Recent Literature

Want to learn more about monarchs? Check back here for reviews and comments on the latest in butterfly research.

Research Review: Citizen science monitoring demonstrates dramatic declines of monarch butterflies in western North America

by Emily Galli
Cal Poly Biological Sciences

Original Article Information: Citizen science monitoring demonstrates dramatic declines of monarch butterflies in western North America. Cheryl B. Schultza, Leone M. Brownb, Emma Peltonc, and Elizabeth E. Croned. Biological Conservation (2017), 1-4.

Population Viability Analyses based on counts of monarch butterflies allows researchers to assess patterns of population change through time and evaluate future persistence. In this study, researchers combined PVA models and data collected from citizen scientists to evaluate the viability of the western population of monarch butterflies over 36 years. An important feature of the analysis was combining irregular sampling from multiple sites to obtain a single estimate of total abundance using state-space models. The average population growth rate was negative, and the average abundance in the 2000's was less than 5% of average abundance in the 1980's. Additionally, the data illustrated that the current quasi-extinction risk of western monarch butterflies is 72% within 20 years. There are two possible explanations for this decline: negative population growth rate, or steady state populations with high variance. Despite wide confidence intervals in some parameter estimates, western monarch monitoring data provide unambiguous evidence for dramatic population declines. In order to acquire viable populations, wildlife managers could target historic abundance and high enough growth rates to avoid near-term extinction. 

Research Review: Migratory monarchs wintering in California experience low infection risk compared to monarchs breeding year-round on non-native milkweed 

by Emily Galli
Cal Poly Biological Sciences

Original Article Information: Migratory monarchs wintering in California experience low infection risk compared to monarchs breeding year-round on non-native milkweed. Dara A. Satterfield, Francis X. Villablanca, John C. Maerz and Sonia Altizer. Integrative and Comparative Biology (2016), 1-10.

This study looks into migratory animals and their influence on ecological processes via the monarch butterfly. Researchers explored seasonal migration's effect on the spread of infectious diseases. Although birds and other migratory species can move pathogens over multiple miles, it appears that in many systems, long- distance migration actually has the opposite effect of reducing opportunities for pathogen transmission. Since the the key factors of numerous seasonal migrations are changing due to human activities, an important task is to examine the consequences of these changes for host–parasite interactions, in part to predict future risks for wildlife and human health. 

Through this study, researchers found that long-distance migration can actually lower the infection risk for animal populations by removing infected individuals during strenuous journeys, thereby spatially separating susceptible age classes, or allowing migrants to periodically escape from contaminated habitats. Seasonal migrations have been changing due to human activities, including climate change and habitat alteration, and for some migratory populations, sedentary behaviors are becoming more common as migrants abandon or shorten their journeys in response to supplemental feeding or warming temperatures. Delving into the effects of decreased movement for host–parasite interactions is needed to predict future responses of animal pathogens to anthropogenic change.

Monarch butterflies (Danaus plexippus) and their specialist protozoan parasite O.e. provide a representative system for examining how long-distance migration affects infectious disease processes in a rapidly changing world. Annual monarch migration from eastern North America to Mexico is known to reduce protozoan infection prevalence, and more recent work suggests that monarchs that forego migration to breed year-round on non-native milkweeds in the southeastern and south central Unites States face extremely high risk of infection. In this specific study, researchers examined the prevalence of OE infection from 2013 to 2016 in western North America, and compared monarchs exhibiting migratory behavior (overwintering annually along the California coast) with those that exhibit year- round breeding. Data demonstrated that infection frequency was over nine times higher for monarchs sampled in gardens with year-round milkweed as compared to migratory monarchs sampled at overwintering sites. This illustrates the importance of animal migrations for lowering infection risk. 

Research Review: Environmental Persistence Influences Infection Dynamics for a Butterfly Pathogen 

by Emily Galli
Cal Poly Biological Sciences

Original Article Information: Environmental Persistence Influences Infection Dynamics for a Butterfly Pathogen. Dara A. Satterfield, Sonia Altizer, Mary-Kate Williams, Richard J. Hall . PLoS ONE (2017), 12(1) 1-16.

