Tropical Milkweed Harmful to Monarch Butterflies?

First we were told to plant tropical milkweed everywhere, as it’s a favorite plant of monarchs, and then we were told NOT to plant tropical milkweed everywhere:

It started with the best of intentions. When evidence emerged that monarch butterflies were losing the milkweed they depend on due to the spread of herbicide-resistant crops in the United States, people across the country took action, planting milkweed in their own gardens. But a new paper shows that well-meaning gardeners might actually be endangering the butterflies’ iconic migration to Mexico. That’s because people have been planting the wrong species of milkweed, thereby increasing the odds of monarchs becoming infected with a crippling parasite.

Habitat loss in both the United States and Mexico has long been the main threat to the North American monarch population. After decades of effort, Mexico curbed deforestation in the butterflies’ winter habitat in the oyamel fir and pine forests of Michoacán and Mexico states. But the loss of milkweed in the United States continues to be a major issue, scientists say. The plant, on which monarchs lay their eggs, used to spring up in between rows of corn, soybeans, and other commercial crops. But today, many farmers plant herbicide-resistant versions of these crops, which allows them to spray their fields with powerful chemicals such as Roundup—killing milkweed in the process. Last year, the number of monarchs that migrated to Mexico was the lowest ever recorded, covering a mere 0.67 hectares of forest, down from a high of 21 hectares in the 1996 to 1997 season. (Scientists in Mexico are planning to announce this season’s count by the end of the month.)

That’s why many monarch buffs swung into action. However, the only species of milkweed widely available in the United States is Asclepias curassavica, which is native to the tropics. Tropical milkweed is pretty, easy to grow, and monarchs love it. “If I were a gardener, I would have done the same thing,” says Dara Satterfield, a doctoral student in ecology at the University of Georgia, Athens.

The problem is that tropical milkweed—at least when planted in warm environments like southern Texas and the U.S. Gulf Coast—doesn’t die back in the winter like native milkweed does. When presented with a place to lay their eggs year-round, many monarchs don’t bother making the trip to Mexico at all. Tropical milkweed is “trapping the butterflies” in these new winter breeding sites, says Lincoln Brower, a monarch biologist at Sweet Briar College in Virginia.

But it turns out that year-round tropical milkweed presents an even more direct threat to the butterflies. Milkweed hosts a protozoan parasite called Ophryocystis elektroscirrha (OE). As caterpillars, monarchs ingest the parasite along with their normal milkweed meals, and when they hatch from their chrysalises they are covered in spores. “It’s a debilitating parasite,” Satterfield says. Infected monarchs are much weaker than their healthy counterparts and don’t live nearly as long. In fact, if an OE-infected monarch tries to migrate, it will probably die long before it arrives in central Mexico, Satterfield says.

Yet the SCIENCE(TM) is not settled:

David James takes issue with the loud and persistent claim that non-native milkweeds pose serious threats to monarch butterflies and the viability of their migrations. When asked if he thinks the technically non-native Tropical milkweed poses a dire threat to monarch butterflies, James’ answer was emphatic.

“No, I do not. Not at all in fact,” said the research scientist and agricultural entomologist at Washington State University.

Having studied monarch butterflies for more than four decades, James focuses on the monarch population of the Pacific Northwest these days. That population, much smaller, less famous and even more at risk than those east of the Rocky Mountains that migrate north from Mexico each spring, moves around various sites along the Pacific coast.

Tropical milkweed, Asclepias curassavica, has been much debated as a significant factor in monarch decline and disease for almost a decade. According to several studies, presence of the easy-to-grow, widely available perennial can encourage monarchs to break their reproductive diapause and stop migrating. The orange or yellow bloomer is so irresistible to monarchs, some research suggests, that it lures monarch females to lay eggs and start the next generation of butterflies in the fall, rather than wait until spring when conditions might be more hospitable.

Also, research suggests the plant’s resilience and appeal contribute to the build-up of the deadly, spore-driven disease, Ophryocystis elektroscirrha, known in the monarch world as OE.

But James contends that the presence of milkweed–native or non-native–is NOT a primary cue for suspending reproductive and migratory behavior. “Changing day lengths and temperatures are,” he said.

James’ study of monarchs began in the 1970s in Sydney, Australia. There, he documented winter breeding monarchs on milkweed right next to large clusters of non-breeding monarchs in nearby trees.

