Abstracts from The Monarch Butterfly: Biology and Conservation
Abstracts from The Monarch Butterfly: Biology and Conservation, edited by Karen Oberhauser and Michelle Solensky, are not included in the book itself, but give good overviews of the content and conclusions of each chapter. To order the book in its entirety, please visit our online store.
Chapter List
Part 1: Breeding Biology
| Chapter 1 - | Overview of Monarch Breeding Biology, by Karen S. Oberhauser |
|---|---|
| Chapter 2 - | Temporal and Geographical Variation in Monarch Densities: Citizen Scientists Document Monarch Population Patterns by Michelle D. Prysby and Karen S. Oberhauser |
| Chapter 3 - | Effects of Female Age, Female Mass and Nutrients from Males on Monarch Egg Mass by Karen S. Oberhauser |
| Chapter 4 - | Natural Enemies and Survival of Monarch Eggs and Larvae by Michelle D. Prysby |
| Chapter 5 - | Effects of Monarch Larval Host Plant Chemistry and Body Size on Polistes Wasp Predation by Linda S. Rayor |
| Chapter 6 - | The Effect of Fire Ants on Monarchs Breeding in Texas by William H. Calvert |
| Chapter 7 - | Effects of Milkweed Latex and Cardenolides on Foraging and Maintenance Behaviors of First Instar Monarch Butterfly Larvae by Tammi Hoevenaar and Stephen B. Malcolm |
| Chapter 8 - | Behavioral and Genetic Components of Male Mating Success in Monarchs by Michelle J. Solensky and Karen S. Oberhauser |
| Chapter 9 - | Survival of Experimental Cohorts of Monarch Larvae Following Exposure to Transgenic Bt Corn Pollen and Anthers by Laura C. H. Jesse and John J. Obrycki |
Part 2: Migration Biology
| Chapter 10 - | Overview of Monarch Migration by Michelle J. Solensky |
|---|---|
| Chapter 11 - | Monarch Butterflies’ Migratory Behavior Persists Despite Changes in Environmental Conditions by Sandra M. Perez and Orley R. Taylor |
| Chapter 12 - | Stopover Ecology of Monarchs in Coastal Virginia: Using Ornithological Techniques to Study Monarch Migration by Andrew K. Davis and Mark. S. Garland |
| Chapter 13 - | Characteristics of Fall Migratory Monarch Butterflies, Danaus plexippus, in Minnesota and Texas by Jane Borland, Carol Johnson, Thomas Crumpton III, Markisha Thomas, Sonia Altizer, Karen Oberhauser |
| Chapter 14 - | Documenting the Spring Movements of Monarch Butterflies with Journey North, a Citizen Science Program by Elizabeth Howard and Andrew K. Davis |
Part 3: Overwintering Biology
| Chapter 15 - | Overview of Monarch Overwintering Biology by Michelle J. Solensky |
|---|---|
| Chapter 16 - | Two Methods Estimating Overwintering Monarch Population Size in Mexico by William H. Calvert |
| Chapter 17 - | Locations and Area Occupied by Monarch Butterflies Overwintering in Mexico from 1993 – 2002 by Eligio García-Serrano, Jaime Lobato Reyes and Blanca Xiomara Mora Alvarez |
| Chapter 18 - | Can't See the Forest for the Butterflies: The Need for Understanding Forest Dynamics at Monarch Overwintering Sites by Andrés F. Keiman and Miguel Franco |
| Chapter 19 - | Design and Implementation of a New Protected Area for Overwintering Monarch Butterflies in Mexico by Monica Missrie |
| Chapter 20 - | Catastrophic Winter Storm Mortality of Monarch Butterflies in Mexico during January 2002 by Lincoln P. Brower, David R. Kust, Eduardo Rendon-Salinas, Eligio García Serrano, Katherine R. Kust, Jacob Miller, Concha Fernandez del Rey and Karen Pape |
| Chapter 21 - | Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America by Dennis Frey and Andrew Schaffner |
| Chapter 22 - | Analysis of the Pattern of Distribution and Abundance of Monarch Overwintering Sites along the California Coastline by K. L. H. Leong, W. H. Sakai, W. Bremer, D. Feuerstein, and G. Yoshimura |
| Chapter 23 - | Environmental Factors Influencing Postdiapause Reproductive Development in Monarch Butterflies by Liz Goehring and Karen S. Oberhauser |
Part 4: Integrated Biology
| Chapter 24 - | Modeling the Distribution and Abundance of Monarch Butterflies by Karen S. Oberhauser |
|---|---|
| Chapter 25 - | Transmission of the Protozoan Parasite, Ophryocystis elektroscirrha, in Monarch Butterfly Populations: Implications for Prevalence and Population-Level Impacts by Sonia M. Altizer, Karen S. Oberhauser, and Kari A. Geurts |
| Chapter 26 - | Spatial and Temporal Population Dynamics of Monarchs Down-Under: Lessons for North America by Myron P. Zalucki and Wayne A. Rochester |
| Chapter 27 - | Simulating the Development and Migration of the Monarch Butterfly by Johannes J. Feddema, Jason Shields, Orley R. Taylor and David Bennett |
Chapter Abstracts
Chapter 1: Overview of Monarch Breeding Biology
