BIOLOGY OF THE MONARCH BUTTERFLY

By Doug Collicutt

DESCRIPTION

The monarch is one of Manitoba's largest butterflies, with a wingspan of up to 100 mm. The body of the butterfly is black with some white spots. The upper surfaces of the wings are orange with black veining. The wing margins are black with white spots. Males can be distinguished from the females by a black patch on their hind wings, called a stigmata. Underneath, the wings are a paler orange, almost beige, with the same black veining and borders. The caterpillar (larva) of monarchs is boldly patterned, too, with black, white and yellow banding.

NAME

The monarch butterfly is an insect (Class: Insecta), belonging to the subgroup of insects that includes the butterflies, moths and skippers (Order: Lepidoptera; from the Greek, "Lepis" = scale, "pteron" = wing). It is a member of a further subgroup called the milkweed butterflies (Family: Danaidae). The monarch is the only species of this group, that contains 300 species world-wide, to occur in Canada.

The common name of this butterfly was assigned by early settlers to North America. There was, at the time, a King William, Prince of Orange, stateholder of Holland, who would later be named King of England. The butterfly's colour lead to the name: monarch.

Its scientific name, Danaus plexippus, is derived as follows. (Or maybe not. I couldn't track down a definitive statement anywhere, so this is an educated guess.)

Genus: Danaus (Latin) from "Daunus", a fabled king of Apulia.

Danaus had 50 daughters who he ordered to kill their husbands on their wedding night. Each was given a dagger to carry out the deed. All but one complied. Perhaps this has to do with the stigmata mark on the hind wings of male monarchs?

Or perhaps it derives from "Danae". In Greek mythology, she is the woman who was imprisoned by her father because it was fortold that her son (Perseus) will kill him. Zeus comes down to the chamber as a golden shower and sleeps with her. They have a baby. Her father seals her and the baby in a chest and sets it a drift.

Species: plexippus (Greek) from "Plexippos", one of the numerous sons of Aegyptus.

Aegyptus's 50 sons married Danaus's 50 daughters. Plexippos is one of the sons killed by Danaus' daughters. Sounds like the stab-mark theory relating to the stigmata on male monarchs may be the key?

As I've mentioned before, scientific nomenclature stemming from earlier, more "romantic" times can be a little strange. At least the scientific name has some relation to the species' common name, in keeping with the theme of royalty. It just doesn't have much to do with the biology of the critter. I might have suggested "Asclepiovorus migratoria-rex". Read on and you'll be able to figure out why.

DISTRIBUTION

Monarchs occur throughout North America south of the boreal forest zone and in central and northern South America. They have been introduced to the Hawaiian Islands and to Australia, and are thought to be spreading around to other islands in the Pacific ocean on their own. There are 3 separate populations in North America: one east of the Rocky Mountains, another west of the Rockies, and the third occupying some Carribean islands. In Manitoba, they occur in the southwestern 1/3 of the province up to the edge of the boreal forest.

HABITAT

Just about anywhere you can find milkweed plants (Genus: Asclepias) and open meadows, you can find monarch butterflies. They frequent prairies, meadows and wetlands, but avoid thick forests. Food for the caterpillars (milkweeds) and for the adults (flower nectar) are found mainly in grasslands and meadows in Manitoba, so that's where monarchs tend to be.

Adult monarch habitat.

Monarch caterpillar heaven.

MIGRATION

Monarchs are thought to be the only butterfly that truly migrates. Other species emigrate out from certain areas as their food supply emerges with the changing seasons, but in no other species do adults actively return to a certain site at the end of the growing season.

Monarchs must travel to and from Manitoba each year, though no one butterfly that leaves this province will ever return. Only it's progeny will make the return trip. Adult monarchs hatched out here in mid to late summer will fly south all the way to the wintering grounds in central Mexico (solid blue arrow in diagram), over 3000 km away! They reach the wintering grounds (dark red) by mid-October, averaging over 50 km per day on their southward migration. In spring, around mid-March, our Manitoba monarchs will begin the journey northward, but probably only make it as far as the southern US, (dotted blue arrows) where they will start a new generation. The first monarchs to make it back to Manitoba (dotted green arrow) in late May or early June are likely the children of the ones that left here the previous fall. Some of the grandhildren of our original Manitoba monarchs (dotted yellow arrow) may make it to Manitoba, too, later in the summer. (The monarchs living west of the Rocky Mountains migrate to sites in southern California to overwinter.)

Why do monarchs migrate? In spring and summer their northward journey gives them access to more and more emerging milkweed plants for their caterpillars and gets the adults into areas where there is less competition for flower nectar. But why south to Mexico in the fall? Are they seeking warmer climes with abundant food? Actually, they are seeking the opposite. What monarchs need to successfully overwinter are cool, moist, protected surroundings. They find these conditions in mountainous regions of central Mexico. Monarchs congregate by the millions, in a few isolated locations in forests within the mountain valleys. These overwintering sites were not even discovered until 1975! And that was only after a lengthy investigation involving a tagging program that saw thousands of monarchs fitted with tiny wing tags!

