Skip to content
Toggle Left Side Navigation

Unless otherwise noted, information contained in each edition of the Kansas School Naturalist reflects the knowledge of the subject as of the original date of publication.


Volume 62, Number 2 - November 2017

Biology of the Monarch Butterfly

 

 v62n2-175px
 Photo Credits: Photos courtesy of Monarch Watch unless otherwise credited.

PDF of this issue


ABOUT THIS ISSUE

ISSN: 0022-877X

Published by EMPORIA STATE UNIVERSITY

Issued by THE DEPARTMENT OF BIOLOGICAL SCIENCES

Editor: JOHN RICHARD SCHROCK

Editorial Committee: R. Brent Thomas, William Jensen, Marshall Sundberg

Circulation (this issue): 10,160

Press Run: 13,000

Press Composition: Ryan O’Meara

Online edition posted by: Terri Weast

Printed by: Mennonite Press Inc.

The Kansas School Naturalist is sent free of charge and upon request to teachers, school administrators, public and school librarians, youth leaders, conservationists, and others interested in natural history and nature education worldwide. Back issues are sent free. A back issue list is sent free upon request and is available online. To be added to the KSN mail list (free), e-mail postal address to ksnaturl@emporia.edu. The Kansas School Naturalist is sent by third class mail to all U.S. zipcodes, and first class air mail to all other countries. The Kansas School Naturalist is published by Emporia State University Department of Biological Sciences. Address print correspondence to Kansas School Naturalist, Department of Biological Sciences, 1 Kellogg Drive, Emporia State University, Emporia, KS 66801-5087. Opinions and perspectives expressed are those of the authors and/or editor and do not reflect the official position or endorsement of ESU. Issues can also be viewed online at: www.emporia.edu.edu/ksn/ The Kansas School Naturalist is listed in Ulrich’s International Periodicals Directory, indexed in Wildlife Review/Fisheries Review, and appropriate issues are indexed in the Zoological Record. The Kansas School Naturalist is issued from one to four times per year. Publication of this issue is partly underwritten by the Ted and Betty Andrews KSN Endowment. Publication and distribution of this issue of the Kansas School Naturalist is also partially underwritten by contributions from readers like you.

Photo Credits: Photos courtesy of Monarch Watch unless otherwise credited.

taylor

 

Author Bio

Dr. Orley “Chip” Taylor is Professor of Entomology Emeritus at the University of Kansas where he directs Monarch Watch. His research has included studies of the reproductive isolating mechanisms in sulfur butterflies, reproductive and life history patterns in plants, and the comparative biology of European and Neotropical African honey bees and the migratory behavior of monarch butterflies. Chip initiated the outreach program Monarch Watch in 1992. This issue of the Kansas School Naturalist heavily uses illustrations from this program.


Biology of the Monarch Butterfly

Introduction

Carolus Linnaeus, the founder of modern taxonomy, first named the monarch butterfly Papilio plexippus in 1758 in his Systema Naturae. “Papilio” means “butterfly” and as more butterflies were described, this species was used to establish the new genus Danaus in 1780.

 The monarch’s scientific name is now Danaus plexippus (Linnaeus) 1758, with the parentheses around Linnaeus’ name indicating his original description and name were in another genus. 

The monarch is a member of the milkweed butterflies in the subfamily Danainae. About 300 species are in this subfamily worldwide. For a long time, these butterflies were in their own family Danaidae. However, they are now placed in the Nymphalidae, a large family of “brush-footed” butterflies. These butterflies walk on their mid- and hind-legs, the front pair of legs being feather-like, not used in walking, but used for plant detection by the females through “drumming.”

Hierarchy of Taxa to Which the Monarch is Assigned Life Cycle

Ranks Taxa
Kingdom: Animalia
- Phylum: - Arthropoda
-- Class: -- Insecta
--- Order: --- Lepidoptera
---- Family: ---- Nymphalidae
----- Subfamily: ----- Danainae
------ Tribe: ------ Danaini
------- Genus: ------- Danaus
-------- Species: ------- Danaus plexippus

Life Cycle

Monarch butterfly development from egg to adult is completed in about 30 days.

