(Or, how to create the perfect parasite-free butterfly garden.)
Sometime in elementary school, an article in my local paper alerted me to the plight of the monarch butterflies.
Milkweed is only food monarch caterpillars eat, the article said, and it’s also the only plant where the adult butterflies lay their eggs. But wild milkweed populations were shrinking, and the monarchs were suffering as a result. All I’d have to do to attract throngs of butterflies was to plant some milkweed in my yard.
I dragged my mom out to the garden store, where I attempted to convince her that planting something with the word “weed” in its name in her well-organized garden would be a great idea. “Think of the butterflies,” I wheedled. She gave in, lest she quash my budding naturalist spirit.
As it turned out, my milkweed took over the green bean patch and only attracted a handful of butterflies. But I had the right idea. Loss of milkweed habitat is hurting monarchs, and replanting it in gardens can help them survive. It’s inexpensive and easy to grow.
So when I moved to Santa Cruz from Wisconsin a few months ago, I was surprised by how complicated the milkweed-planting advice had suddenly become. Suddenly it seemed that experts couldn’t agree on what type of milkweed was best – and whether one should plant it at all.
The reason, in part, is a small football-shaped parasite called Ophryocystis elektroscirrha (OE). Infected butterflies spread OE spores on milkweed plants. Caterpillars who eat the leaves also ingest the parasite and become sick, and the cycle continues.
Some scientists are concerned that tropical milkweed planted in gardens is increasing monarchs’ susceptibility to this devastating parasite, particularly in places with warm winters. Tropical milkweed is the most common variety sold in garden stores. In much of California and the southern United States, though, tropical milkweed can live year-round instead of dying back in the winter like the natives do. The OE parasite can then build up on milkweed plants over generations of monarchs.
Dara Satterfield, a graduate student at the University of Georgia, studies the relationship between this parasite, milkweed, and migratory monarch behavior. Not all monarchs migrate: butterflies that hatch in the spring and summer will lay their eggs right where they are. Every fourth generation, though, makes an epic migration south to overwintering sites. Santa Cruz hosts an overwintering colony of monarchs every year.
Satterfield says there’s evidence that the tropical milkweed is messing with monarchs’ migration by providing a year-round food source so tempting that some monarchs don’t leave. Instead of migrating in the fall, some butterflies will lay eggs right where they are, essentially establishing year-round breeding colonies.
Year-round breeding also increases OE rates – the long migration usually kills off the sickest butterflies and prevents them from passing on the parasite to the next generation. Without migration, more OE-infected butterflies can reproduce.
The case against tropical milkweed is strongest in the southeastern United States. But California also has temperate winters and so is at risk for the same types of problems.
“At some sites in California with year-round tropical milkweed, 100 percent of monarchs are infected with OE,” Satterfield says. She recommends that Californians plant native milkweed species instead, or cut back tropical milkweed in the fall so that it mimics the natives’ growth pattern.
There’s another challenge in Santa Cruz, and in coastal California in general: these areas host overwintering monarchs, not just summer-breeding ones. Satterfield and her colleagues haven’t yet determined what happens when migrating monarchs find tropical milkweed growing near their winter sites, but suspect that it could be problematic.
“It is possible that this will encourage monarchs to become reproductively active too soon,” she says. “They could encounter tropical milkweed and start breeding during the winter instead of roosting in trees and waiting until the spring to reproduce as they historically have done.”
To be cautious, Satterfield recommends that people not plant milkweed within a few miles of a monarch overwintering site, but no official guideline has been established. Residents living adjacent to overwintering sites can instead plant nectar-producing flowers that provide a food source for the adult butterflies.
Not everyone agrees that tropical milkweed is a risk in California–some butterfly advocates fear that vilifying tropical milkweed will result in reduced habitat for monarchs, and that the benefits of any sort of milkweed outweigh the potential risks. Researchers are still working to gather more conclusive evidence for the area. Satterfield believes that keeping milkweed seasonal is important, whether that’s done by planting native species or cutting back tropical ones in the winter. But, she emphasizes, milkweed provides critical habitat for monarchs, and people who live in areas where it’s helpful should plant it if they’re able.
Back in Wisconsin, OE affects some monarchs. But there’s no way overwintering butterflies or any type of milkweed could survive the arctic chill of our winters, so the gardening guidelines are more straightforward.
I’m moving around too much right now to cultivate a garden, and my track record with potted plants has been notoriously poor. Someday when I have a yard again, though, I’ll give milkweed another go.