Many people are attracted to monarch biology by their spectacular migration and the unequaled phenomenon of millions of butterflies blanketing a few hectares of land in central Mexico. However, every adult that carries out this migration and joins millions of conspecifics in Mexico or thousands in California began its life as an egg on a milkweed plant and faced a myriad of environmental challenges as it developed. In addition, most generations of monarchs do not undergo the long-distance migrations to and from the overwintering sites. Pre-adult stages and the adult behaviors that produce them provide the subjects of this section: mating, egg laying, and the many challenges faced by eggs and larvae as they develop into adults.
Chapter 2: Temporal and Geographical Variation in Monarch Densities: Citizen Scientists Document Monarch Population Patterns
In the Monarch Larval Monitoring Project (MLMP), volunteers from the US and Canada monitor monarch egg and larval densities weekly throughout the breeding season. They also measure milkweed density and condition, record presence of other invertebrates on the plants, and collect and rear larvae to estimate parasitism rates. From 1997 to 2002, volunteers and participating scientists monitored 264 sites in 30 states and provinces. Sites include natural areas, gardens, roadsides, and agricultural fields. Preliminary analyses of what will eventually be a much longer-term data set show significant year-year variation in egg densities, with 1998 being a particularly low year; regional differences in monarch densities, with lower densities in the Northeast than the Upper Midwest; and significant fall reproduction in the southern US. These data can be used to validate monarch population models and to complement other long-term monarch population records, providing a more complete understanding of fluctuations in monarch populations. In addition to its scientific outcomes, this ‘citizen science’ project raises public awareness of monarch ecology and conservation and provides an important educational opportunity for students, teachers and the general public.
Chapter 3: Effects of Female Age, Female Mass and Nutrients from Males on Monarch Egg Mass
As part of a study of fecundity and lifespan in monarch butterflies (Oberhauser 1997), I measured the mass of eggs laid by females throughout their lives. Females in this study received varying amounts of spermatophore material from males. On average, individual eggs weighed 0.46 mg. Individual egg mass decreased over the female lifespan, and was positively correlated with female size. Females that received more nutrients from males (via spermatophores) laid more eggs, but not larger eggs. These results suggest that females utilize nutrients from different sources differently in egg production; nutrients acquired during the females’ larval development affect egg size, while nutrients acquired from male spermatophores affect egg number but not egg size.
Chapter 4: Natural Enemies and Survival of Monarch Eggs and Larvae
I investigated the effects of natural enemies on the survival of monarch eggs and larvae in three field studies. In an exclosure experiment, monarch survival was significantly higher with terrestrial predators excluded. Excluding both aerial and terrestrial predators raised survival more, but this pattern may reflect a cage effect since the exclusion of aerial predators also led to the monarchs being better protected during storms that occurred during the experiment. In the second study, I investigated the effects of Formica montana, an aphid-tending ant species abundant on milkweed, on monarch survival. I attached monarch eggs to ramets with ants and aphids, aphids only or neither ants nor aphids, and tracked survival over 6 days. Survival was seven times higher for monarchs on ramets without ants than for monarchs on ramets with ants, but it was not affected by aphid presence. Finally, I measured parasitism rates in natural monarch populations. Tachinid flies represented an important source of mortality; of the fourth and fifth instars collected, 15% were parasitized by tachinids in 1999 and 23% in 2000. There was an association between parasitism and the habitat from which the larvae were collected, with higher parasitism rates in larvae from cornfields than from non-agricultural areas. These studies demonstrate that natural enemies significantly affect monarch survival. Further research is needed to determine whether natural enemies actually regulate monarch populations and to investigate interactions between natural enemies and other ecological factors.