After mating, female monarchs search among the grasses and flowering plants for milkweeds. These are the only plants on which they lay their eggs. Females usually lay a single egg on the underside of a leaf near the top of the plant.

Female monarchs lay several hundred eggs during their lifetime. The eggs hatch in four days. Eggs are approximately 1.2mm high and 0.9mm wide.

The larva that develops within the egg eats its way through the egg shell and then proceeds to eat the rest of the shell before it starts to feed on a milkweed leaf.

Monarchs do all of their growing during their five larval/caterpillar stages (instars). Each time the caterpillar grows too large for its skin, it molts (sheds) its skin. The soft new skin soon becomes harder. The caterpillar typically eats the shed skin before it resumes feeding on a leaf.

Approximate length of the body at each stage: 1st instar, 2-6mm; 2nd instar, 6-9mm; 3rd instar, 10-14mm; 4th instar, 13-25mm; and 5th instar, 25-45mm. The caterpillar stages take a total of 10 to 14 days.

The transition from larva to adult (metamorphosis) occurs during the pupal stage. During the 10-14 days of the pupal stage the larval structures and tissues break down while groups of cells dedicated to become adult structures and organs such as legs, wings and ovaries develop rapidly. The result is a fully formed adult beneath the pupal skin (cuticle) that completes the process of emergence by causing the pupal skin to split. The new adult hangs from this skin, expands its wings and is ready to fly in a few hours.

Male and female monarchs can be easily distinguished. Males have a black spot on a vein on each hind wing that is not present on the female. These spots are actually pouches made of specialized scales designed to deliver pheromones during courtship, though this function has been lost in monarchs. The ends of the abdomens are also distinctive; males have triangular shaped claspers and females have a conspicuous notch just before the end of the abdomen. Females are darker than males with wider black veins on their wings.

p01-a-125px p01-b-125px p01-c-125px

p02-hi-250px

Do the Adults Eat?

No growth occurs in the adult stage, but monarchs need to obtain nourishment to maintain their body and fuel it for flight. Nectar from flowers is about 20% sugar and provides most of their adult food. Monarchs visit many different flowering species for nectar. They use scent (olfaction) and vision to find flowers. They use taste receptors on their front feet to “taste” host plants, but they do not dip their feet into nectar.

How Big Are Monarch Butterflies?

With a wingspan of usually more than 3.5 inches (95mm) and an average mass (weight) of 0.5 grams, monarchs are among the largest butterflies in North America. During the migration, monarchs may gain in mass by feeding on nectar from fall flowers and often weigh 0.6 -0.7 grams at the start of the overwintering period.

How Do Monarchs Reproduce?

In the spring and summer, the primary job of the adult stage is to reproduce—to mate and lay the eggs that will become the next generation.

Females begin laying eggs soon after mating.

Adults live from two to five weeks during the reproductive, summer generations. The generation of monarchs which emerges in late summer (August) and early fall (September-October) migrates to overwintering grounds, either in central Mexico for eastern monarchs or in California for western monarchs. Overwintering adults live seven to nine months. These migratory butterflies are non-reproductive and exhibit many behaviors not seen in reproductive monarchs such as clustering in large numbers and strong directional flight to the SW. After the winter period (late October–March), monarchs migrate northward to reproduce during spring and summer.

Battle of Plants vs. Insects

Over 50 years ago, botanist Peter Raven and entomologist Paul Ehrlich described how many plants and insects co-evolve. Some plants evolve chemical defenses that repel or harm insects that eat them. In turn, many insects evolve defenses or a tolerance to these chemicals.

Monarch larvae feed on milkweeds in the genus Asclepias and several other genera of vine-like milkweeds in North America. Most milkweeds are perennial plants, the individual plants living for more than one year. They grow each spring from rootstock rather than seeds alone. In the Midwest, milkweeds were historically widespread on prairies, but habitat destruction has reduced their range and numbers.

p02-low

Though most members of the milkweed genus Asclepias are tropical, there are approximately 110 species in North America known for their milky sap or latex contained in the leaves. A few species are toxic to vertebrate herbivores if ingested due to the cardenolide alkaloids contained in the leaves and stems. When monarch larvae eat milkweed, they also ingest the plants’ toxins, called cardiac glycosides.

