Tuesday, May 24, 2022

Climate change on course to hit U.S. Corn Belt especially hard, study finds

"It's important to begin thinking about how to transition out of our current damaging monoculture paradigm toward systems that are environmentally sustainable, economically viable for farmers and climate-smart," says Emily Burchfield, assistant professor in Emory's Department of Environmental Sciences.

By Carol Clark

Climate change will make the U.S. Corn Belt unsuitable for cultivating corn by 2100 without major technological advances in agricultural practices, an Emory University study finds. 

Environmental Research Letters published the research, which adds to the evidence that significant agricultural adaptation will be necessary and inevitable in the Central and Eastern United States. It is critical that this adaptation includes diversification beyond the major commodity crops that now make up the bulk of U.S. agriculture, says Emily Burchfield, author of the study and assistant professor in Emory’s Department of Environmental Sciences. 

“Climate change is happening, and it will continue to shift U.S. cultivation geographies strongly north,” Burchfield says. “It’s not enough to simply depend on technological innovations to save the day. Now is the time to envision big shifts in what and how we grow our food to create more sustainable and resilient forms of agriculture.” 

Burchfield’s research combines spatial-temporal social and environmental data to understand the future of food security in the United States, including the consequences of a changing climate. 

More than two-thirds of the land in the U.S. mainland is currently devoted to growing food, fuel or fiber. And about 80 percent of these agricultural lands are cultivated with just five commodity crops: Corn, soy, wheat, hay and alfalfa. Previous research based on biophysical data has established that climate change will adversely affect the yields of these crops. 

Building predictive models

For the current paper, Burchfield wanted to investigate the potential impacts of climate change on cultivation geographies. She focused on the six major U.S. crops that cover 80 percent of cultivated land in the United States: Alfalfa, corn, cotton, hay, soy and wheat. She drew from historical land-use data classifying where these crops are grown and publicly available data from the U.S. Department of Agriculture, the U.S. Geographical Survey, the WorldClim Project, the Harmonized World Soil Database and other public sources. 

Using these data, she built models to predict where each crop has been grown during the 20 years spanning 2008 to 2019. She first ran models using only climate and soil data. These models accurately predicted — by between 85 and 95 percent — of where these major crops are currently cultivated. 

Burchfield ran a second set of models that incorporated indicators of human interventions — such as input use and crop insurance — that alter biophysical conditions to support cultivation. These models performed even better and highlighted the ways in which agricultural interventions expand and amplify the cultivation geographies supported only by climate and soil. 

Burchfield then used these historical models to project biophysically driven shifts in cultivation to 2100 under low-, moderate- and high-emission scenarios. The results suggest that even under moderate-emission scenarios, the cultivation geographies of corn, soy, alfalfa and wheat will all shift strongly north, with the Corn Belt of the upper Midwest becoming unsuitable to the cultivation of corn by 2100. More severe emissions scenarios exacerbate these changes. 

“These projections may be pessimistic because they don’t account for all of the ways that technology may help farmers adapt and rise to the challenge,” Burchfield concedes. She notes that heavy investment is already going into studying the genetic modification of corn and soy plants to help them adapt to climate change. 

“But relying on technology alone is a really risky way to approach the problem,” Burchfield adds. “If we continue to push against biophysical realities, we will eventually reach ecological collapse.” 

The need for diverse landscapes

She stresses the need for U.S. agricultural systems to diversify beyond the major commodity crops, most of which are processed into animal feed. 

“One of the basic laws of ecology is that more diverse ecosystems are more resilient,” Burchfield says. “A landscape covered with a single plant is a fragile, brittle landscape. And there is also growing evidence that more diverse agricultural landscapes are more productive.” 

U.S. agricultural systems incentivize “monoculture farming” of a handful of commodity crops, largely through crop insurance and government subsidies. These systems take an enormous toll on the environment, Burchfield says, while also supporting a meat-heavy U.S. diet that is not conducive to human health. 

“We need to switch from incentivizing intensive cultivation of five or six crops to supporting farmers’ ability to experiment and adopt the crops that work best in their particular landscape,” she says. “It’s important to begin thinking about how to transition out of our current damaging monoculture paradigm toward systems that are environmentally sustainable, economically viable for farmers and climate-smart.” 