In their Mexican retreat, the monarchs have fairly stable weather conditions, with cool, but not freezing temperatures (monarchs are not freeze-tolerant), high humidity and protection from damaging winds. They converge on very small patches of forest and cluster together, often covering the individual trees. They rest and conserve their energy, drawing slowly on their stores of body fat; produced from the nectar of flowers they visited on their journey south. And there they remain, rousing occasionally to flutter about and perhaps sip some water from local ponds, then return to their hibernation, until about mid-March. With the return of spring, they rouse and begin their journey northward.

How do monarchs migrate? It's thought that monarchs use the position of the sun, combined with an innate circadian rhythm, and the earth's magnetic field to determine north and south while on their journey. Monarchs are strong fliers, but also make use of favorable winds, and may even make use of rising columns of air, "thermals", to gain altitude, then glide for long distances without expending energy in flapping. In North America, the funneling effect of the shape of the southern U.S. and central Mexico likely helps monarchs end up in the same places each year.

However, there is still much to be learned about monarch migration. For example, how do changing environmental conditions in late summer turn off reproduction in the emerging adults and "flip their switches" from "fly north" to "fly south"? The real secrets of why and how monarchs migrate to the exact spots that they do, still remains trapped within their genes.

FOOD

Food for adult monarch butterflies consists mainly of flower nectar. They fuel their great travels and reproductive efforts by sipping this sugary solution from obliging plants. The plants are, of course, taking advantage of the monarchs and other insects to do the job of pollination. As a large butterfly, monarchs tend to make use of larger flowers. Most of their favorites fall within the Asteraceae family of plants, including such things as fleabanes (Erigeron spp.), asters (Aster spp.), sunflowers (Helianthus spp.) and blazingstars (Liatris spp.), but they are not really that selective when it comes to flower nectar. Any flower that has available nectar could be visited by a monarch. (For more information on nectar sources for butterflies in general, check out our ° Butterfly Gardening ° article in this issue.)

The most important food for monarchs is that which the caterpillars eat, milkweed (see ° "If You Grow It, They Will Come" ° below). Monarch caterpillars, like the caterpillars of most butterflies and moths, have evolved fairly restrictive diets. Monarch caterpillars eat the flowers and leaves, and sometimes the seed pods, of milkweed plants and that's about it. They are one of the few caterpillars that does eat members of the milkweed family. These plants contain a thick foul tasting milky liquid (hence the name) that flows into areas of the plant that have been damaged (eaten or broken). This liquid is primarily intended as a deterrent to insects eating the plant. Monarchs can get partly around this defense, by chewing holes in the base of the main veins of leaves before they eat them, thus preventing excess "milk" from pouring into the leave. The plants have other chemical defenses, though, one of which the monarchs even turn to their own use.

Caterpillar on whorled milkweed (Asclepias verticillata).

TOXIC DEFENSE AND MIMICRY

Milkweed tissues contain compounds called cardenolides, that are quite toxic. Intended by the plant as a further chemical deterrent to being eaten, monarch caterpillars are immune to these and actually accumulate the toxins in their own tissues, rendering them toxic, too. The bold patterning of monarch caterpillars advertises this fact. Should a bird attempt to eat a monarch caterpillar, it will become ill (usually vomiting out the caterpillar) and will refrain from eating any more. Bold warning coloration, reinforced by the occasional bad meal, leaves monarch caterpillars save from most avian predators. If attacked, a monarch caterpillar will roll up in a ball and fall off the milkweed plant to the ground below, where it will remain curled up until the danger has passed. In this position its bright warning colours are even more pronounced.

No defense is perfect, however, and just as the monarch caterpillars have evolved resistance to the milkweed's toxins, so too have other insects developed resistance to the caterpillar's defense. Several kinds of parasitic flies and wasps, plus a variety of micro-organisms take their toll on monarch caterpillars.

Monarch butterflies retain the toxic defense that originated in their caterpillar stage. Their bold black and orange coloration advertises this, and potential predators learn to avoid this species. This defense is so good that it even offers protection to another unrelated species of butterfly, the viceroy (Limenitis archippus). Viceroy butterflies, which are also quite common in Manitoba, are not toxic or bad tasting, but they have evolved to look very similar to monarchs, thus gaining some protection from birds and other predators that have learned not to eat monarchs. This sneaky trick is an example of "Batesian mimicry", a fairly common technique employed in the insect world.

 

Can you see the characteristics in the hind wing of the viceroy (on the left) that distinguishes it from the monarch (on the right)?