Cardiac glycosides are heart poisons that can seriously affect vertebrates. They are related to digitalis, a chemical from the foxglove plant that is used in medicine to treat heart disease but can also be poisonous in large doses.

Monarchs sequester these compounds in their wings and exoskeletons, making the monarch larvae and adults unpalatable to many predators. Birds and some vertebrate predators appear to avoid monarchs once they learn that the larvae and adults taste bad and/or make them vomit. There is considerable variation in the amount of toxins in different species of milkweeds. Some northern species contain almost no toxins while others seem to contain far more toxins.

Where Do Monarchs Occur?

Monarchs are a New World species distributed from at least 50°N in Canada down to Colombia, Venezuela, and coastal Ecuador and Peru. The  intentional and accidental introductions of milkweeds and monarchs, due to the activities of colonists, missionaries, and others—along with the development of steamships in the late 1800s—led to the establishment of monarchs in Australia, New Zealand, Hawaii, and numerous islands in the South Pacific as well as a few islands in the eastern Atlantic (the Canaries and Azores).

How Are Monarchs Protected from Predators?

As the larvae feed on milkweeds, they acquire some of the cardiac glycosides and sequester them into the cuticle of the developing adult late in the pupal stage. Cardiac glycosides are bitter and these toxins can induce vomiting in predators.

Most insect eating (insectivorous) birds only need to eat a monarch once to get the message that they are not good to eat. Most insects that incorporate toxins from noxious plants advertise their distastefulness, by being brightly colored. Contrasting colors such as the black, orange, and white coloration of the monarch adult and the bright black, yellow, and white coloration of the monarch larvae advertise the monarchs’ distastefulness. This form of advertising is known as aposematic (or warning) coloration.

p03-250px

How Long Do Monarchs Live?

Migratory monarchs that survive overwintering to reproduce in the spring are 7-9 months old and may be the longest lived of all butterflies. In contrast, reproductive monarchs breeding during the summer months only live 2-5 weeks. The difference in longevity is ascribed to the higher metabolic rates and levels of activity of reproductive monarchs as well as the greater hazards faced during reproduction. Continuous use of powered flight during reproduction also contributes to wear and tear on the wings and perhaps the flight muscles.

What is the Monarch Mating Behavior?

Monarch butterflies are notably promiscuous with lifetime mating frequencies approaching 8 for each sex. Since it is usually the sperm from the most recent mating that is used to fertilize eggs, females produce offspring that have been fathered by a series of males through their egg laying history. This high degree of multiple mating may be advantageous since it increases the genetic variability of a female’s offspring in a highly variable (heterogeneous) environment.

Do They Have a Random Breeding System?

Because each overwintering colony in Mexico represents monarchs from all regions of the eastern United States and Canada, matings that begin toward the end of winter, combined with further matings as the butterflies move northward, suggest that monarchs have a panmictic (random) breeding system resulting in complete genetic mixing of all portions of the eastern population.

This type of breeding structure has the effect of maintaining a high degree of genetic variability while keeping the population from differentiating into geographically distinct sub-populations.

p04

 

05

Migration

Monarch butterflies migrate from the eastern provinces of Canada and the eastern United States each fall to the Transvolcanic Range of central Mexico where they overwinter as clusters in high elevation oyamel fir forests. This migration involves hundreds of millions of butterflies and is one of nature’s most intriguing and spectacular displays.

Annual Cycle

The monarch’s annual cycle can be broken down into two phases, a migratory phase and a reproductive phase. Because the monarch population utilizes much of the North American continent, these phases overlap on the calendar but not by latitude; at each location only one of these two phases predominates at any given time.

Migratory Phase

At the northern limit of milkweeds (50°N = Winnipeg), the migration starts in mid- August and continues until the last of the monarchs arrive at the overwintering sites near the end of the first week of December.

Reproductive Phase

Reproduction begins shortly after the overwintering monarchs begin moving north at the end of February and continues until November when the last of the last generation monarchs join the migration in Texas or Mexico.

Do All Monarchs Migrate?

No. Some monarchs in Florida and Mexico—areas without a winter and with milkweeds available yearlong—appear to live out their life cycle without migrating.

How Fast Do They Fly?