Burchfield plans to expand the modeling in the current paper by integrating interviews with agricultural policy experts, agricultural extension agents and famers. “I’d especially like to better understand what a diverse range of farmers in different parts of the country envision for their operations over the long term, and any obstacles that they feel are preventing them from getting there,” she says. 

Related:

Data-driven study digs into the state of U.S. farm livelihoods

Diverse landcover boosts yields for major U.S. crops, study finds 

Monday, May 23, 2022

Paint color-matcher quantifies iron levels in soil

Debjani Sihi, right, demonstrates using the Nix Pro to measure iron content in soil with colleagues Gaurav Jha (left) and Biswanath Dari. Iron is a vital micronutrient to grow plants and "a fundamental mineral species that dictates many other soil functions, like carbon storage, greenhouse gas emissions and nitrogen and phosphorous recycling," says Sihi, a biogeochemist in Emory's Department of Environmental Sciences.  (Photo by Aneesh Chandel)
 

By Carol Clark

A handheld color-sensing tool, commonly used to match paint shades, is also effective at quantifying the iron content in soil by analyzing its color, a study finds. Agricultural & Environmental Letters published the research showing that the inexpensive color sensor, known as the Nix Pro, can rapidly and accurately quantify soil iron. 

“We found that the Nix Pro is easy to use in the field on soil samples and can give an accurate estimate for iron content within seconds using a technique that we developed,” says Debjani Sihi, corresponding author of the paper and assistant professor in Emory’s Department of Environmental Sciences. “We think that this device holds the potential to become a really handy, cost-effective tool for farmers.” 

Gaurav Jha, who did the work as a post-doctoral researcher at the University of California, Davis, is first author of the study. 

Iron is a vital micronutrient, explain Sihi, a biogeochemist who studies environmental and sustainability issues at the nexus of soil, climate, health and policy. Just as people need iron to make a protein in red blood cells that carries oxygen through the body, plants need iron to move oxygen through their systems and produce the chlorophyll that makes them a healthy, green color. 

Soil iron levels are also a key factor in climate change. “It’s a fundamental mineral species that dictates many other soil functions, like carbon storage, greenhouse gas emissions and nitrogen and phosphorus cycling,” Sihi says. 

And yet, iron is often deficient in soil, especially in agricultural lands. “If a farmer knows that their soil does not contain the right amount of iron for their crops, they can amend the soil before planting,” she says. 

Problems with testing for soil iron 

The problem farmers face is that determining iron content in soil is expensive and/or time consuming. One method is to gather soil samples from across a landscape. The samples are then sent to a laboratory for analyses via a benchtop atomic-absorption spectrometer or inductively coupled plasma. Laboratory analysis yields a precise percentage of iron content for each sample but it can be costly and often requires weeks to learn a result. 

A thick book of soil color samples, known as the Munsell charts, provides an alternative. Comparing the color of a soil sample with the swatches on the Munsell charts can guide an effort to classify the soil color. The downside to this method is that it requires practice, it is labor-intensive and the data is qualitative and imprecise. 

During a coffee break chat among Sihi, Jha and co-author Biswanath Dari, from North Carolina Agricultural and Technical State University, the idea came up of trying the Nix Pro’s color sensing on soil. 

The Nix Pro is a palm-sized light-emitting spectrometer that measures the reflectance of a surface to quantify its color composition. It’s commonly used by paint stores, printing shops and graphic design firms for color matching. A Nix Pro costs just $349 and is easy to use, with a cell phone app providing a near-instantaneous result for a sample. 

The Nix Pro soil sensor, center, is a handheld device that is more automated and user-friendly for measuring soil iron content than a book of color samples known as the Munsell charts. (Photo by Gaurav Jha)

“The color of soil can tell you a lot of stories about an environment,” Sihi says. 

Dark brown soil contains a lot of organic material while whitish soil indicates that most of the organic material has washed away. Dark red soil, including Georgia’s famous red clay, indicates strong iron-oxide content. 

