eggs on milkweed plants, perhaps 300 to 400 eggs per female over two to five weeks. Larvae (also called caterpillars) hatch within about four days and begin feeding on leaves. As they grow, the larvae molt or shed their exoskeleton four times, reaching a length of about 2.5 to 4.5 cm in around 9 to 14 days. The fully grown larva spins a silk pad on a leaf or stem, from which it hangs upside down and molts into an immobile pupa (also called a chrysalis). Contrary to popular belief, the larva does not dissolve into liquid at this stage before re-forming as an adult. Instead, many of the adult tissues and structures develop gradually as the larva grows, with the most dramatic change — the formation of the wings — taking place during the pupal stage. Adults emerge after about 9 to 15 days and continue the stepwise migration north for two or three generations. Adults of these generations live for two to five weeks. During this time they mate and reproduce. Adults of the final generation emerge in a state of reproductive diapause, meaning they will not reproduce until the following spring. This helps them to save their energy for the 3,000 to 4,000 km flight south, leaving enough in reserve to survive the winter. Overwintering adults are mostly dormant, active only occasionally on warmer days. They resume reproduction in the spring at the beginning of the northward migration.

Conservation

The monarch is an endangered species, according to both the Committee on the Status of Endangered Wildlife in Canada and the International Union for Conservation of Nature. Its populations have declined greatly since the late 1980s: over 95 per cent in the case of western monarchs and at least 70 per cent in the case of eastern monarchs. This is mainly due to habitat loss and degradation.

About 90 per cent of North America’s monarchs overwinter in central Mexico, mainly within the Monarch Butterfly Biosphere Reserve. Over time, illegal logging and forest fires have reduced the extent of this overwintering habitat. In 2016, Canada, the United States and Mexico jointly committed to protecting at least six hectares of occupied overwintering habitat in Mexico. Six hectares is considered enough to mitigate the risk of eastern monarch extinction in the short term. At overwintering sites in California, western monarchs are monitored primarily by volunteers participating in two annual counts: the Thanksgiving Count starting in November and the New Year’s Count starting in December. Since they began in 1997 and as of 2022, the three lowest counts were recorded in 2018, 2019, and 2020.

Monarchs reproduce and develop on milkweed, but due to widespread herbicide use, more frequent droughts and increasingly erratic spring weather, fewer and fewer of these plants are available for them to live on. Much effort to protect monarchs therefore involves planting milkweed. Milkweed can be easily incorporated into pollinator gardens and planted along roadsides, or other open areas protected from mowing and pesticide use.

Ecology

Monarchs are eaten by birds and other insects, including ants, spiders and wasps, and act as hosts for parasitic wasps and flies. As a form of defense, monarchs can incorporate toxic chemicals into their bodies, which they obtain from their host plants. These chemicals, called cardiac glycosides, are foul-tasting to predators, a fact which is advertised by the monarch’s bright, contrasting colours (known as aposematism). Not all milkweeds produce these toxins, however, leaving some monarchs poorly defended. These individuals nevertheless benefit from their warning colouration, since potential predators are more likely to find something else to eat than risk eating something poisonous. The viceroy butterfly (Limenitis archippus) has a very similar colour pattern to the monarch and is itself foul-tasting, which amplifies the signal so that predators are even more likely to avoid eating both species. Despite this, some birds have learned to find and eat only poorly-defended monarchs (e.g. the black-backed oriole), while others are unaffected or undeterred by the toxins (e.g. the black-headed grosbeak).

Relationship with Humans

Monarchs have long captivated humans. Their annual migration is significant in the mythology of the Indigenous peoples of central Mexico, for whom the returning butterflies signify the souls of ancestors returning from the dead, as well as the start of the corn harvest. Monarchs also draw ecotourism at important points along their migration route, such as at Point Pelee National Park in southern Ontario, and at overwintering sites in California and Mexico. Because of its popularity, the monarch inspires support for biodiversity conservation in general.