LIFE CYCLE

The life cycle of monarchs involves what is termed "complete metamorphosis". That is, they pass through 4 distinct stages that include: egg, larva (caterpillar), pupa (chrysalis) and winged adult. Complete metamorphosis involves the transformation from the larval to the adult stage via a resting or pupal stage. Other insects have "incomplete metamorphosis", where the change from larval to adult stages is more gradual, with no distinct pupal stage. A good example of incomplete metamorphosis would be grasshoppers, which develop progressively, with each larval (or nymphal) stage still looking like a grasshopper.

In their Mexican wintering grounds and along the migration route back to Manitoba, the male and female butterflies will mate. Males have special patches on their hind wings, called "stigmata", containing scent scales, called "androconia" which release pheromones (a pheromone is an airborne substance, a chemical or scent, used in communication) that may help to attract females or aid in species recognition. After a brief aerial courtship the pair will mate. They mate by joining the tips of their abdomens together and the male passes sperm to the female. Mated pairs stay joined for several hours and can even fly off together if disturbed. In this case only the one adult will fly and the other will be carried along. (I don't know which, male or female, or both alternately, does the flying in such cases.)

When their eggs are ready to be laid, females search out milkweed plants on which to lay them. They find their way unerringly to milkweeds, most likely navigating by scent until they find a patch. Apparently the females will also "test" individual plants by "scratching" them with a foot to determine if they are, in fact, milkweeds. (Butterflies and many other insects have chemical receptors - noses - in their feet.) The eggs are laid, singly, on the undersides of leaves or flower buds. A female may lay several eggs individually throughout a patch of milkweed, then will fly off to find more plants. One female may lay hundreds of eggs over her life time. The life span of adult monarchs hatched in the spring and summer may be only a few weeks, as opposed to the adults that migrate south, then start the migration northward the following spring. These individuals may live for 8 or 9 months.

A single egg on the underside of a leaf.

The eggs hatch after about 7-10 days, depending on the temperature (slower when colder). At hatching the caterpillars are only about 4 mm long, but like all caterpillars, they are eating machines and they grow rapidly. The caterpillars will shed their skins 4 times, passing through 5 stages, called "instars". In 2 or 3 weeks they will reach full size, about 50 mm long, having increased in weight about 3000 fold! When they are ready to pupate they "go walkies" and leave the plants where they have been feeding to search out a safe place to pupate.

Once the caterpillar has found what it considers to be a safe place, usually under a leaf, branch or building overhang, it will spin a small silk pad and attach itself to the pad by its hind feet. The caterpillar then "goes J" and hangs upside down from the silk pad. About a day later it sheds its skin to enter the chrysalis (pupa). The metamorphosis occurring with the chrysalis can take from 7-21 days depending on the temperature. (In 1997, I witnessed one butterfly emerge in mid-September after being in the chrysalis for 21 days!) In very warm conditions an entire monarch generation, from egg to adult, may take only 1 month. In Manitoba, with our cooler temperatures, generation times are more likely to be 5-6 weeks. You can tell when the adult butterfly is going to emerge, because a day or so before it does, the chrysalis becomes translucent and you can see the dark butterfly body within.

I have found chrysalises in the oddest places.

Adult butterflies emerge from the chrysalis with shriveled wings and squat, puffy bodies. They will hang beneath the chrysalis remains and pump fluid into the wings to inflate them. Newly emerged butterflies secrete an unpleasant smelling liquid, called" meconium", that contains the waste materials from the chrysalis stage. After a couple of hours the butterfly's wings will be fully formed and dried, and it will be able to fly off in search of food and a mate.

Further south in North America, adults that have pupated and emerged when day length is still increasing and temperatures are warm will be sexually mature. They will seek out mates, breed and continue the northward migration. As summer progresses and day length begins to decrease, and temperatures start to drop, changes will occur in the developing monarchs. Emerging adults will not be sexually mature and they will begin to migrate southward. Only with the lengthening of days the following spring in the mountains of central Mexico will these butterflies become sexually mature.

In Manitoba, monarchs don't get here until June, and the first batch of adults that are hatched here, by mid to late July, may or may not be sexually active. If we have a long warm summer, and the monarchs have gotten here early, then we can have two generations of adult monarchs emerge. In shorter, cooler summers, we may have only one generation emerge. In either case, the last batch of adults to emerge from their chrysalises in Manitoba will make the astonishing journey all the way to Mexico and back to, at least, southern Texas the following spring. Only the great- or great-great-grandchildren of our Manitoba born monarchs will return to this province in successive years.

To watch a slide show of the transformation of caterpillar to butterfly, click below.