If researchers want to learn about the water table in red clay soil, they can look at the colors in a cross-section of the ground. Past flooding will turn some of the soil layers from red to a grayish color because the water has washed out much of the iron. Tree roots from the past will appear as orange patches. “The orange color indicates where the tree roots borrowed oxygen from the surrounding air and oxidized the iron,” Sihi says. 

Sihi studies redox reactions involving electron receptors like oxygen and iron, which can reveal such hidden stories in an environment. Ferrous oxide, for instance, is known as iron 2 due to the number of electrons lost in the oxidation process. Ferric oxide, or iron 3, contains iron oxide that lost three electrons. 

Soil oscillating between periods of heavy rain to no rain undergoes fluctuating redux, or shifts in the levels of iron 2 and iron 3 as the water tables go up and down. “Fluctuating redox is a big threat multiplier for greenhouse gas emissions into the atmosphere,” Sihi says. 

‘An extremely strong result’ 

Due to the broad importance of iron, the researchers focused on iron content as the first soil test for the Nix Pro. They first collected data on soil samples from New Mexico using the Nix Pro app for three different color spaces: cyan, magenta, yellow and black; lightness and darkness; and red, green and blue. The Nix Pro app allowed them to export the data into an Excel document. 

Researchers from New Mexico State University helped calibrate and validate the three color models generated by the Nix Pro data by comparing them with results for total iron content generated through analysis in a NMSU laboratory. The results showed that all three of the models were significant in predicting soil iron content, with the cyan, magenta, yellow and black model (CMYB) delivering the strongest result. 

Finally, they further validated the CMYB model by using it to estimate total iron content in soil samples from an adjacent field. The results showed that the Nix Pro CMYB model was 80 percent accurate compared to laboratory results for the same samples. 

“That’s an extremely strong result,” Sihi says. “We’re comparing the power of a small, handheld tool with a really fancy lab instrument and it’s holding up.” 

The team now plans to test the Nix Pro on soils from other regions to determine if the method will work universally as an iron soil predicter. They hope other researchers will also apply their method to speed up the data-gathering process. Sihi and her colleagues plan to further expand their project by experimenting with the Nix Pro’s possible efficacy for other agricultural applications related to soil composition. And this summer, they will conduct a greenhouse gas experiment to evaluate if the Nix Pro color sensor can be used to identify nitrogen deficiency in corn plants through the characteristic symptom of yellowing leaves. 

Additional authors of the current paper include Harpreet Kaur, April Ulery and Kevin Lombard (NMSU); and Mallika Nocco (University of California, Davis). 

Related:

Climate change on course to hit U.S. Corn Belt especially hard, study finds

Data-driven study digs into the state of U.S. farm livelihoods

The growing role of farming and nitrous oxide in climate change

Thursday, May 5, 2022

Ancient DNA gives new insights into 'lost' Indigenous people of Uruguay

A sculpture commemorates the Indigenous people of Uruguay in the capital of Montevideo. Archeological evidence for human settlement of the area goes back 10,000 years. (Photo by Maximasu via Wikimedia Commons)

By Carol Clark

The first whole genome sequences of the ancient people of Uruguay provide a genetic snapshot of Indigenous populations of the region before they were decimated by a series of European military campaigns. PNAS Nexus published the research, led by anthropologists at Emory University and the University of the Republic, Montevideo, Uruguay. 

“Our work shows that the Indigenous people of ancient Uruguay exhibit an ancestry that has not been previously detected in South America,” says John Lindo, co-corresponding author and an Emory assistant professor of anthropology specializing in ancient DNA. “This contributes to the idea of South America being a place where multi-regional diversity existed, instead of the monolithic idea of a single Native American race across North and South America.” 

The analyses drew from a DNA sample of a man that dated back 800 years and another from a woman that went back 1,500 years, both well before the 1492 arrival of Christopher Columbus in the Americas. The samples were collected from an archeological site in eastern Uruguay by co-corresponding author Gonzalo Figueiro, a biological anthropologist at the University of the Republic. 

The results of the analyses showed a surprising connection to ancient individuals from Panama — the land bridge that connects North and South America — and to eastern Brazil, but not to modern Amazonians. These findings support the theory proposed by some archeologists of separate migrations into South America, including one that led to the Amazonian populations and another that led to the populations along the East coast. 

