The link between OE disease and tropical milkweed in coastal monarchs: by Dara Satterfield
From 2012-2015, I spearheaded a study showing that non-migratory monarchs that breed during the winter in the southern U.S. suffer high levels of parasite infection. In our study, every instance of winter-breeding along the southern Atlantic and Gulf coasts occurred on tropical milkweed – often the only milkweed species sold at nurseries. In this blog post, I would like to share our key findings and explain how this study developed. The results suggest that seasonal (not year-round) milkweeds in eastern North America better support monarch health. More information about native milkweeds is available here.
Let me start from the beginning -
“But why are you searching for butterflies during the winter? That doesn’t make any sense,” my dad said as I packed up my car for a trip to Savannah, GA. That was exactly the point. As an ecology Ph.D. student, I was interested in the incredible 2000-mile migration that monarchs undertake annually from eastern North America to central Mexico. But some monarchs, volunteers reported, were breeding year-round along the coast in the southern U.S. This was what drew me to Savannah in January 2012.
In Savannah, I searched for monarch caterpillars among the tropical milkweed plants in a pollinator garden bordering a golf course. I admired the caterpillars’ vibrant stripes of yellow, white, and black as they methodically) munched on the milkweed leaves. By the time I counted the last larva for the day (61 total) it was late afternoon and the temperature had fallen to 35 F.
Figure above - (Left) A monarch caterpillar eating tropical milkweed in January in Savannah, (Right) Monarchs that breed during the winter rather than migrate are at high risk of acquiring OE disease (right). (L photo by Dara Satterfield; R photo, from Jaap de Roode).
Temperatures dropped below freezing that night. I drove up to the milkweed patch the next morning to find wilted milkweed leaves, dark green masses that flopped limply from the branches. Only 22 monarch larvae were still alive. More than a dozen fifth-instar caterpillars lay dead on the ground.
This garden was one of the numerous locations that reported monarchs on Journey North during the winter of 2012 along the southern Atlantic and Gulf coasts.
The freezing was disheartening enough. But back at our lab at the University of Georgia, I discovered that most of the adult monarchs I sampled from the same site were heavily infected with a parasite known as Ophryocystis elektroscirrha (OE), a debilitating protozoan that can shorten monarch life span, impair flight ability, and cause death during eclosion.
In fact, my co-adviser Dr. Sonia Altizer had been receiving emails since the early 2000s from gardeners in Texas and northern Florida who noticed sick monarchs in their yards during in the winter – a time when most monarchs are tucked into butterfly clusters on fir trees at their overwintering sites in the mountains of Mexico.
Dr. Altizer’s research had previously shown that the monarchs’ migration helps to limit the spread of disease caused by OE. The strenuous migration weeds out infected monarchs that are unable to complete the journey, and because infections build up in monarch breeding habitats over time, the migration allows butterflies to leave behind contaminated milkweeds for part of the year. Then they return to new, parasite-free milkweed in the spring. In between, the parasites die or are greatly reduced in number. In these ways, the migration maintains monarch health.
But what happens when the monarchs’ migration unravels? We were concerned that the non-migratory monarchs reported by volunteer observers in coastal Texas, Louisiana, northern Florida, and Georgia would be at high risk of acquiring OE infections.
To investigate this, we conducted a comparative study of OE infection in migratory and non-migratory (“winter-breeding”) monarchs. Over two years, from fall 2011 to spring 2013, our team of ecologists worked with 107 Monarch Health citizen scientists across the eastern U.S. and Canada to test over 5,000 monarchs for OE parasites. Parasite testing involves pressing a clear sticker against monarch’s abdomen. The sticker pulls off some monarch body scales and can also remove some parasite spores for visualization, but does not harm the butterflies. At our lab, we observed the samples under 60X magnification to measure the proportion of monarchs that were infected from each population. We examined samples from (1) migratory monarchs before the fall migration (a total of 2,566 monarchs from 89 summer-breeding sites, the yellow dots in the figure below), (2) migratory monarchs after the fall migration (a total of 2,390 monarchs sampled from 2 wintering sites in Mexico, in green), and (3) non-migratory monarchs in the southern U.S. (a total of 667 monarchs sampled from across 30 sites, in blue).