This research paper explores the infection dynamics of a monarch butterfly pathogen. A myriad of infectious pathogens, including those infecting insects, are transmitted in dormant stages shed into the environment where they must persist until encountering a susceptible host. Understanding how abiotic factors influence environmental persistence, and how these conditions influence pathogen spread are key for predicting patterns of infection risk. In this study, researchers looked at the effects of environmental transmission for infection dynamics of they deadly protozoan parasite O.e. (Ophryocystis elektroscirrha) that infects monarch butterflies (Danaus plexippus).

In the beginning of the study, researchers first conducted an experiment to observe the persistence of O.e. spores exposed to natural conditions. Results demonstrated that pathogen doses maintained high infectivity even after many days in the environment, although they did yield infections with lower parasite loads after environmental exposure. Because pathogen longevity exceeded the time span of the experiment, researchers developed a model to further explore environmental persistence for this host-pathogen system. Model analysis data demonstrated that longer spore persistence led to higher infection prevalence and slightly smaller monarch population sizes. The model indicated that typical parasite doses shed onto milkweed plants must remain viable for a minimum of 3 weeks for prevalence to increase during the summer-breeding season, and for 11 weeks or longer to match levels of infection commonly reported from the wild.

Data showed that transmission stages of O.e. are long-lived, and also indicated that this is a necessary condition for the to persist in local monarch populations. This is very useful, as it provides a modeling framework for future work examining the dynamics of an ecologically important pathogen in an iconic insect. 

Research Review: Unraveling the annual cycle in a migratory animal: Breeding-season habitat loss drives population declines of monarch butterflies. 

by Emily Galli
Cal Poly Biological Sciences

Original Article Information: Unraveling the annual cycle in a migratory animal: Breeding-season habitat loss drives population declines of monarch butterflies. Tyler Flockhart , Jean-Baptiste Pichancourt, D. Ryan Norris and Tara G. Martin. Journal of Animal Ecology (2014), 10(1111) 1365-2656.

This study aims to demonstrate that even migratory animals can have threats that occur at multiple periods of the annual cycle, separated by thousands of kilometers and span international borders. In this case, monarch butterfly populations of eastern North America have declined over the last 21 years. There are three hypotheses that have been posed to explain the decline: habitat loss on the overwintering grounds in Mexico, habitat loss on the breeding grounds in the United States and Canada, and extreme weather events.The objectives of this study were to assess population viability, determine which life stage, season and geographical region are contributing the most to population dynamics and test the three hypotheses that explain the observed population decline.

The researchers developed a model that integrated patterns of migratory connectivity and demographic rates across the yearly cycle. They then used disturbance analysis to determine the sensitivity of population abundance to changes in vital rate among life stages, seasons and geographical areas. In addition, this study compared the effects of each threat to the full model with all factors operating at the same time. Finally, the researchers generated predictions to assess the risk of host plant loss as a result of genetically modified crops on current and future monarch butterfly population size and extinction probability.

The resulting model predicted population declines of 14% and a quasi-extinction probability (<1000 individuals) >5% within a century. Monarch abundance was more than four times more sensitive to perturbations of vital rates on the breeding grounds than on the wintering grounds. Simulations that considered only forest loss or climate change in Mexico predicted higher population sizes compared to milkweed declines on the breeding grounds. The model predictions also suggest that mitigating the negative effects of genetically modified crops results in higher population size and lower extinction risk. Reducing the negative effects of host plant loss on the breeding grounds is the top conservation priority to slow or halt future population declines of monarch butterflies in North America. 

Research Review: Host Diet Affects the Morphology of Monarch Butterfly Parasites

by Emily Galli
Cal Poly Biological Sciences

Original Article Information: Host Diet Affects the Morphology of Monarch Butterfly Parasites. Kevin Hoang, Leiling Tao, Mark D. Hunter, and Jacobus C. de Roode. Journal of Parisitology (2017), 103(3), 228-236.

In recent decades, much of the research on Monarch butterfly and parasite interactions has been focused on numerical traits of parasites affecting the butterflies, including parasite load, size, and shape. This study breaks from that norm and focuses rather on how the host diet affects the morphology of its parasites. A parasite's morphology can have large effects on its fitness, such as initial colonization of hosts, avoidance of host immune defenses, and the availability of resources for parasite replication.This article gives some insight into understanding factors that affect parasite morphology, which can be helpful in hypothesizing the consequences of host–parasite interactions.