“We’ve now provided genetic evidence that this theory may be correct,” Lindo says. “It runs counter to the theory of a single migration that split at the foot of the Andes.” 

Archeological evidence 

The archeological evidence for human settlement of the area now known as Uruguay, located on the Atlantic coast south of Brazil, goes back more than 10,000 years. European colonizers made initial contact with the Indigenous people of the region in the early 1500s. 

During the 1800s, the colonizers launched a series of military campaigns to exterminate the native peoples, culminating in what is known as the massacre at Salsipuedes Creek, in 1831, which targeted an ethnic group called the Charrúa. At that time, the authors write, the term Charrúa was being applied broadly to the remnants of various hunter-gatherer groups in the territory of Uruguay. 

“Through these first whole genome sequences of the Indigenous people of the region before the arrival of Europeans, we were able to reconstruct at least a small part of their genetic prehistory,” Lindo says. 

The work opens the door to modern-day Uruguayans seeking to potentially link themselves genetically to populations that existed in the region before European colonizers arrived. “We would like to gather more DNA samples from ancient archeological sites from all over Uruguay, which would allow people living in the country today to explore a possible genetic connection,” Lindo says. 

Focusing on little-explored human lineages 

The Lindo ancient DNA lab specializes in mapping little-explored human lineages of the Americas. Most ancient DNA labs are located in Europe, where the cooler climate has better preserved specimens. 

Less focus has been put on sequencing ancient DNA from South America. One reason is that warmer, more humid climates throughout much of the continent have made it more challenging to collect usable ancient DNA specimens, although advances in sequencing technology are helping to remove some of these limitations. 

“If you’re of European descent, you can have your DNA sequenced and use that information to pinpoint where your ancestors are from down to specific villages,” Lindo says. “If you are descended from people Indigenous to the Americas you may be able to learn that some chunk of your genome is Native American, but it’s unlikely that you can trace a direct lineage because there are not enough ancient DNA references available.” 

Further complicating the picture, he adds, is the massive disruption caused by the arrival of Europeans given that many civilizations were destroyed and whole populations were killed. 

By collaborating closely with Indigenous communities and local archeologists, Lindo hopes to use advanced DNA sequencing techniques to build a free, online portal with increasing numbers of ancient DNA references from the Americas, to help people better explore and understand their ancestry. 

Co-authors of the current paper include Emory senior Rosseirys De La Rosa, Andrew Luize Campelo dos Santos (the Federal University of Penambuco, Recife, Brazil), Monica Sans (University of the Republic, Montevideo, Uruguay), and Michael De Giorgio (Florida Atlantic University). 

The work was funded by a National Science Foundation CAREER Grant. 

Related:

Ancient DNA lab maps little-explored human lineages

Ancient DNA from Sudan shines new light on Nile Valley past

'Potato gene' reveals how ancient Andeans adapted to starchy diet 

Wednesday, April 6, 2022

Computerized, rolling DNA motors move molecular robotics to next level

"I love the idea of using something that's innate in all of us to engineer new forms of technology," says Emory graduate student Selma Piranej, shown with a cell phone microscope set up to observe the rolling DNA-based motors.
 

By Carol Clark

Chemists integrated computer functions into rolling DNA-based motors, opening a new realm of possibilities for miniature, molecular robots. Nature Nanotechnology published the development, the first DNA-based motors that combine computational power with the ability to burn fuel and move in an intentional direction. 

“One of our big innovations, beyond getting the DNA motors to perform logic computations, is finding a way to convert that information into a simple output signal — motion or no motion,” says Selma Piranej, an Emory University PhD candidate in chemistry, and first author of the paper. “This signal can be read by anyone holding a cell phone equipped with an inexpensive magnifying attachment.”

“Selma’s breakthrough removes major roadblocks that stood in the way of making DNA computers useful and practical for a range of biomedical applications,” says Khalid Salaita, senior author of the paper and an Emory professor of chemistry at Emory University. Salaita is also on the faculty of the Wallace H. Coulter Department of Biomedical Engineering, a joint program of Georgia Tech and Emory. 