Monarchs have two flight modes: powered flight and gliding. Powered flight can be broken down into short escape flights of greater than 12 mph, directional flight toward another individual or resource at 10–12 mph or less, and normal flight of less than 10 mph when looking for flowers, or host plants in the case of females. Migrating monarchs employ both powered and gliding flights; minimizing the former and maximizing the latter to save energy and reduce wear and tear on the wings and flight muscles on the 1200-2000+ mile continental traverse to Mexico.

Gliding flight is composed of soaring (i.e., catching thermals in the manner of hawks and vultures to gain altitude) and then gliding S/SW with the aid of the wind. Monarchs glide forward 3-4 feet for every foot they drop in altitude. If utilizing favorable tail and quartering winds, they can maintain altitude by flapping the wings once every 20–30 feet.

Temperature and Winds

Minimum conditions for flight are full sun, light winds, and temperatures greater than 55°F. Headwinds greater than 10 mph and temperatures greater than 88°F impede the migration. Unlike many insects, monarchs can migrate at relatively low temperatures (mid 60°sF) and with completely overcast skies.

How Long Do They Migrate?

The travel times for individual monarchs from regions of origin to the overwintering sites in Mexico are not known but probably range from 2-2.5 months.

How High Do They Fly?

Although the monarch migration is visible at ground level, observations by pilots of gliders and commercial aircraft have reported monarchs from the boundary layer (+/-1200ft) to over 10,000 ft under certain conditions. What appear to be monarchs have also been seen on NEXRAD radar. From these observations, it is clear that much of the monarch population can pass through an area without being seen by those of us at ground level. How often these high elevation flights occur is unknown.

p06-250px

Arrival at the Overwintering Sites

Monarchs begin arriving in the vicinity of the overwintering sites in the last few days of October each year. They are first seen as a high-flying and highly dispersed butterfly-filled sky, a loose milling group of butterflies. They appear to be searching for a location to land and in time the monarchs descend and begin to form loose clusters on oyamel fir trees, usually on ridge tops. Over several weeks these clusters form and reform often in the same general area as in previous years. Presumably, they settle in areas based on microclimate, but some scientists speculate that odors from previous clusters might be involved. One would like to think that monarchs cluster in protected areas but they have been known to select sites with poor protection from the elements and suffer high mortality as a result.

Why Suspend Reproduction?

Migratory monarchs are non-reproductive and generally remain so until mid-February when sexual activity begins as monarchs are about to leave the colonies on their journey northward. This state of suspended reproduction is termed “reproductive diapause.”

During diapause, juvenile hormone (jh) necessary for reproduction is reduced and nearly absent from these butterflies, indicating a major shift in their physiology. In the late summer, only the newly emerged butterflies enter reproductive diapause. Butterflies that are already reproductive at the time the migration starts do not enter diapause and do not join the migration. Although there is some evidence that quality of late summer milkweeds eaten by larvae and the temperatures experienced by pupae and adults have a role in diapause, the factors that contribute to the initiation of this non-reproductive condition are not fully understood.

How Fast is the Migration?

The migration starts slowly in the far North, picks up a bit of speed in middle latitudes of the United States, and then slows again as the butterflies approach the overwintering sites. The migration advances at a rate of 25-30 miles per day on average in the central portion of the continent and is reasonably predictable. Therefore, it is possible to predict the date of arrival of the leading edge of the migration to within three days for any latitude from northern Minnesota to the Mexico border.

How Do Monarchs Navigate?

Migrating monarchs in the interior of the continent fly in directions (bearings, headings) that seem to be geographically appropriate, given the need to reach Mexico. For example, monarchs leaving Minnesota fly almost due south while those in eastern Kansas have bearings averaging 210 degrees and those just north of Atlanta fly toward 260 degrees.

The unanswered navigation question is how the butterfly determines these geographically appropriate directions. It seems likely that monarchs integrate locally acquired signals to set direction. But the signals used and the manner of integration are unknown. Components of the navigational system that are known involve a time-compensated sun compass linked to the circadian clock, and a protein (Cry1) that is sensitive to blue light and ultraviolet wavelengths.

Magnetoreception (the ability to detect a magnetic field) also seems to be involved in navigation.