POPULATION FLUCTUATION

Monarchs are endangered! Monarchs are endangered! The sky is falling! The sky is falling! Beware of what you hear from the media on the subject of monarch populations. There has been considerable coverage in past years of the "plight" of the monarch in its wintering sites in central Mexico. "Rampant logging", "unheard-of frosts" and other "sky is falling" scenarios have been touted in the media, with suggestions that our beloved monarchs may soon go the way of the dodo. While the monarch's migratory nature and dependence on fairly isolated wintering sites do have the potential to be problematic for the species, don't count them out just yet. There is still considerable controversy over the true nature of habitat required for monarchs to overwinter successfully, and they have an enormous potential for recovery from minor or major population losses on the overwintering sites. Remember, monarchs produce at least 3 or 4 generations every year after leaving Mexico. Each generation has the potential to increase the species overall numbers dramatically. Weather and habitat conditions the monarchs face on the way north to Manitoba, and south again to Mexico, probably play at least as great a role in determining the fate of the species, as do events and weather in Mexico.

Over my life I have seen years when virtually no monarchs could be found in southern Manitoba and some where they seemed to be everywhere. And I've never seen a correlation between dire warnings about the overwintering monarchs and the populations that reach here the following spring. While we should always be vigilant and mindful of the conservation of this and every species, I think that monarchs are more resilient than the media gives them credit for! The real answers to the mysteries of monarch populations are a lot of research away.

GENETIC BOTTLENECK!

Recent genetic studies of monarchs have revealed that the entire North American population of this species is very closely related. Some scientists are speculating that monarchs faced near-extinction in the not-too-distant past, perhaps only a few thousand years. It's thought that the overall population was reduced to a very low number, and hence contained little genetic diversity. All three North American populations, which are now geographically isolated, are thought to have stemmed from this small number of monarchs that survived this population bottleneck.

IF YOU GROW IT, THEY WILL COME.

If you really want to do something for monarch butterflies, grow some milkweed! My house in Winnipeg is 1 km from the nearest natural habitat containing milkweed plants, yet each year several female monarchs will find the patches of milkweeds I grow in my gardens, specifically for them. I have counted as many as 25 adult monarchs that were raised and hatched from my small garden! Only in years where monarchs have been almost absent from Manitoba in general, has my garden failed to produce several new adults.

Manitoba has 6 species of milkweed plants and they are all actively sought out by female monarchs for egg deposition. (There are many more species of milkweeds further south in North America, as there are for most types of plants and animals.) Here's a quick glimpse at Manitoba's Milkweeds:

Showy Milkweed (Asclepias speciosa) - found occasionally in fields and prairies

Silky Milkweed (Asclepias syriaca) - common in prairies, meadows, and near farm fields

Whorled Milkweed (Asclepias verticillata) - common, though seldom seen, in fields and prairies

Dwarf Milkweed (Asclepias ovalifolia) - common in prairies and near forest edges

Swamp Milkweed (Asclepias incarnata) - found occasionally in wetlands and ditches

Green Milkweed (Asclepias viridiflora) - uncommon, in sandy prairies

Hairy Milkweed (Asclepias lanuginosa) - uncommon, in sandy prairies (sorry, no image yet)

Seeds and plants of whorled, dwarf and swamp milkweeds are available from some Manitoba growers, listed in the article ° "Grow Your Own Prairie Wildflowers "° in our Spring Issue. Showy and silky milkweeds are still considered noxious weeds under the Provincial Weed Act and should not be propagated. They are too aggressive for most garden settings, anyway.

MONARCHS IN THE CLASS ROOM?

Unfortunately, monarch butterflies don't arrive in Manitoba until June, very late in the school year, and too late to raise caterpillars to adulthood in the class room. Unless you're at summer school! In which case, or if you just want to raise the caterpillars at home anyway, check out our Raising Caterpillars article (coming soon!) in this issue.

Do you know what would be really great to do in schools, or rather, in the school yard? Grow a butterfly garden! Milkweed plants are easy to grow from seed. They are perennials that, once established, will persist for many years. Together with a variety of other plants, to provide food for caterpillars of other species, and some nectar producing flowers, you could create an ideal monarch butterfly nursery right in your school yard. And what Manitoba school yard couldn't do with a little sprucing up? For some "How-To" information check out our Butterfly Gardening and Growing Prairie Wildflowers articles in our summer and spring issues, respectively.

ACKNOWLEDGMENTS

Some of the information used in this article was drawn from the following publications.

The Butterflies of Manitoba. 1989. By P. Klassen, A. Westwood, W. Preston and W. McKillop. Published by the Manitoba Museum of Man and Nature.

The Peterson Field Guide Series: A Field Guide to the Butterflies of North America, East of the Great Plains. 1979. By A. Klots. Published by Houghton Mifflin Company.

Some excellent monarch web sites:

° Monarch Watch Web Site °

° Monarch Lab °

More on butterflies in NNZ:

Mourning Cloaks, Butterflies are Free?, Butterflies of Manitoba & Butterfly Gardening