The motors can sense chemical information in their environment, process that information, and then respond accordingly, mimicking some basic properties of living cells. 

“Previous DNA computers did not have directed motion built in,” Salaita says. “But to get more sophisticated operations, you need to combine both computation and directed motion. Our DNA computers are essentially autonomous robots with sensing capabilities that determine whether they move or not.” 

The motors can be programmed to respond to a specific pathogen or DNA sequence, making them a potential technology for medical testing and diagnostics. Another key advance is that each motor can operate independently, under different programs, while deployed as a group. That opens the door for a single massive array of the micron-sized motors to carry out a variety of tasks and perform motor-to-motor communication. 

“The ability for the DNA motors to communicate with one another is a step towards producing the kind of complex, collective action generated by swarms of ants or bacteria,” Salaita says. “It could even lead to emergent properties.” 

The high speed of the rolling DNA-based motor allows a simple smart phone microscope to capture its motion through video.
 

DNA nanotechnology takes advantage of the natural affinity for the DNA bases A, G, C and T to pair up with one another. By moving around the sequence of letters on synthetic strands of DNA, scientists can get the strands to bind together in ways that create different shapes and even build functioning machines. 

The Salaita lab, a leader in biophysics and nanotechnology, developed the first rolling DNA-based motor in 2015. The device was 1,000 times faster than any other synthetic motor, fast-tracking the burgeoning field of molecular robotics. Its high speed allows a simple smart phone microscope to capture its motion through video. 

The motor’s “chassis” is a micron-sized glass sphere. Hundreds of DNA strands, or “legs” are allowed to bind to the sphere. These DNA legs are placed on a glass slide coated with the reactant RNA, the motor’s fuel. The DNA legs are drawn to the RNA, but as soon as they set foot on it they erase it through the activity of an enzyme that is bound to the DNA and destroys only RNA. As the legs bind and then release from the substrate, they keep guiding the sphere along. 

When Piranej joined the Salaita lab in 2018, she began working on a project to take the rolling motors to the next level by building in computer programming logic. 

“It’s a major goal in the biomedical field to take advantage of DNA for computation,” Piranej says. “I love the idea of using something that’s innate in all of us to engineer new forms of technology.” 

DNA is like a biological computer chip, storing vast amounts of information. The basic units of operation for DNA computation are short strands of synthetic DNA. Researchers can change the “program” of DNA by tweaking the sequences of AGTC on the strands. 

“Unlike a hard, silicon chip, DNA-based computers and motors can function in water and other liquid environments,” Salaita says. “And one of the big challenges in fabricating silicon computer chips is trying to pack more data into an ever-smaller footprint. DNA offers the potential to run many processing operations in parallel in a very small space. The density of operations you could run might even go to infinity.” 

Synthetic DNA is also biocompatible and cheap to make. “You can replicate DNA using enzymes, copying and pasting it as many times as you want,” Salaita says. “It’s virtually free.” 

Limitations remain, however, in the nascent field of DNA computation. A key hurdle is making the output of the computations easily readable. Current techniques heavily rely on tagging DNA with fluorescent molecules and then measuring the intensity of emitted light at different wavelengths. This process requires expensive, cumbersome equipment. It also limits the signals that can be read to those present in the electromagnetic spectrum. 

"Developing devices for biomedical applications is especially rewarding because it's a chance to make a big impact in people's lives," says Piranej, shown during a trip to San Franciso.

Although trained as a chemist, Piranej began learning the basics of computer science and diving into bioengineering literature to try to overcome this hurdle. She came up with the idea of using a well-known reaction in bioengineering to perform the computation and pairing it with the motion of the rolling motors. 

The reaction, known as toehold-mediated strand displacement, occurs on duplex DNA — two complementary strands. The strands are tightly hugging one another except for one loose, floppy end of a strand, known as the toe hold. The rolling motor can be programmed by coating it with duplex DNA that is complementary to a DNA target — a sequence of interest. When the molecular motor encounters the DNA target as it rolls along its RNA track, the DNA target binds to the toe hold of the duplex DNA, strips it apart, and anchors the motor into place. The computer read out becomes simply “motion” or “no motion.” 

