Copyright Baton Rouge Advocate
For years, home gardeners have been told to do one thing to save monarch butterflies: plant milkweed, the only plant monarch caterpillars eat. And for years, that’s what Linda Barber Auld, known as “NOLA BugLady,” did. Her garden was full of it, as well as other butterfly-friendly vegetation. But things changed in 2020 after she saw scientific data suggesting that milkweed is spreading a disease that can cause butterflies to emerge from their chrysalises wingless and deformed. “I came home and I ripped out all the milkweed in my yard,” she said, both the native and tropical varieties. That disease — ophryocystis elektroscirrha, or OE, for short — is widespread across North America, but monarch infection rates are particularly high in New Orleans and along the Gulf Coast. The vast majority of monarchs in New Orleans are infected with the protozoan pathogen, according to data collected largely by citizen scientists and compiled by Project Monarch Health at the University of Georgia. Some researchers warn OE spores can build up on milkweed that persists through winter — especially tropical milkweed, which is not native to Louisiana. “The very thing that people are doing to help the monarchs is the thing that’s causing them problems,” said Andy Davis, an assistant research scientist at the University of Georgia who studies monarchs. ‘Heavily infected’ Monarchs are an emblematic North American butterfly, with deep orange wings overlaid with an intricate webbing of black markings. Breeding monarchs live only a few weeks, but, each year, a “super generation” of the insects is born. These monarchs can migrate thousands of miles and live for up to eight months. Scientists believe the North American monarch population has declined substantially due to habitat loss and the spread of OE. While there is some disagreement on how much the population has declined and what is responsible, the U.S. Fish and Wildlife Service thinks the bugs are in enough trouble that it recommended listing them as threatened. That’s primarily due to the effects of habitat loss, pesticides and climate change, but OE plays a role, too — especially in the Gulf South. If only mildly infected, a butterfly can live a fairly normal life. But a serious infection can be deadly, especially if a monarch catches it as a caterpillar, before it goes through the process of metamorphosis and grows into a butterfly. OE can spread when adult butterflies mate or it can be passed down from a female to her offspring. Increasingly, it’s spreading by accumulating on milkweed plants — particularly on non-native tropical milkweed, which is hardy enough to survive Louisiana’s mild winters — that many females are using to lay their eggs and where it infects monarch caterpillars. Once infected, monarchs are less likely to survive their annual long-distance migration. Their wings can crumple and fold over, and their bodies can be covered in a gooey substance. Every year, around August or September, butterflies east of the Rocky Mountains fly south from Canada to Mexico, and as they do, those infected with OE die off. Those that survive to mate are then less likely to be infected. But if milkweed is available year-round, monarchs are less likely to attempt migration. That’s led to the development of populations that don’t migrate at all and repeatedly lay eggs on the same milkweed plants, where OE thrives. Anna Timmerman, an LSU AgCenter agriculture and natural resources extension agent, said that a few years ago, she only discouraged gardeners from planting tropical milkweed. Now, she takes a more aggressive approach. “At this point, we have a nonmigratory resident population, which is heavily infected,” she said, and so she discourages gardeners from planting even native milkweeds, like aquatic milkweed, which naturally thrives in Louisiana’s cypress swamps. Karen Oberhauser, a professor emerita at the University of Wisconsin–Madison and longtime butterfly researcher, said there are strong disagreements within the monarch research community about whether gardeners should still be planting milkweed. On the one hand, data show that monarch populations have declined due to habitat loss, as fields once filled with native milkweeds have been razed for development or for agricultural uses. On the other hand, planting milkweed, particularly non-native varieties like tropical milkweed, has allowed OE to spread widely. “Milkweed being present where and when it’s not naturally present can pose problems,” Oberhauser said. If gardeners are going to plant milkweed in their yards, Oberhauser recommends seeking out native varieties. In New Orleans, that’s primarily aquatic milkweed. “Most people would agree that milkweed is beneficial if it’s available when it’s naturally available,” she said. “Often, that’s hard in a garden, because a garden isn’t always natural.” When people plant milkweed, it may survive longer than it would in the wild because it’s being watered or tended to, she said. It might not die back in the wintertime as it naturally would, allowing butterflies to make use of it at times when they wouldn’t normally have access to it. Another thing to consider is whether the plant has been sprayed with herbicides, pesticides or fungicides, said Ray Moranz, a conservation specialist with the Xerces Society. “We have found that a very large percentage of the milkweed plants sold at nurseries had significant amounts of pesticides in them,” he said. While Moranz discourages planting tropical milkweed, he thinks there’s likely still value in planting native milkweeds. “There’s not a lot of data demonstrating that they are actually going to cause as much of a problem as tropical milkweed,” he said.
