A Kingdom for a Monarch.
by Mark Duckworth

Photo by Mark Duckworth

Click for a full-size picture.

The North American Monarch butterfly earned its name from early English and Dutch European settlers who observed a large, black and orange insect, not seen in their homelands. They were so impressed that they named it  " Monarch " after William of Orange  ( 1650-1702 ) the then Stadholder of the Netherlands and later King of England.

Throughout the summer, Monarchs can be seen across the USA and Southern Canada. They produce two or three generations during this time. Newly emerged adults become sexually mature in three or four days after feeding extensively on nectar from flowers and sap from damaged plants. Once a male locates a female, a brief courtship flight ensues, during which he wafts her with sexual pheromones. These are produced in glands located on the hind wings and in pencil-like protrusions extended from the abdomen. A virtual rape then occurs with the female grappled to the ground where copulation takes place. The couple then stay quietly together. Monarchs may mate many times during their adult life, an unusual phenomenon, as most insects normally only mate once or twice. Fertile females lay a number of eggs, dotted on the leaves of Milkweed (Asclepias) and more rarely Dogbane (Apocynum). Egg laying will then continue on each plant they encounter throughout their lives. In laboratory experiments egg laying has been found to be stimulated by the colour green and carbohydrates, with female Monarchs laying on pieces of sugar coated green plastic.

Eggs hatch in about four to eight days with the caterpillars eating their eggshell first. Larvae deprived of this first meal are found to be weak and often die. They are coloured in white, yellow and black bands with two tentacle-like protrusions from behind the head and two smaller ones towards the end of the abdomen. The rather podgy youngsters have a voracious appetite and grow to maturity in under 14 days, often stripping the Milkweed, leaving only the thick stems. This plant however, can withstand such attacks and recovers quickly. They are also very toxic, something that the Monarchs make good use of, storing the cardiac glycosides (heart poisons) in their bodies, giving good protection from predators and explaining their conspicuous appearance. Mature larvae tend to wander from the food-plant just before pupation. Once a suitable place has been found the larva spins a silk button from which it hangs, head down like a letter J. It then sheds its skin for the fifth and final time to reveal the chrysalis.  The pupa once hard takes on a jewelled appearance, bright green, with gold dots. The green is caused by pigments, the gold by total internal reflection and refraction  (structural colouration). It has a squat and rounded shape with a rim, that 12 days later will give the emerging adult a foothold.

Summer Monarchs live between 2 and 6 weeks depending on conditions, however those which hibernate may live up to eight months. This species is ancestrally tropical and must return to milder climates to survive the winter. First signs occur during late August as Monarchs stop breeding and begin laying down body fat. They fly in a southerly direction, roosting together at night in small groups at first. These soon grow into huge swarms, often completely covering the same roost trees each year.  These mass migrations are split into distinctive groups. Populations west of the rocky mountains and Mississippi river to the Pacific coast gather in southern California along the coastal mountain range between San Francisco (122` west 38` north) and Los Angeles (119` west 33` north). They spend the winter in a state of semi hibernation, gathering in groups of tens of thousands on Blue Gum trees (Eucalyptus globulus) and Monterey pines. Of the 45 known sites, the most famous location is at Pacific Grove. Nick-named "Butterfly town", local residents have an annual carnival to celebrate the Monarchs' arrival which has become a major tourist attraction. This marvel however is dwarfed by the Eastern migration, both in distance travelled and sheer numbers.

With the exception of comparatively small numbers of Monarchs overwintering in Southern Florida, the entire eastern population of Southern Canada and the USA fly to Mexico. The location of these roosts remained a mystery until recently. Research into the biology and migration of Monarchs is known to have started in about 1857 with reports of mass movements of butterflies heading south. In 1907 a naturalist, Jennie Brooks from Kansas published an article in "Harper's Magazine" in which she predicted that Monarchs migrated to Mexico. With the mystery still unsolved, in 1940 a Canadian Zoologist Fred Urquhart and his wife Nora began a project to plot the migration to a hopeful conclusion. This involved enlisting many volunteers to tag the Monarchs forewing with a coded label in the hope of recapture further south.

The results showed that the butterflies were capable of flying up to 3,000 miles, with mass migration from Minneapolis/St Paul (93` west 45` north) in NW Minnesota to Eagle Pass (100`30" west 28`40" north) on the Texas/Mexico border in just 30 days. An average speed of 50 miles a day over the 1,500 mile journey. However it was not until 1975 that participants Kenneth and Cathy Brugger discovered millions of Monarchs at an altitude of 10,000 feet in the mountain forests at Sierra Chincue near Angangueo, Michoacan (100`20" west 19`40" north), Mexico. They completely covered the Oyamel fir trees (Abies Religiousa). Estimates carried out by Lincoln Brower, Professor of Zoology at the University of Florida in Gainesville, in 1977 put the population at approximately 14 million individuals in a site of less than 4 acres.  There are now 14 known roosts in these mountain forests within 100 miles north west of Mexico City, their total area taking up less than 18 square miles. This is the true winter metropolis for Monarchs, with each roost containing tens or even hundreds of millions of butterflies. It is still a mystery as to how they find these roosts, as unlike birds which may live for several years, the Monarchs only live for a few months, and so have never been to the roost previously. 

The criteria for each site appears to be the same. A good tree canopy to provide protection from adverse weather conditions, water in the form of streams and damp ground, as well as an abundance of flowers. They begin arriving in early November. On sunny days some fly down to the forest floor to take in water or feed on the flowers. Generally by mid December all arrivals are settled in for the winter. Activity begins again in February when the adults make full use of the spring sunshine, feeding on the abundance of flowers and mating many times before abandoning the sites by the end of March. Though some Monarchs have been known to return as far north again as Boston MA (71` west 42`20" north) most will not survive beyond Texas. It will be their progeny that strive to the northern limits in their relentless search for fresh milkweed.  Monarchs face many hazards during their lives, both natural and manmade. Though toxic and aposematic, natural predation occurs to all stages in the form of insects, spiders, mice or birds, as well as attacks from parasites and various pathogens. At the winter roosts, work is being carried out into the effects of certain predators on populations. A study at the Sierra Chincue site showed that there are 37 species of omnivorous and insectivorous birds, two feeding extensively on Monarchs. The amount of cardenolides a butterfly may possess is variable. In some it is strong enough to induce immediate vomiting, whilst others have no toxin, exhibiting a polymorphism for palatability known as automimicry. Black-headed Grosbeaks may be able to detect the more edible adults as well as being capable of ingesting large amounts of toxin. However the Black-backed orioles vomit on only small amounts. Cardenolides are found in the Monarchs' cuticle rather than throughout the body. Orioles peck open the cuticle and eat only the internal organs and body fat. They then discard the " unzipped " body shell and wings. Estimates on Monarchs killed by bird predation at this site alone are put at 2.03 million during the winter, about 9% of the site's population.  There is also a mouse that takes advantage of  the potential food supply. The Black-eared mouse ( Peromyscus melanotis ) migrates into the roost during the winter and breeds extensively, feeding on the Monarchs at night. They tend to eat wet Monarchs close to the ground and during the course of the winter take approximately another 5% of the population.