“When I first saw this concept work during an experiment, I made this really loud, excited sound,” Piranej recalls. “One of my colleagues came over and asked, ‘Are you okay?’ Nothing compares to seeing your idea come to life like that. That’s a great moment.” 

These two basic logic gates of “motion” or “no motion” can be strung together to build more complicated operations, mimicking how regular computer programs build on the logic gates of “zero” or “one.” 

Piranej took the project even further by finding a way to pack many different computer operations together and still easily read the output. She simply varied the size and materials of the microscopic spheres that form the chassis for the DNA-based rolling motors. For instance, the spheres can range from three to five microns in diameter and be made of either silica or polystyrene. Each alteration provides slightly different optical properties that can be distinguished through a cell phone microscope.

The Salaita lab is working to establish a collaboration with scientists at the Atlanta Center for Microsystems Engineered Point-of-Care Technologies, an NIH-funded center established by Emory and Georgia Tech. They are exploring the potential for the use of the DNA-computing technology for home diagnostics of COVID-19 and other disease biomarkers. 

“Developing devices for biomedical applications is especially rewarding because it’s a chance to make a big impact in people’s lives,” Piranej says. “The challenges of this project have made it more fun for me,” she adds. 

Related:

NIH grant funds Emory work on indoor air sensor for SARS-CoV-2 

New DNA motor breaks speed record for nano machines

Nano-walkers take speedy leap forward with first rolling DNA-based motor 

Thursday, March 17, 2022

Monarch butterflies increasingly plagued by parasites, study shows

"Our findings suggest that tens of millions of eastern monarch butterflies are getting sick and dying each year from these parasites," says Emory evolutionary biologist Jaap de Roode, senior author of the study.

By Carol Clark

Monarch butterflies, one of the most iconic insects of North America, are increasingly plagued by a debilitating parasite, a major new analysis shows. The Journal of Animal Ecology published the findings, led by scientists at Emory University. 

The analysis drew from 50 years of data on the infection rate of wild monarch butterflies by the protozoan Ophryocystis elektrosirrha, or O.E. The results showed that the O.E. infection rate increased from less than one percent of the eastern monarch population in 1968 to as much as 10 percent today. 

“We’re seeing a significant change in a wildlife population with a parasitism rate steadily rising from almost non-existent to as high as 10 percent,” says Ania Majewska, first author of the paper and a post-doctoral fellow in Emory’s Department of Biology. “It’s a signal that something is not right in the environment and that we need to pay attention.” 
 
The O.E. parasite invades the gut of the monarch caterpillars. If the adult butterfly leaves the pupal stage with a severe parasitic infection, it begins oozing fluids from its body and dies. Even if the butterflies survive, as in case of a lighter infection, they do not fly well or live as long as uninfected ones. 

The rise in parasitism, the researchers warn, may endanger the mass migration of the monarchs, one of the most spectacular displays in the animal kingdom, involving hundreds of millions of butterflies. Each fall, the western monarch population flies hundreds of miles down the Pacific Coast to spend the winter in California. Meanwhile, on the other side of the Rocky Mountains, eastern monarchs fly from as far north as the U.S.-Canadian border to overwinter in Central Mexico, covering as much as 3,000 miles. 

“Our findings suggest that tens of millions of eastern monarch butterflies are getting sick and dying each year from these parasites,” says Jaap de Roode, Emory professor of biology and senior author of the study. “If the infection rates keep going up, fewer and fewer monarchs will be able to survive to migrate to their overwintering sites.” 

One contributor to the rise in the parasitism rate is the increased density of monarchs in places where they lay their eggs, the study finds. The researchers posit that the increased density may be due to many factors, including the loss of wildlife habitat; the widespread planting of exotic, non-native species of milkweed; and by people raising monarchs in large numbers in confined spaces.

Co-authors of the paper are Sonia Altizer and Andrew Davis of the University of Georgia Odum School of Ecology. 

"Monarchs are incredible animals," says Ania Majewska, first author of the study, shown in the Monarch Butterfly Biosphere Reserve in Mexico.

Majewska began studying monarchs eight years ago while a graduate student at the University of Georgia. She joined the De Roode lab at Emory in 2019, funded by the NIH program Fellowships in Research and Science Teaching. 

