Graduate student Dara Satterfield came to town in late January for the second time in 12 months to take the pulse of the Monarch butterfly population at the San Antonio River Museum Reach Milkweed Patch. There, dozens of Tropical milkweed plants play year round host to Monarch butterflies. Satterfield works closely with local volunteers like Mary Kennedy and Mobi Warren of the Monarch Larvae Monitoring Project (MLMP), a citizen scientist program, to monitor milkweed patches far and wide for egg, caterpillar, chrysalis and butterfly action that might shed light on the state of the Monarch butterfly migration.
Satterfield, like other scientists, believes the increased availability of Asclepias curassavica, commonly known as Tropical milkweed, coupled with our warmer winters, may have an unhealthy impact on Monarch butterflies and their migration. The science is undetermined on that question.
A native of Marietta, Georgia, Satterfield attends the University of Georgia where she is a PhD candidate whose dissertation focuses on the relationship between migration and infectious disease in wildlife. Monarchs are her species focus. Satterfield works closely with Monarch scientist Dr. Sonia Altizer, the foremost expert in the country on Ophryocystis elektroscirrha, or OE. The unpronouceable protozoan disease infects Monarchs and other milkweed feeders, often resulting in butterfly crippling or death.
As winters get warmer, Asclepias curassavica, the only milkweed species commercially available, is less likely to die back in winter. Some scientists hypothesize that A. curassavica entices Monarchs to forego migration and winter in the U.S. This could create an unhealthy hotbed of lingering OE spores for caterpillars and butterflies that remain in the local area. Since the spores transfer from creature to creature via physical contact with each other or the plants on which they rest or eat, scientists worry that local OE-infested Monarchs will infect and breed with populations that are passing through, possibly jeopardizing the migration.
The situation exacerbates an already serious decline in the Monarch migration. Drought, genetically modified crops, late summer wildfires, and generally inhospitable conditions pose multiple threats to Monarchs and their migration. It’s likely that 2012 will be the worst year, numbers wise, in Monarch butterfly migration history since records have been kept.
“Monarchs are famous for this migration so when we see what appears to be a break in their migratory pattern, we want to know why and what the implications are,” said Satterfield.
But butterfly breeders and enthusiasts argue that OE is like staphylococcus–present in the general population and becoming a threat only under stressed circumstances. Organizations like the International Butterfly Breeders Association promote best practices for limiting OE in captive environments through education of its membership and seminars on managing and limiting its presence. And yet others believe that OE is simply a part of the evolutionary cycle, killing those butterflies less fit than others.
For butterfly hobbyists and gardeners, the “curassavica question” presents a quandary: should we be planting Tropical milkweed or not?
“This is a very sensitive subject in the Monarch world,” said Satterfield. “We just don’t have the data right now.” It will take three or four more years to complete Satterfield’s research. She advises that natives are always better–which is true, IF you can find them and meet their persnickety growing needs. Organizations like Monarch Watch and the Xerces Society‘s have launched milkweed restoration campaigns, but wide availability of native stock is still a dream.
If you DO plant curassavica, many scientists suggest cutting it to the ground in winter–unless yours is a research site like the Milkweed Patch on the San Antonio River Museum Reach. Scientists encouraged the City of San Antonio and San Antonio River Authority to leave our milkweed patch alone as an experiment. Interestingly, the San Antonio patch brags a lower incidence of OE (15%) than in other monitoring sites observed (47%) by the Monarch Larvae Monitoring Project. The 47% figure is SIX TIMES the rate of OE in Monarchs that migrate to Mexico.
Satterfield suggested that a deluge of milkweed beetles at the Patch this winter decimated not only the foliage, but OE spores. The hungry orange-and-black bugs pruned much of the diseased growth, creating a better balance for the butterflies. Apparently Mother Nature has her own way of managing the OE problem.
“This may have removed any OE spores on the milkweed plants, which probably helped to keep the prevalence of OE low this year,” said Satterfield. She added that since our Milkweed Patch is further inland and enjoys cooler temperatures than coastal situations in Houston and Florida, the Monarch population was lower and perhaps more hearty.
In the meantime, gardeners are left to make their own calls. Is it better to have a questionable milkweed source in your yard to provide Monarchs with nectar and host plant, or not?
Let’s see: Tropical milkweed is easy to grow, widely available, prolific bloomer, favorite host plant for Monarchs and a great nectar source for all butterflies. I know where I come down on that. You’ll see Tropical milkweed in my yard. But I’ll be sure to keep it out of wildscapes and ranch situations, and slash it to the ground in the winter.