Chapter 5: Effects of Monarch Larval Host Plant Chemistry and Body Size on Polistes Wasp Predation
Predatory wasps exert a major selective force on lepidopteran larvae. Paper wasps are efficient, visual, generalist predators that continuously search for edible larvae, cover a vast area in their search for prey, and overlap with monarchs in all regions where the butterflies breed. In choice tests conducted in a greenhouse, I examined the predatory behavior of the abundant social paper wasp, Polistes dominulus on monarch larvae raised on different milkweed host plants ( Asclepias curassavica, A. syriaca, A. incarnata, A. tuberosa, A. verticillata). My results demonstrate that (1) predatory wasps readily capture and consume large numbers of monarch larvae raised on any of the host plants; (2) wasps tend to prefer monarch larvae raised on host plants that contained lower levels of cardenolides, but cardenolide level is not the only factor affecting wasp predation; (3) wasps prefer more palatable larvae ( Pieris sp. or Trichoplusia ni) over monarchs, but monarchs reared on A. curassavica are preferred over buckeye ( Junonia coenia) larvae, which contain iridoid glycosides; (4) late third through early fifth instar monarch larvae are at highest risk from wasp predation. In summary, Polistes wasps are likely to be a significant source of mortality to monarch larvae throughout the monarch butterflies’ entire breeding range.
Chapter 6: The Effect of Fire Ants on Monarchs Breeding in Texas
Four fire ant exclosures were built at two sites near Austin, Texas, one on clay-based soils of the Blackland Prairie and the other on sandy soils of the Post Oak Savannah in 1997. Three exclosures were built at the Post Oak Savannah site in 1998. I applied various methods to exclude fire ants and other terrestrial predators from these exclosures. Monarch immatures increased in number during late March and early April as ovipositing monarch adults arrived from Mexico, and then declined to zero or near zero by late May. The number of monarchs surviving to the fifth instar was higher inside the exclosures. Fire ants were seldom completely eliminated from the exclosures, but they were held to a low percentage of those outside, especially during the early stages of the experiments. The method for excluding ants improved in 1998, and the percentage of ants inside was lower during that year. The exclusion of the ants during the crucial first, second and third instar stages is likely the cause of the higher production of monarchs inside the exclosures. These findings suggest that fire ants affect the reproductive success of monarchs in North America, but since the fire ants likely out-competed and preyed upon other monarch predators, pre-fire ant predation rates may have been similarly high.
Chapter 7: Effects of Milkweed Latex and Cardenolides on Foraging and Maintenance Behaviors of First Instar Monarch Butterfly Larvae
Plant defense theory has been built on observations that specialist insect herbivores circumvent the defenses of their host plants either physiologically or behaviorally. Monarch larvae tolerate the cardenolide defenses characteristic of their milkweed host plants, and also deploy feeding behaviors, such as trenching, to reduce or stop the flow of latex produced by milkweeds as both a mechanical and a biochemical defense against herbivory. However, recent evidence shows that both cardenolides and latex negatively affect monarch larvae. We tested the effects of these plant defenses on first instar feeding and maintenance behaviors. We compared behaviors of larvae feeding on leaves of either Asclepias curassavica, with high constitutive cardenolide content and latex volumes, or A. incarnata, with low cardenolide content and latex volume. We also tested the effects of latex on feeding behaviors by feeding larvae leaves that were either intact or had the midrib severed to stop latex flow. Our results showed that neither plant species nor leaf treatment had a significant impact on larval growth or feeding behaviors. We conclude that first instar larvae of monarch butterflies are well-adapted specialists that are able to handle the variable cardenolide and latex defenses of A. incarnata and A. curassavica without any measurable negative impact on their performance.
Chapter 8: Behavioral and Genetic Components of Male Mating Success in Monarchs
Male monarchs vary greatly in their ability to obtain mates. We investigated genetic and behavioral components of male mating success, which we define as the number of matings achieved by a male during a 10-day period of access to reproductive females. We found significant additive genetic variance associated with male mating success. Successful males not only mated more often (by definition), but were more likely to copulate with a female during any given mating attempt. However, successful males were not better able to discriminate between males and females, nor did they attempt to mate more frequently, leaving the proximate mechanism for variation in male mating success unidentified. There is a heritable component to male mating success, but the specific morphological, behavioral, or physiological trait or traits underlying this variation have yet to be revealed.