“Monarchs are incredible animals,” she says. “Each one is only as heavy as a paperclip but they can fly so far and they are incredibly resilient.” 

The Monarch Butterfly Biosphere Reserve in Mexico, where the eastern monarchs overwinter in pine and fir forests, is a World Heritage Site and an important generator of tourism income. “The trees can become so heavy with monarchs that sometimes branches break off and fall,” Majewska says. “When sunlight hits the clusters the monarchs explode like confetti. It’s a magical sight.” 

The butterflies arrive in large numbers in Mexico near the Day of the Dead, when families gather around the gravesites of their loved ones. For traditional cultures in the region, monarchs have come to represent the souls of ancestors returning to visit for the celebrations. 

In addition to the monarch migration’s natural beauty, economic and cultural significance, it plays an ecological role, as the butterflies pollinate plants and provide food for wasps, ants and other invertebrate predators. 

Birds, however, tend to avoid the monarchs, as the butterfly’s striking coloration of orange, black and white is a warning sign that it may be poisonous to them. 

Monarchs overwintering on a tree in Mexico. (Photo by Jaap de Roode)

When monarchs leave their overwintering sites in the spring, they fly north and lay their eggs. Their caterpillars feed on any of dozens of species of milkweed plants, including some species that contain high levels of cardenolides. These chemicals do not harm the caterpillars, but make them toxic to some predators even after they emerge as adults from their chrysalises. 

In 2010, Jaap de Roode discovered that monarchs also use cardenolides as a kind of drug. Experiments in his lab showed that a female infected with the O.E. parasite prefers to lay her eggs on a toxic species of milkweed, rather than a non-toxic species. Uninfected female monarchs, however, showed no preference. While cardenolides do not cure the caterpillars of parasites, they can lessen the severity of an infection. 

For the current paper, the researchers wanted to investigate the O.E. infection rate in monarch populations over time. They accessed multiple available data sets, which were mostly for the eastern monarchs. The data included samples going back to 1968 collected by the late Lincoln Brower, an entomologist who specialized in monarchs, and by other researchers through the decades. 

The results showed the rise in the parasitism rate remained low in the eastern monarch population for several decades before shooting up beginning in the 2000s, then making a slight dip in recent years.

Monarchs caterpillars feed exclusively on milkweed plants. (Photo by Jaap de Roode)

Among the factors that may be contributing to the increased monarch density associated with the rise in parasitism, the researchers note, is the loss of natural habitat and agricultural practices that have reduced the places where milkweed is found. Milkweed used to proliferate amid crops in the Midwest, for instance, but farmers increasingly use genetically engineered herbicide-resistant crops. That allows them to spray their fields to eradicate weeds. 

Since milkweed is the sole source of food for monarch caterpillars, and fewer of the plants are available, the female monarchs must cluster more densely to lay their eggs and the butterfly “nurseries” become more crowded with caterpillars. 

“One thing that the COVID-19 pandemic taught us is that social distancing can help reduce the spread of an infectious disease,” de Roode says. “The same holds true for monarchs and the O.E. parasite.” 

Around the year 2000, the researchers note, conservation groups began planting exotic species of milkweed to try to support the monarch population, which has been declining. Ironically, this conservation effort may have fueled more parasitism. “The exotic species of milkweed tend to have more cardenolides than native species,” Majewska explains, “so infected female monarchs may be seeking the exotic species out, adding to the density problem.” 

The researchers further hypothesize that people raising monarch caterpillars in large numbers — to support conservation efforts or for commercial purposes — may be keeping them in crowded conditions that foster the spread of the parasite. 

“Ultimately, a continuing rise in the monarch’s parasitic infection rate could cause the species to suffer significantly,” Majewska says. “If tens of millions of them are dying annually from parasitic infections, then an extreme weather event during the winter in Mexico might reduce the population to a level that could be dangerous for their genetic diversity.” 

“Parasitism is often overlooked in conservation efforts,” de Roode adds, “but our findings show how parasites can have a massive impact on wildlife.” 

The research was supported by the National Institutes of Health and the National Science Foundation.

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