In this paper, the effects of host diet were studied in relation to the spore morphology of the protozoan parasite O.e. (Ophryocystis elektroscirrha), a specialist parasite of the Monarch butterfly (Danaus plexippus). Using a single clone of the parasite O.e., lab-reared monarchs, and three low-to-high cardenolide lineages of milkweed from across the USA, the scientists tested the effects of monarch milkweed diets on O.e.

Cardenolides are chemicals present in the latex milkweed produces, which is highly toxic to most animals and parasites. However, Monarch butterflies can eat milkweed without being harmed by the chemicals. Fro previous research, scientists believe that the buildup of cardenolides help Monarchs circumvent the detrimental consequences usually caused by parasites like O.e. 

As hypothosized, the study's results demonstrated that infected monarchs reared on milkweeds of higher cardenolide concentrations had smaller parasites, a possible hidden characteristic of cardenolides that may have significant future
implications for monarch–parasite interactions.

Research Review: Secondary Defense Chemicals in Milkweed Reduce Parasite Infection in Monarch Butterflies, Danaus plexippus

by Ashley Fisher
Cal Poly Biological Sciences

Original Article Information: Secondary Defense Chemicals in Milkweed Reduce Parasite Infection in Monarch Butterflies, Danaus plexippus. Camden D. Gowler, Kristoffer E. Leon, Mark D. Hunter, Jacobus C. de Roode. Journal of Chemical Ecology (2015), 41(6), 520–523

It's a well known fact that Monarch larva feed exclusively on milkweed. That's all they eat….but why? This article gives some insight as to why Monarchs have become milkweed specialists. The scientists who wrote this paper believed the ingestion of the cardenolides in the milkweed, help larva avoid the negative effects of parasites.

Cardenolides are a chemical contained in the latex milkweed produce, and is toxic to most animals and parasites. Monarchs are unique in the fact that they can eat milkweed without being harmed or killed by the toxin. Instead, researchers believe the buildup of these toxins within the larva help the organism avoid the negative effects of parasites. 

To verify this, the authors of this paper made a prediction they could test: If a monarch is infected with parasites, the more cardenolides it ingests, the longer it will live. 

To test this prediction, researchers separated monarch butterfly larvae into groups.  In all groups, the larvae were infected with parasites at the beginning of the experiment. Then, each group was assigned a different diet of milkweed. Some of the milkweed diets contained high amounts of cardenolides whereas others contained very low amounts of cardenolides.  Then, the researchers recorded how long the larva/monarchs from each group lived. 

Just as the researchers predicted, monarchs that ate milkweed high in cardenolides lived longer than the monarchs that ate milkweed with lower cardenolide concentrations. These results suggest cardenolides help MB larva resist the negative effects of parasites.


Research Review: Genetic Factors and Host Traits Predict Spore Morphology for a Butterfly Pathogen

by Emily Hermann
Cal Poly Biological Sciences

Original Article Information: Genetic Factors and Host Traits Predict Spore Morphology for a Butterfly Pathogen. Sarah E. Sander 1, Sonia Altizer, Jacobus C. de Roode and Andrew K. Davis. Insects (2013), 4(3), 447-462

Ophryocystis elektroscirrha (O.E.) is a protozoan parasite that commonly resides on the abdomen of adult Monarchs, gets brushed off onto milkweed leaves as they emerge, and infects the larval stages of butterflies as they eat the leaves. Therefore, spores are very easily transferable across individuals.

In order to learn more about this lethal parasite, Sander and fellow researchers looked at the varying morphologies of the spore ranging in length and color/opaqueness. By identifying 30 replicate spores per host across different geographic sites, they were able to identify a range of results. The geographic locations included areas such as Western N. America, Eastern N. America, Hawaii, and South Florida.

The purpose of the study was to test morphological attributes of the spores to compare with the host characteristics. The researchers used digital imaging and morphometric assays to determine if there is a correlation across the organisms. The results supported a relationship between host traits, like color and wing size, affecting parasite development and morphology.


