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Science
Volume 40 Issue 17 • Aug 26-Sept 1, 2010
now in our 40th season

End of Summer Splendor

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

As the summer winds down and glimpses of fall appear, Nantucket is adorned with regal and beautiful creatures fitting above the sandplain grasslands that have appeared to feed upon the milkweed, wild asters, Joe Pye weed, and goldenrod on their migration down south.  Monarch butterflies, Danaus plexippus (Linnaeus, 1758), begin their annual migration from Canada and the United States to central Mexico in late summer with the peak in travelers for our area typically passing through in September (http://www.monarchwatch.org/tagmig/peak.html).  Their trip may be as long as 3,000 miles, an amazing feat considering their size and the weather they have to endure.  No other butterfly manages to make such a long two-way trip.  Other butterfly species may travel long distances one way, but the monarch will go south in the winter in order to survive over the winter months, traveling together en masse after making that trip north the previous spring.  There are populations of monarchs in California, Florida, and Texas that don't migrate, but the majority of monarchs do.  The primary range for monarchs is in the Americas, but populations exist in New Zealand and Australia and in small islands and limited areas in Europe such as Canary Islands, the Azores, and Madeira.

MonarchFor the past few years, the Nantucket Land Council has been sponsoring a monarch butterfly tagging program that they share with area school kids. This program started with the help of island lepidopterist and Biodiversity Initiative scientist, Dr. Mark Mello of the Dartmouth MA based Lloyd Center for the Environment.  Some of the city kids I bring from off island are also lucky enough to participate in capturing and tagging these delicate insects with the expertise and guidance of NLC’s resource ecologist, Emily MacKinnon.  Each butterfly is tagged with a tiny, round, 3/8” sticker, which contains an identification number, phone number and an email address where recaptured butterflies can be reported.  It is amazing how much information can be read from this tiny little adhesive backed sticker.  Tags are placed on the mitten shaped cell (discal cell) on the underside of the hindwing near the center of lift and gravity where they won’t interfere with flight or add too much weight.  After the butterfly is tagged and released, the data is sent to Monarch Watch (http://www.monarchwatch.org/), a group dedicated to monarch research, and added to a national database.  If a tagged butterfly is recaptured or, more likely, found dead, researchers learn about its life including its flight path, distance traveled, and life span.  This is a wonderful program and I would encourage anyone who works with children to check out the monarch watch web site and order some tags and use the many resources available on the web site in addition to spending some time learning about the biology and life history of these beautiful creatures from the Land Council.

Monarch butterflies have very memorable coloring which helps to protect them from predation.  The upperside of male monarchs are bright orange with wide black borders and thin black veins. Males are slightly larger than females and they sport a noticeable patch of scent scales called the "androconium" in the center of each hind wing from which pheromones are released (may be where the men’s scent “Axe” got its inspiration).  The upperside of the female monarch is orange-brown with wide black borders and blurred thick black veins; the females have no scent pouches on their hindwings. Both sexes have white spots on borders and apex.  The Viceroy butterfly (Limenitis archippus) is a Mullerian mimic; it has similar coloration and is also distasteful. The monarch wing span is 3 3/8 - 4 7/8 inches (8.6 - 12.4 cm). Like all insects, the Monarch has six legs, however it uses only four of its legs as it carries its two front legs against its body.

According to the book “Insect Potpourri: Adventures in Entomology” by Jean Ruth Adams in a chapter dedicated to monarchs and written by Douglas Sutherland, the oldest known monarch specimen was found in the collection of a English pharmacist named James Petiver around 1698-1709.  Danaus plexippus was described and named from Pennsylvanian specimens in the 1758 10th edition of Systema Naturae by the Swedish botanist Carolus Linnaeus.  Early colonial Americans called it many things including archippus butterfly, milkweed butterfly, King Billy, and the wanderer.  It is thought that the colonists were so impressed by this species that they named it “monarch” in honor of “King William, Prince of Orange, stateholder of Holland, and later King of England.”  Both genus and species name is linked to the Greek mythology tale of Danaus.

The monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae), in the family Nymphalidae. It is perhaps the best known of all North American butterflies.  Their obsession with milkweed plants is a survival mechanism. Most milkweeds contain cardiac glycosides (http://www.answers.com/topic/cardiac-glycoside) which are chemicals that affect heart rhythms.  The cardiac glycosides are stored in the bodies of both the caterpillar and adult in various sections and concentrations.  These poisons are distasteful and emetic to birds and other vertebrate predators. After tasting a monarch, a predator might associate the bright warning colors of the adult or caterpillar with an unpleasant meal, and avoid monarchs in the future.