Chapter 9: Survival of Experimental Cohorts of Monarch Larvae Following Exposure to Transgenic Bt Corn Pollen and Anthers
When mortality from predation is minimized via mechanical exclusion, we demonstrate a consistent trend ( p = 0.1) of increased mortality when monarch larvae are exposed to Bacillus thuringiensis (Bt) corn pollen and anthers naturally deposited on milkweed plants within a field. The presence of transgenic Bt corn anthers on Asclepias syriaca, the common milkweed, within cornfields has previously been noted, but not quantified. In this field study, we recorded anther densities ranging from 0 to 104 anthers / plant on 86-100% of milkweed plants in transgenic Bt cornfields. The frequent occurrence of anthers on milkweeds is significant because anther tissue contains higher concentrations of Bt toxin compared to pollen from Bt corn. Results from these field studies indicate that multiple year field studies are needed to quantify the potential effects of wide scale planting of Bt corn on monarch larvae. Also, prior to concluding that Bt corn poses no risk to monarch larvae (Sears et al. 2001), we argue that it is necessary to examine within field mortality resulting from deposition of corn tissues that include anthers and pollen.
Chapter 10: Migration
Unlike most temperate insects, monarch butterflies cannot survive a long cold winter. Every fall, North American monarchs fly south to spend the winter at roosting sites. In the spring, these overwintering monarchs fly north toward their breeding range. The monarch is the only butterfly to make such a long, two-way migration, flying up to 4830 kilometers (3000 miles) in the fall to reach their winter destination (Urquhart and Urquhart 1978). Monarchs that live east of the Rocky Mountains generally fly to overwintering sites in the mountains of central Mexico, while monarchs west of the Rocky Mountains typically overwinter along the California coast (Figure 1a). It has been proposed that this western North American population is not truly migratory but rather undergoes an annual range expansion and contraction of California monarchs (Wenner and Harris 1993). That is, these monarchs may be year-round residents of California whose offspring are able to spread to surrounding states during the mild summer weather but are forced to return to California or perish when the inhospitable northern winters return. This issue is still being debated and offers great potential (and substantial challenges) for study by West Coast residents.
In Australia, monarchs sometimes exhibit seasonal movement, but they move from inland to coastal areas in a north to northeasterly direction during the fall and winter (James 1993). Because the most spectacular monarch migrations (in terms of distance and numbers of migrants) occur in the eastern North American population, much of the research on monarch migration has focused on this population. Amazingly, these butterflies fly from their summer breeding range, which spans more than 100 million ha, to winter roosts that cover less than 20 ha, often to the exact same trees, year after year. Since the discovery of these winter roosts in Mexico by the scientific community in 1975 (Urquhart 1976), researchers have struggled to understand the cues that cause monarchs to begin their migration, the mechanisms they use to orient and find the overwintering sites and the patterns of fall and spring flight.
Chapter 11: Monarch Butterflies' Migratory Behavior Persists Despite Changes in Environmental Conditions
Throughout the fall, monarchs from the eastern US and Canada migrate to Mexico. It is unclear whether the signals initiating migration, most likely a suite of environmental factors, are necessary for maintaining migratory behavior throughout the fall season or whether migratory condition is a state that, once triggered, remains “turned on." We investigated this question in two ways: by artificially changing the environmental conditions of migratory butterflies and by changing the natural environmental conditions (i.e., displacing the butterflies south). Migratory butterflies manipulated in both ways continued to show significant directionality, in contrast to non-migratory butterflies that showed no directionality. This is the first study to demonstrate that monarch butterfly migratory behavior, once triggered, is resistant to changes in environmental conditions.
Chapter 12: Stopover Ecology of Monarchs in Coastal Virginia: Using Ornithological Techniques to Study Monarch Migration
Monarch butterflies, like migratory birds, make frequent stops during their migration to their overwintering destinations, but little is known about where and when monarchs choose to stop. We used methods from ornithological studies to examine several factors influencing monarch stopover decisions at a site in coastal Virginia. To determine when they stop, we compared the daily numbers migrating to the number of grounded monarchs at a known accumulation site. To determine how long they stop, we used mark, release, recapture methods; out of 688 tagged monarchs, 27 were recaptured. The median stopover duration of these individuals was 2 days, with no significant difference in stopover duration between males and females. We then examined the influence of wind condition on the numbers of migrating and roosting monarchs, probability of stopover, and the wing conditions of grounded monarchs. Wind conditions had a small effect on the numbers of roosting but not migrating monarchs. Although not significant, higher proportions of monarchs were later recaptured when they were initially tagged during unfavorable than favorable wind directions. Finally, the highest proportions of monarchs with damaged wings were captured during favorable wind directions, but this difference was not significant. We discuss the implications of these results and issues future research projects should address.