Research Review: Molecular Ecology article on Cardenolide and Butterflies

by Dr. Francis X. Villablanca 
Cal Poly Biological Sciences
Monarch Alert Project Director

Original Article Information: The evolution of cardenolide-resistant forms of Na+,K+-ATPase in Danainae butterflies. Matthew L. Aardema, Ying Zhen, and Peter Andolfatto. Molecular Ecology (2012) 21, 340–349.

In an article published in the journal Molecular Ecology, researchers report on a monarch butterfly gene that has experienced two mutations. Previous research proposed that both of these mutations may provide monarch butterflies with resistance to cardenolide.

First, something about cardenolides and then back to mutations. Cardenolides are a type of steroid. Many plants contain its derivatives, collectively known as cardenolides, including many that contain structural groups derived from sugars. Cardenolide glycosides are often toxic; specifically, they can stop hearts.

Some plant and animal species use cardenolides as a defense mechanism, most notably the monarch butterflies. Adult monarch butterflies store the cardenolides they have built up as larvae while feeding on milkweeds (plants in the genus Asclepias). The cardenolide content in monarch butterflies deters birds and other vertebrate predators. Notable exceptions are the Black-backed Orioles (Icterus abeillei) and Black-headed Grosbeaks (Pheucticus melanocephalus), and some rodents in the genera Peromyscus and Microtus, all of which are cardenolide-tolerant butterfly predators.

Ok, back to genetics. The gene under scrutiny is named alpha subunit of Na+, K+-ATPase, or ATPa for short. Two of its mutations are associated with the monarch butterflies’ own tolerance to the cardenolides. The shorthand names for the two mutations are N122H and Q111L. The number (122) refers to the mutation’s position in the gene. The letters that bookend the number (N122H) are the specific amino acids within the gene that have been mutated (for example a mutation from N to H). 

The authors argue that earlier research supporting the interpretation that both mutations provide an advantage to monarch butterflies is wrong. 

Their logic is as follows: the Q111L mutation occurred in the distant past of the Lepidoptera (the order of insects that include moths and butterflies). In fact, long before milkweed butterflies (and therefore resistance to milkweed) could have evolved. So, they propose, this mutation could not provide an advantage because at the time the mutation occurred, there was no need for it.

In contrast, the more recent N122H mutation is present in three species of Danaus, (the genus of butterflies including Tigers, Milkweeds, Monarchs, and Queens) and not in other Lepidopterans. Interestingly, this same mutation is present in a beetle that is also resistant to cardenolides. The authors regard this as evidence of convergent evolution (independent evolutionary lineages ending up with the same traits in response to the same natural selection pressure) between the beetle and the three Danaus milkweed butterfly species. The recentness of the mutation and the fact that it also occurs in other cardenolide-resistant invertebrates seems like solid evidence.

The authors are likely correct in their interpretation of the importance of the N122H mutation (the recent one that shows convergence). Yet their interpretation of the importance of the Q111L mutation is not as clear. It is safe to say the mutation is not an “adaptation” because it occurred before milkweed butterflies ever arose and ever encountered milkweed. In other words, Lepidoptera would not have made use of this mutation.

Yet, the mutation could still provide an advantage if it is an “exaptation.”  An exaptation is a trait that currently provides an advantage, but the advantage is for a function that is different from its original function. You might think of this as sort of a “pre-adaptation” because it provides an advantage, but long after the trait arises.

The best way to test the authors’ ideas is to use genetic engineering technology to make “gene knock out” monarch butterflies that have the N122H mutation, or the Q111L, or both. In this manner the effects of each of these mutations could be tested directly by seeing how they affect the ATPa.  This would be like redoing nature’s experiment to see if the mutation actually has the effect we think it has.

This article is important because it shows us how something as simple as a single mutation could allow an organism to evolve an entirely new strategy for dealing with its predators. (Go nature!)  It also clearly shows us that scientists need to know what has already been discovered, but also be willing to test those discoveries in order to “prove it to themselves” and make sure the original conclusions were correct.  In the end, the data don’t interpret themselves; we have to interpret them.

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