The life cycle of a monarch includes a change of form called complete metamorphosis (http://en.wikipedia.org/wiki/Monarch_(butterfly). The monarch goes through four very different stages.  The eggs are laid by the females during spring and summer breeding months.  The creamy white, sometimes very pale yellow eggs hatch (after four days), revealing worm-like larvae to begin the caterpillar stage.  The caterpillar is banded with yellow, black, and white stripes.  The head is also striped with yellow and black.  There are two pairs of black filaments, one pair on each end of the body.  The caterpillars consume their egg cases (yum! – hey at least they are tidy, unlike most kids), and then feed on milkweed, ingesting all that useful cardiac glucoside.  You can find some on Nantucket right about now if you look on the underside of the milkweed plants found all over the island.  During the caterpillar stage, monarchs store energy in the form of fat and nutrients to carry them through the non-feeding pupa stage.  The caterpillar stage lasts around two weeks.  In the pupa or chrysalis stage, the caterpillar spins a silk pad on a twig or leaf and hangs from this pad by its last pair of prolegs.  It hangs upside down in the shape of a 'J', and then molts, leaving it encased in an articulated green exoskeleton.  At this point, hormonal changes occur, leading to the development of a butterfly (metamorphosis).  The chrysalis darkens (the exoskeleton becomes transparent) a day before it emerges, and its orange and black wings can be seen.  The mature butterfly emerges after about two pupal weeks and hangs from the split chrysalis for several hours until its wings are dry (often in the morning). Meanwhile fluids are pumped into the crinkled wings until they become full and stiff. Some of this orangey fluid (called meconium) drips from the wings.  Finally (usually in the afternoon) the monarch spreads its wings, quivers them to be sure they are stiff, and then flies away, to feed on a variety of flowers, including milkweed flowers, red clover, and goldenrod.

Their long distance migration is perhaps the most fascinating part of the monarch life history.  On their trip south, monarchs stop to feed on flower nectar and to roost together at night.  They often gain weight as the fly south. At the Mexico wintering sites, butterflies roost in trees and form huge aggregations that may have millions of individuals.  During the winter the butterflies may take moisture and flower nectar during warm days.  As winter ends, the temperature begins to rise, and the days grow longer, the monarchs become more active, beginning to mate and prepare for their trip north.  The mating ritual occurs in two stages, an aerial stage and a ground stage.  The male nudges, and then tackles the female in midair; they then land on the ground to exchange phone numbers and make baby butterflies.  They leave their roosts during the second week of March, flying north and looking for milkweed plants on which to lay their eggs.  These monarchs have already survived the previous fall’s southern migration and the winter chill.  They have escaped predatory birds, weathered storms, and are the only monarchs left that can produce a new generation.  If they return too early, before the milkweed is up in the spring, they will not be able to lay their eggs and continue the cycle.  The migrating females lay eggs on the milkweed plants they find as they fly, recolonizing the southern United States before they die.  Soon the first spring caterpillars hatch and metamorphose into orange and black adults.  It is these newly emerged monarchs, the offspring of the butterflies that made the fall journey, that recolonize their parents' original homes.  Summer monarchs live a much briefer life than the overwintering generation; their adult lifespan is only three to five weeks compared with eight or nine months for the overwintering adults.  Over the summer there are three or four generations of monarch butterflies, depending on the length of the growing season.  Since each female lays hundreds of eggs, the total number of monarch butterflies increases throughout the summer.  Before the summer ends, there are once again millions of monarchs all over the U.S. and southern Canada.

As fall approaches, the monarchs are born that will make that long trip south, waiting to mate until the following spring.  These late summer monarchs will travel hundreds and even thousands of miles to their winter grounds in Mexico and California.  These monarchs need a lot of energy to make their trip.  They store fat in their abdomens that will help them make the long trip south and will help them survive the winter. During their five months in Mexico from November to May, monarchs remain mostly inactive.  They will remain perfectly still hour-after-hour and day-after-day.  They live off of the stored fat they gained during their fall migration.  When they first arrive at their winter locations in November, monarchs gather into clusters in the trees.  By December and January, when the weather is at its coldest, the monarchs will be tightly packed into dense clusters of hundreds or even thousands of butterflies.  By mid-February these clusters of butterflies begin to break up and the monarchs will begin to gather nectar.  In the spring they will reproduce and their offspring will make the return trip to the north, completing the cycle a relay race among multiple generations.  The original butterfly dies along the way, but the offspring it leaves behind continues on to the north where the cycle will start again in the fall.  This is not unlike the pilgrimage many summer visitors are making right now as they head back to school and work. 

To see these beautiful butterflies, check out Smooth Hummocks or the Land Bank and Nantucket Conservation Foundation land off Madaket Road near the water fountain and Barrett Farm road turnoff or anywhere large open meadows and sand plain grasslands are covered with goldenrod and asters in bloom.  Monarchs are facing a series of threats from severe storms that may be linked to climate change related temperature swings, to outbreaks of beetles taking over their winter habitat to deforestation.  If your community’s school has the resources, encourage them to participate in the tagging program which is both inexpensive and very rewarding.

 

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