Figure above - (Left) Our study compared infection rates among migratory monarchs during the summer (sampling locations indicated by yellow points) and winter (in Mexico, green points) to winter-breeding monarchs along southern coasts (blue points). (Top right) Samples from wild monarchs are viewed under the microscope for OE spores (bottom right).
As the samples poured in from volunteers that had “winter-breeding” monarchs, I stayed late at night in the lab examining the tape samples through the microscope. Sample after sample was covered in parasites.
The difference in infection rates (the fraction of sampled monarchs that were heavily infected with OE) between migratory and winter-breeding monarchs was drastic. In the first year of the study (2011-2012), 49% of winter-breeding monarchs were infected with OE. Only 8% of migratory monarchs in Mexico, on the other hand, were heavily infected. The second year of the study (2012-2013) showed a similar pattern. Altogether, it was a striking difference: Monarchs that breed in the southern U.S. during the winter are 5 times more likely to harbor protozoan infections than migratory monarchs.
This was the first critical evidence that winter-breeding within the range of North American migratory monarchs was leading to high levels of disease. It is important to point out that citizen scientists motivated the research by reporting their initial observations and helped document the problem by tracking disease through Monarch Health.
Observations from the citizen scientists also made it clear that winter-breeding went hand in hand with the presence of tropical milkweed. Site surveys provided by volunteers indicated that in every single case where monarchs were breeding during the winter in our study, it occurred on tropical milkweed (Asclepias curassavica). There were no exceptions. It is important to note that most native milkweeds (of which there are dozens of species used by monarchs in North America) die back during the fall as monarchs begin their journeys southwards. However, tropical milkweed (an exotic species to most of North America) often grows year-round in warm, coastal areas. Year-round availability of tropical milkweed enables monarchs to breed during the fall and winter months, a behavior that increases OE infection risk for those butterflies. To make matters worse, winter-breeding monarchs also suffer mortality from freezing temperatures, like the Savannah caterpillars. In other cases, caterpillars become too dense on tropical milkweed and can run out of food.
Figure above - Monarch larvae often become overcrowded on exotic milkweed. Here, 11 caterpillars on a single stalk compete for the last leaves left in the entire garden. October 2014. Houston, TX. Photo by Dara Satterfield
In 2015, we published the findings about OE in one of the leading scholarly journals for the biological sciences - link here to see the paper. It wasn’t good news. But it was valuable information. The results were hard to hear: A beautiful and popular milkweed plant –a single exotic species– is leading to high monarch disease levels when it grows year-round. A small number of people didn’t accept the news at all or denigrated the science. Others asked good questions about our findings. Many more people just asked how they could help or started shopping for native milkweeds, of which there are over 100 species.
Although it was a painful realization, the connection between disease and winter-breeding was important for us to understand. This new piece of knowledge means that we can better protect the migration and health of North American monarchs by providing them with seasonal (rather than year-round) milkweeds. And this result would still be unknown to science if not for dedicated gardeners and citizen scientists who reported what was happening in their yards and helped to conduct scientific research.
It will be this same group of passionate people who help to reduce disease risk for coastal monarchs. Numerous organizations are working to grow and protect native milkweeds and to share recommendations to replace tropical milkweed, or in the meantime, adhere to a rigorous strategy of cutting it back each month during the winter.* Conserving wildlife is a challenge that requires adapting as new information comes to light. We are grateful to the citizen scientists who work arduously to uncover this scientific knowledge.
*Except in south Florida, where monarchs have been non-migratory for many decades. In all other locations in the eastern U.S., we recommend replacing exotic milkweeds with native species as they become available. In the meantime, gardeners can cut tropical milkweed to 6” each month during Nov.-Feb. to keep it “seasonal.” For states in the southeast, one native milkweed that is relatively easy to grow is swamp milkweed, Asclepias incarnata.
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