Chapter 13: Characteristics of Fall Migratory Monarch Butterflies, Danaus plexippus, in Minnesota and Texas
We compared wing condition, size (wet mass and wing length) and reproductive status of migrating monarch butterflies from Texas, Wisconsin and Minnesota during the 1998-2001 fall migrations. There was a tendency for monarchs captured in the north to be lighter than those captured in Texas, suggesting that nectaring along the migratory path makes monarchs heavier. Winglength decreased with date of capture. We found greater wing scale loss and more broken wings in Texas migrants than in Minnesota in all years, but individuals captured early in Texas tended to show more wing damage than those captured late. Non-destructive palpation indicated that only 3% of Minnesota females, vs. 18% of Texas females, had mated. More females collected early during the migration through Texas had mated. Mated females had significantly more wing damage than those without spermatophores. Males were consistently larger and displayed more wing tatter than females.
Chapter 14: Documenting the Spring Movements of Monarch Butterflies with Journey North, a Citizen Science Program
The fact that monarch butterflies migrate each spring from their overwintering sites in Mexico to their summer breeding range across eastern North America is well established. However many questions remain about annual variation in the timing of the spring migration, rate of travel, and patterns of movement. Here we describe a monarch ‘citizen science’ program designed to provide answers to such questions. Journey North is a continent-wide, volunteer-based, Internet program where individuals report their first sighting of an adult migrant monarch each spring. We provide an overview of the results of the program for the first 6 years and present an example of how the data can be used to document long-term variation in certain spring migration parameters.
Our most striking finding was the regularity of the migration pattern from year to year. However, we did find significant differences among years in the average arrival date of four different latitude categories. Further, the average arrival dates at three latitudes were more than 5 days earlier than previously published estimates of first oviposition dates based on research using larval census methods. Finally, we found significant annual variation in the duration of migration, as determined by the time needed to reach the upper range of the monarchs breeding distribution. Possible reasons for these results are discussed.
Chapter 15: Overwintering Biology
Although monarchs inhabit many areas of the world, the most spectacular overwintering colonies occur in North America. There are two major regions in North America in which monarchs regularly congregate during the winter: central Mexico and coastal California (Brower 1995). Monarchs also reside in southern Florida throughout the year, but the population receives an influx of migratory individuals from the eastern migratory each fall (Knight 1997; Altizer 2001). The degree to which monarchs from this part of the US move back into the larger population is not understood.
Monarchs that spend the summer breeding season west of the Rocky Mountains overwinter along the coast of southern California. Here, they roost in wooded areas most often dominated by eucalyptus trees, Monterey pines, and Monterey cypresses located in sheltered bays or farther inland that provide moderated microclimate extremes and protection from strong winds. More than 300 different aggregation sites have been reported (Frey and Schaffner, this volume; Leong et al., this volume).
North American monarchs that spend the summer breeding season east of the Rocky Mountains overwinter in oyamel fir forests in the Transvolcanic mountains of central Mexico. The location of these overwintering sites was unknown to the scientific community until 1975 when associates of Dr. Fred Urquhart located colonies on Cerro Pelon and Sierra Chincua (Urquhart 1976; Brower 1995). While scientists have learned much about the phenomenon of monarch overwintering in the past few decades, several basic questions remain. Measuring the density of an organism that congregates by the millions, and perhaps billions, presents a formidable challenge. In addition to estimating population size, scientists also seek to understand the characteristics of the overwintering sites that are most important to monarch survival and factors that influence the patterns of colony formation and dispersal (Figure 1).
Chapter 16: Two Methods Estimating Overwintering Monarch Population Size in Mexico
I used two methods to estimate monarch abundance at two Mexican overwintering colonies. A mark, release and recapture study using Petersen and Jolly-Seber indices yielded an estimate of about 10.4 million monarchs per hectare early in the season, and 33.8 million late in the season. I used forest parameters including tree diameter and branch mass; numbers of monarchs on tree trunks and branches of different sizes; proportions of trunks, crowns and trees occupied by monarchs; and the number of trees in the colony. This method yielded an estimate of 10.3 million monarchs per hectare. While both methods introduce some error, it is important to have a way of estimating the size of overwintering monarch populations. Applications of these data are discussed.