Distance Per Day

Although the overall migration advances only 25-30 miles per day, some tagged individual monarchs have covered distances of hundreds of miles in a few days. Most of the long-distance records are associated with weather fronts and other favorable wind conditions. One late season monarch averaged 61 miles per day for 2 weeks while flying from Virginia to Texas.

Composition of Overwintering Colonies in Mexico

Two techniques which identify the geographic origin of each monarch, one using stable isotopes and the other using tagged monarchs, show that the monarchs at each colony come from all portions of the eastern breeding range. For example, more than 2800 monarchs tagged at one Kansas site in a 4-hour period were recovered at three overwintering sites in numbers proportional to all tags recovered from these sites. In other words, if 7% of all tags were recovered from one colony, then roughly 7% of all the tags recovered from that Kansas site were found at that colony. This result, as well those from the isotope study, shows that the butterflies are randomized as they arrive at the overwintering sites and that none of the colonies are representative of a particular region of the northern breeding range.

p08-250px

Overwintering

Monarchs overwinter in dense clusters on oyamel firs in a semi-dormant state. They become active when the conditions allow (sunny with temperatures greater than 55°F) and seek water. Water is needed to metabolize the fats stored in the abdomen into sugars that circulate in the blood. These sugars are used to keep the metabolic machinery running (e.g., respiration) during extended cold periods. In effect, the monarchs live off stored fats through the winter months.

Winter Mortality

The biggest threats to overwintering monarchs are winter storms. These storms, though rare, sweep in from the north or the Pacific, bringing moisture-laden warm air. As the moist air rises in the mountains, it cools and rains begin, often wetting the butterflies - a lethal precondition if the skies clear and the temperatures drop into the mid 20sF. A storm that began with 48 hours of rain on the 12th of January in 2002 killed an estimated 80% of the overwintering population and two storms in January-February 2004 killed at least 70% of the monarchs.

Although the cardiac glycosides (i.e., toxins) in monarchs give them protection from most birds, two birds—the black-headed grosbeak and black-backed oriole—are able to feed on monarchs in the oyamel forests. Losses to birds are usually less than 10% of the population. Some monarchs die due to broken wings that prevent them from clustering or reaching water sources, others die due to a lack of fat reserves and still others, often quite full of fat, die for unknown reasons. During mild winters the overall mortality is surprisingly low (less than 20%).

What Are the Oyamel Forests of Mexico?

Oyamel fir trees are the dominant, and often the only, tree species at the sites where monarch colonies form in the mountains of central Mexico. Although monarchs cluster on numerous tree species during their migration southward, the clusters are never as dense as those seen in Mexico. Even though specific colony sites may be chosen for the microclimates they provide, the oyamel, due to its structure, seems to afford a high degree of protection to the monarchs.

The structure of the needles and branches on oyamels seem to allow for the formation of dense clusters and the growth of the forest itself provides a protective canopy that insulates the butterflies from both high and low temperatures.

In addition, incident radiation absorbed by the trees during the day is reradiated at night, further moderating the microclimate in the vicinity of the butterflies. Within dense oyamel forests, temperatures seldom exceed 65°F during the day or drop below 30°F in early morning during the heart of the winter.

These dynamics change, and both higher and lower temperatures are reached as the forests are degraded. Protection of these oyamel forests is therefore essential for the protection of the monarch migration.

Where Are the Breeding Areas?

The monarch breeding areas in eastern North America are recolonized by two generations of monarchs; the overwintering butterflies that move north in March and April and their offspring that move north in May and June.

Monarchs overwintering in Mexico begin moving north at the end of February and continue to arrive in the southern United States beginning the first week of March. These arrivals continue for 5 weeks or more. The females mate and lay eggs as they continue on a path that is generally N/NE, with some females scattering eggs over 1000 miles. Most of these monarchs die before reaching 40°N and before May.

The first of their offspring reach maturity and begin flying N/NE by the end of April. Due to the wide range of latitudes, and the time span over which the eggs were laid by the returning monarchs, these offspring continue to reach maturity and fly northward over a 6-week period, mating and laying eggs as they do so, thus re-establishing the breeding population to the limits of milkweed (50°N).