The juxtaposition challenged conventional monarch wisdom–that reproductive and non-reproductive monarchs can live side by side–led him to earn a PhD in entomology, and write more than a dozen papers on the migrating insects.

“The science behind assertions that Tropical milkweed can terminate migration and reproductive dormancy in eastern U.S. monarchs is unfortunately flawed,” said James. “A lot more work needs to be done to prove that Tropical milkweed by itself can terminate dormancy and reproduction in eastern U.S. monarchs,” he said, adding that data on this topic does not exist for western U.S. monarchs.

James’ early research suggests that non-reproductive and migratory monarch populations in Australia are not adversely affected by the presence of non-native and other ”tropical” milkweeds such as Gomphocarpus fruticosus, an African milkweed sometimes called Swan plant or Balloon plant.

“In fact,” said James, the presence of milkweed appears to be a prerequisite for the choice of an overwintering site by monarchs in eastern Australia. All overwintering sites (occupied by non-reproductive monarchs) are characterized by milkweed presence.”

Yet Tropical Milkweed might medicate Monarchs against OE:

Monarch butterflies use medicinal plants to treat their offspring for disease, before they even hatch, a new study finds.

Monarch caterpillars feed on any of dozens of species of milkweed plants, including some species that contain high levels of a group of chemicals callled cardenolides. These chemicals do not harm the caterpillars, but make them toxic to predators even after they emerge as adults from their chrysalises.

As caterpillars, the monarchs are susceptible to gut invasions by parasites, which persist when the caterpillars become adults. An infected female passes on the parasites when she lays her eggs.

“Several years ago we did experiments in which we reared monarch caterpillars on two different species of milkweed, and found that tropical milkweed reduced parasite infection, parasite growth and the disease suffered by the monarchs, ” said Jaap de Roode, an evolutionary biologist at Emory University in Atlanta. “I then wondered if monarchs could take advantage of this, by preferentially using the tropical milkweed if they were infected.”

De Roode and his fellow researchers created an experiment in which they raised monarchs and bred them in the lab. When new butterflies were born, some were infected with the parasites.

Then, they mated uninfected females with infected males, placing the females in a cage to lay their eggs. “The cage had both swamp milkweed and tropical milkweed, which is much more toxic than swamp.  After the female laid their eggs, we counted them,” De Roode said. “The infected females laid more of their eggs on the tropical milkweed, while the uninfected females showed no preference, which suggests that infected females were medicating their offspring.”

Or maybe not:

Many people have cited other recent work from Jaap De Roode’s lab at Emory University showing that tropical milkweed can have a medicinal effect on monarchs infected with OE, and that infected female monarchs seek out highly toxic milkweed like tropical milkweed to lay their eggs. This is interesting and important work. However, it cannot be emphasized enough that tropical milkweed does not ‘cure’ monarchs of infection. If this were true, we would not see such high levels of infection in monarchs sampled in the winter-breeding tropical milkweed patches in the wild. In some of these patches, every single monarch was heavily infected. Tropical milkweed, like other toxic milkweed species, reduces disease severity (spore load) in infected monarchs – sometimes by half – and thus allows infected monarchs to live longer. But living longer can give infected monarchs more time to spread parasites. In other words, feeding on toxic milkweeds is beneficial to individual infected monarchs because they have a better chance of surviving long enough to mate and lay eggs; but if they do reproduce, their offspring will also become infected.  In this way, tropical milkweed could lead to high levels of infection in the wild. This is somewhat like parents giving a child Tylenol and sending her to school when she wakes up feeling ill, resulting in the transmission of disease to her classmates.

Conclusion

I have tropical milkweed, both in our yard and in the plant nursery. We have a healthy skepticism of organized science; however, the safest bet might be to just plant native milkweeds if you’re worried about monarchs. On the other hand, tropical milkweed attracts a wide range of insects and is still a high-value – and often preferred – species for monarchs.

Perhaps the weak shall die and the strong survive, as monarch adapt to a changing climate and plants. Or perhaps this is a tempest in a teapot.

In the 70’s, we were told the earth was going to freeze over. By the 90’s, we were told we were going to roast.

The world is complicated place!

Just do your best with the information you have. And don’t feel too guilty or frightened about the various studies. They’re always changing.