Chapter 17: Locations and Area Occupied by Monarch Butterflies Overwintering in Mexico from 1993 – 2002.
Since the 1993 – 1994 overwintering season, we have monitored 22 monarch overwintering colonies in the Monarch Butterfly Reserve, Mexico. The main objective of this study was to assess the environmental status of the overwintering sites, with the goal of generating conservation strategies. Location, size and mortality of each colony within and outside of the reserve’s protected area were estimated according to the field methods described by García-Serrano (1999). The total area covered by all colonies ranged from 2.3 ha (2000-2001) to 17.6 ha (1996-1997), with population estimates ranging from 23 million to 176 million butterflies, respectively. Mortality rates ranged from 0.17% (1996-1997) to 27.7% (1997-1998) and appeared to be negatively correlated with colony surface area.
Chapter 18: Can't See the Forest for the Butterflies: The Need for Understanding Forest Dynamics at Monarch Overwintering Sites
The oyamel fir (Abies religiosa) forests along the border of the states of Mexico and Michoacan states in Mexico are essential for the successful establishment of monarch overwintering colonies. These forests have traditionally been a source of energy, building materials, food and income to the indigenous communities, and the pressure to exploit them on a larger scale increases with population growth and the lack of employment opportunities in the area. It is therefore necessary to design strategies of conservation and forest utilization that are compatible with the monarch migratory phenomenon.
Monarch butterflies form colonies in mature forest patches with relatively closed canopies, but the amount of deviation from a closed-canopy, self-thinning forest that monarch colonies can tolerate to overwinter successfully is unknown. This knowledge is necessary before management practices are implemented that may have potentially tragic consequences for monarch overwintering colonies. Here, we reconstruct the dynamics of occupation (biomass accumulation) and self-thinning (density-dependent mortality) in the absence of human-induced disturbance. This will help us to understand impacts of human-induced thinning on overwintering monarchs.
Chapter 19: Design and Implementation of a New Protected Area for Overwintering Monarch Butterflies in Mexico
One goal of nature reserves is to target unique communities, ecosystems, or species for protection. However, the size, shape and precise location of protected areas often represent a compromise between ecological, political and economic interests. The history and recent revision of protected areas for monarch butterflies overwintering in Mexico represents an important example of how recently developed ecological methods, especially geographic information systems (GIS) systems and iterative algorithms, can be used to define the optimal borders of protected areas based on biological knowledge of the target organism. Here, I provide a general overview of the 4-year process that led to the conversion of the Monarch Butterfly Special Biosphere Reserve created in 1986 into the new Monarch Butterfly Biosphere Reserve. I describe the technical studies performed to design new boundaries for the protected area as well as the negotiations that led to the creation of the new reserve, decreed on 10 November 2001 by Mexican President Ernesto Zedillo. The creation of the reserve included important financial contributions that resulted in economic incentives for local communities to increase awareness and engage in sustainable forest use.
Chapter 20: Catastrophic Winter Storm Mortality of Monarch Butterflies in Mexico during January 2002
A wet winter storm impacted the entire monarch butterfly overwintering region in central Mexico on 12–14 January 2002 and caused the severest winter kill since the discovery of the colonies by the scientific community in the late 1970’s. We documented mortality in four colonies and analyzed 58 samples of downed butterflies in two colonies. We estimated 70 to 80% mortality in two major overwintering colonies on the Sierra Campanario and Sierra Chincua massifs, totaling nearly a quarter of a billion dead monarchs. We attribute initial underestimates of the mortality and confusion in early press reports to failure to realize that previously frozen monarchs continue to fall from their clusters for many days after a freezing event. We found an average of nearly 5000 dead monarchs per m 2, suggesting that the historically accepted density estimate of 1000 live monarchs per m 2 in the overwintering colonies is incorrect. We revise this estimate to 6500 live monarchs per m 2 (65 million per ha). We also documented that the survivors in four colonies moved from their original colony positions to nearby new locations after the storm. This behavior explains several erroneous interpretations of this and previous mass winter mortality events. In moving, three of the four colonies reformed in patches of degraded forest. The call for more adequate protection of the forests is louder than ever.