Movement northward appears to cease a week or more before the summer solstice. The number of monarch generations produced at each latitude in the summer is a function of temperature and the length of the period before the migration begins. For example, in Winnipeg, if the monarchs arrive early in June in a warm year, there can be two generations, one in July and another maturing in August in time to begin the migration in the middle of the month.

These butterflies would be three generations removed from the previous migration. There is only one generation at this latitude during cold summers and the migrants would be two generations removed from the previous migration. This is the extreme case, as most monarchs that join the migration are produced at lower latitudes and represent three or four and sometimes 5 generations.

p09-250px

 

p10-500px

Population Size

Monarch populations are measured as the number of hectares (1 hectare = 2.47 acres) of trees occupied by clustering butterflies in mid-December of each year. The size of the population has varied from 0.67 to 18.2 hectares over the last two decades; averaging close to 9 hectares in the 90s. However, the population is now much lower, averaging 3.34 hectares from 2004-2016. The population is clearly declining and habitat loss, specifically the loss of milkweeds due to changes in agriculture and land management, appears to account for the decline. Year to year variation in weather patterns also contribute to the numbers of monarchs recorded each winter. Estimates of the number of monarchs per hectare have varied from 7 to 75 million. Recent assessments suggest that the number per hectare averages close to 21 million.

Because of the wide variance in these estimates, we prefer to talk about the population in terms of the numbers of hectares occupied. In any case, it seems safe to say that hundreds of millions of monarchs spend the winter in Oyamel forests in central Mexico.

Population Cycle

The population is at its maximum size during the middle of the migration (September) but diminishes due to the hazards of the migration and losses during the winter. The population of adult monarchs is at its lowest in late March and April.

How Long Is a Generation?

Temperature determines generation length (egg to adult), which can be as short as 25 days or as long as 50 days. Generation length is about 40 days in March-April in much of the South while summer generations in the northern breeding areas are usually 30-36 days. The generation length is longer in the south because it is colder in March and April in Texas and Oklahoma than it is in the Upper Midwest during the summer.

How Many Generations per Year?

Most of the monarchs joining the migration each fall are 3-4 generations removed from those that made the journey the previous year. During warm years, a few of the migrants from the mid latitudes of the U.S. are 5th generation butterflies.

Why Are Milkweeds Important?

Female monarchs only lay eggs on milkweeds and a few other plants in the same plant family (Apocyanacae). While there are over 110 species of milkweeds in North America, many are rare or confined to remote habitats outside of the main monarch breeding areas.

Monarchs utilize more than 30 milkweed species as hosts for their larvae. Predominant among these is the common milkweed (Asclepias syriaca), a weedy species that readily becomes established in disturbed sites. Estimates are that 90% of the monarchs that overwinter in Mexico each year have fed on this species as a larva.

p11-250px

Why Are Nectar Plants Important?

Adult butterflies need sources of carbohydrates, amino acids, and sometimes salts. Monarchs visit both butterfly and bee flowers.

Butterfly flowers, those that co-evolved with butterflies, e.g., Pentas, Lantanas, butterfly bushes, have dilute nectars (10-15% dissolved solids, mostly carbohydrates) that contain amino acids. Amino acids are used to sustain bodily functions and aid in egg development.

Bee flowers, such as clovers and other legumes, tend to have richer nectars (20-40% dissolved solids) but little in the way of amino acids. Without sources of carbohydrates and amino acids monarchs could neither migrate nor reproduce. Monarchs, in contrast to many butterflies, seem to have little need for salts.

Why Are Shelter And Water Important?

To successfully overwinter, monarchs need protection from wind, rain and freezing conditions and they need occasional access to water. Without the protection of the canopy provided by intact oyamel fir forests, the monarch migration would collapse. The forest also protects the watershed and provides the sources of water needed by monarchs to metabolize their fats.

p12-200px

How Are Monarchs Tagged?

We use a tagging system in which the tag is placed over the large, mitten shaped cell (discal cell) on the underside of the hindwing. The recovery rate for these circular tags is much higher than for monarchs tagged with rectangular tags that overlap the wing margins. The discal cell position is closer to the center of lift and gravity for the butterfly and a tag placed there will not impede flight. More importantly, this tagging method appears to be less harmful to the butterflies.