Chapter 21: Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America
Monarch butterflies overwinter across an extensive landscape in western North America. This overwintering range is predominately located within a few kilometers of the Pacific Ocean and extends approximately 1000 km from Mendocino County, California to Ensenada, Baja California, Mexico. Despite a moderating maritime influence, monarchs experience a range of macroclimate conditions and occur in a variety of vegetative community types. We use information from historical databases and technical reports to describe temporal and spatial patterns of abundance at these habitats. We analyzed abundance patterns on three temporal scales: within a single season, during a 4-year span (1997–2000), and over approximately 20 years. We evaluated the relation between overwintering site attributes and historical abundance patterns. Ownership (private sector or public lands) and orientation of shoreline were not associated with large-scale abundance patterns. Location along the coastline and tree species used for roosting were associated with abundance patterns. Relative abundance of sites were correlated on a year-year basis, supporting the importance of system-wide influences on late summer recruitment.
Chapter 22: Analysis of the Pattern of Distribution and Abundance of Monarch Overwintering Sites along the California Coastline
Based on information from the California Department of Fish and Game Natural Data Base, most overwintering monarchs and winter sites (transitional and climax) are along the central coast of California. We investigated environmental factors that may explain the distribution and abundance of these winter aggregations and described associated geographic features using Geographic Information System (GIS) technology.
We found that the central coast had high ambient moisture, copious amounts of morning dew and moderate winter temperatures. Factors that seemed to limit aggregations in the north were high rainfall and low temperatures. Factors limiting the aggregations in the south were low relative humidity, low moisture and high temperatures.
GIS analyses revealed three geographic features associated with monarch winter aggregations: (1) they were within an average of 2.4 km of the coastline and under the influence of maritime climate; (2) they frequently occurred on slopes with best exposure to the sunlight (south to west orientation); and (3) large winter sites (≥ 25,000 butterflies) were associated with lower slopes of valleys, bays or coastal inlets that suggest shelter from strong northwesterly winds and optimal exposure to sunlight.
Chapter 23: Environmental Factors Influencing Postdiapause Reproductive Development in Monarch Butterflies
We studied diapause development in female monarchs collected near the end of the overwintering period in Mexico. At this time, most females had undeveloped ovaries, but many had mated. Females collected in copula were more likely to have oocytes present, and to have mated previously. When exposed to warm temperatures, females developed mature oocytes rapidly, with most becoming fully reproductive in 7 to 12 days, suggesting that they were in a state of postdiapause quiescence at the time of collection. Exposure to an increasing daylength treatment did not affect the rate of reproductive development, but access to host plants and mating speeded development. In addition, females exposed to host plants were more likely to mate. This response to environmental cues may serve to synchronize oogenesis with oviposition opportunities.
Chapter 24: Modeling the Distribution and Abundance of Monarch Butterflies
The Complexity of Monarchs’ Annual Cycle
Monarch butterflies utilize diverse habitats spanning most of temperate North America during their annual migratory cycle, and populations fluctuate dramatically within and between years. In the course of an annual cycle ¾which includes breeding in the US and southern Canada, migrating over a broad latitudinal range, and overwintering in central Mexico and coastal California ¾monarchs exhibit micro-scale movement as foraging larvae; meso-scale movement as foraging, breeding and overwintering adults; and macro-scale movement as adult migrants (Ackery and Vane-Wright 1984; Vane-Wright 1993). During each of these stages, monarch distribution and abundance are affected both by current environmental conditions and events that occurred during preceding stages. For example, monarch numbers in June in Minnesota may be affected by weather conditions in Texas during April and May, since these monarchs probably spent their egg, larva, pupa and early adult stages far south of Minnesota. A late freeze in Minnesota (not unheard of in June!) will also affect monarch populations, as will monarch or milkweed diseases, other milkweed-consuming herbivores, and monarch predators. In addition to these natural factors, monarch populations are vulnerable to land use change (Malcolm and Zalucki 1993; Hoth et al. 1999) and, due to their susceptibility to temperature extremes, human-induced climatic change (Rawlins and Lederhouse 1981; Zalucki 1982; Malcolm et al. 1987; Zalucki and Rochester 1999; York and Oberhauser 2002).
The above factors require that the study of monarch populations involves a large range of temporal and spatial scales, and a variety of research approaches including laboratory, field, and mathematical studies. The first three sections of this book presented results of empirical studies conducted in the laboratory and the field. These studies were designed to document and, in some cases explain, how monarchs interact with their environment throughout their annual cycle of breeding, migrating and overwintering. In this section, we present papers that have combined empirical data with mathematical models to further our understanding of monarch population dynamics.
Chapter 25: Transmission of the Protozoan Parasite, Ophryocystis elektroscirrha, in Monarch Butterfly Populations: Implications for Prevalence and Population-Level Impacts
Monarch butterflies are susceptible to infection by the neogregarine protozoan parasite, Ophryocystis elektroscirrha. This parasite is transmitted maternally (from infected females to their offspring), and may also be spread by paternal and horizontal (between unrelated individuals) transmission. The relative importance of these transmission modes may vary among populations as a result of differences in host migratory and breeding behavior. We used laboratory studies and mathematical models to evaluate the contribution of different transmission modes to parasite spread and impacts on monarch abundance. In transmission studies with captive monarchs, rates of maternal and paternal transmission were high (75-96%), and parasite-induced reductions in host fitness were more severe for maternally than paternally infected offspring. Paternal transmission was more variable than maternal transmission, and infection rates depended on male parasite loads and the time between female mating and oviposition. Field comparisons showed that horizontal transmission from spores accumulating on milkweed plants varied among populations. A high proportion (98%) of monarchs reared on milkweed collected in southern Florida emerged heavily infected, whereas only 3.6% of monarchs raised on common milkweed from Minnesota and Wisconsin became infected.
We explored the consequences of varying paternal and maternal transmission for disease prevalence and host population impacts using a deterministic model of disease spread. Simulation results demonstrated that across a wide range of host demographic parameters, small changes in paternal transmission generated large differences in parasite prevalence (assuming high rates of maternal transmission). In addition, multiple transmission modes allowed parasites to persist at extremely high prevalence (50-100%) and lead to reductions in host population size. These results suggest that variation in parasite transmission may generate large differences in parasite prevalence observed among wild monarch populations, and that under certain conditions O. elektroscirrha may have substantial negative effects on monarch abundance.
Chapter 26: Spatial and Temporal Population Dynamics of Monarchs Down-Under: Lessons for North America
Monarch butterflies spread across the Pacific in the mid-1800s, reaching Australia around 1870. The species is common in disturbed areas along the eastern seaboard and around the southwestern region of Western Australia. Monarchs have a distinctive seasonal pattern of distribution and abundance determined by Australia’s more tropical climes.
The developmental zero and thermal requirements of monarchs have been used to interpret and model monarch phenology in Australia and North America. We used a simulation model to predict the spatial phenology of monarchs in North America and to quantify the effects of variation in climate and the timing of spring migration on that phenology. Life table work indicates the importance of various factors, including host plant species, defense characteristics and patch size; weather; predators and parasitoids. It is not clear how much of the variation in abundance among years may relate to large-scale changes in these factors.
We have modeled the effect of climate on monarch seasonal geographic range and abundance in Australia, and this bioclimatic modeling successfully predicted the seasonal distribution in eastern North America. Here, this approach is extended to indicate the effect of climate variation and change on seasonal and long-term population trends in monarch abundance. This will be crucial if we are to begin to separate out the effects of large-scale habitat changes in North American breeding areas from changes at overwintering sites on the status of monarch abundance.
Chapter 27: Simulating the Development and Migration of the Monarch Butterfly
We simulate the spatial and temporal dynamics of the migration of the eastern North American population of the monarch butterfly. We simulate the initiation of the migration based on sun angle, the development of subsequent generations of butterflies based on a heating degree-day model, the movement of each generation based on longevity and the return migration based on sun angle. The model calculates solar angle from latitude, and uses daily temperature input from over 3000 cooperative weather stations east of the Rocky Mountains in the US to calculate heating degree-days.
The model was run for 1994 through 1999 and validated using first sightings data posted to the Journey North website (Journey North 2001). Generally, the model effectively predicts arrivals in the region where first sightings involve return migrant individuals from Mexico. Arrival times for subsequent generations are not as accurate. Overall the model predicts arrival dates to within about 10 days of observed arrival days over the entire summer breeding range. Finally, the model was used to estimate the total number of generations produced each year. There was not a significant correlation between model results and overwintering population size. We discuss the model’s shortcomings and features that could improve it.