Tagging was originally used by Dr. Fred Urquhart and his wife Nora of the University of Toronto. They hoped information from recaptures of the tagged butterflies could lead them to where monarchs overwintered. Later, after the colonies became known to scientists, the goal became to establish where monarchs came from that reached the overwintering sites. The long-range tagging program at Monarch Watch has revealed much more. Specifically, tagging has helped define the migration window (time period for each location) as well as the timing and pace of the migration as the monarchs move S/SW across the continent in the fall. The tagging also shows that the probability of reaching Mexico is related to geographic location and the date of tagging.

What Is Mark-Recapture?

Many animals are studied using tagging for mark-recapture. The concept involves a logic of mathematics. For instance, we may capture and mark ten specimens and release them. Later, if we capture specimens in the same field, and find that one-out-of-20 are marked, we could calculate that the one we recaptured represents ten and therefore there are 200 specimens in the population at large. This estimating method assumes that there is no import or export of individuals from elsewhere, no deaths or new births, etc. In the case of the migratory monarch butterfly, these assumptions are all violated. However, with monarchs, the recapture of marked specimens reveals much about their origins, timing of the migration, direction of flight, speed and survival.

When Were the Monarch Colonies Discovered?

Although the existence of overwintering monarch colonies was known to the residents of the mountains in the Mexican states of Mexico and Michoacan, these colonies were not known to scientists until 1975.

Tagging had led Dr. Urquhart to believe monarchs overwintered in Mexico and through an ad, he recruited Ken Brugger, an engineer working in Mexico City, to search for overwintering monarchs. Ken and his wife Cathy (Catalina) discovered a monarch colony on Cerro Pelon on 2 January 1975, and subsequently another colony on Sierra Chincua on 2 February 1975. Further exploration showed that monarch colonies formed most years at elevations of 10,500-12,000 ft on 12 mountains in an area of about 75 sq km at roughly 19.5°N.

What Are the Threats To the Monarch Migration?

Resources for monarchs are diminishing. Shelter and water needed by the overwintering butterflies are declining in Mexico and illegal logging has already eliminated a number of former colony sites. Continued thinning of the forests and outright deforestation reduces the availability of water for both the butterflies and the people.

In the United States, 6000 acres are converted to development each day, eliminating milkweeds and nectar sources for monarchs. Chemically intensive agriculture that utilizes insecticides and herbicides also eliminates monarchs and their milkweed hosts. The use of “Roundup Ready” soybeans, genetically engineered to resist “Roundup” (glyphosate, the worlds’ most widely used herbicide), has resulted in the loss of at least 100 million acres of monarch habitat in row crops (corn and soybeans) since 1997.

The recent interest in biofuels has brought marginal acres into production often eliminating habitats filled with milkweeds and nectar plants. Land management practices that favor grasses rather than flowering plants, such as frequent mowing, are also a factor.

Climate change also poses threats to monarchs throughout their annual cycle. In spite of their tropical origins, monarchs need moderate temperatures to survive the winter in Mexico, to recolonize the United States and Canada each spring, and to produce large populations for the return migration at the end of the summer. The high temperatures that appear to be in our future seem likely to negatively impact monarchs as well as the milkweeds and nectar plants on which they depend.

What Needs To Be Done?

Sustaining the monarch migration will require the cooperation of all three countries (U.S.A., Canada, Mexico) that are home to monarchs for some portion of the year. Recent studies indicate that, along with protection of the overwintering sites in Mexico, continuation of the monarch migration into the future will require the restoration of 1.8 BILLION milkweed stems on a wide variety of landscapes.

Creating Habitats for Monarchs

The Monarch Waystation program was started by Monarch Watch in 2005. We recognized that habitats for monarchs were declining. Our intent was to both inform the public of the loss of habitat and to engage citizens in monarch conservation by creating “waystations” that contained milkweeds and nectar plants for monarchs. These waystations or “habitat islands” are intended to fill gaps in resources for monarchs which have resulted from the expansion of agriculture and the sprawling development of our cities. For more information on Monarch Waystations visit: monarchwatch.org/waystations/.

p13-250px

 

back-cover-500px


The Kansas School Naturalist Department of Biology 
  College of Liberal Arts & Sciences 
Send questions / comments to
Kansas School Naturalist.
 Emporia State University