How Milkweed Location Influences Monarch Egg-Laying and Survival
By Paige Embry
A monarch butterfly (Danaus plexippus) weighs about as much as a paper clip, but as summer ends these tiny animals begin a migration that may cover up to 3,000 miles (4,800 kilometers). The eastern population heads to Mexico and the western one splits between heading to coastal California or further south into Mexico. After huddling through the winter, the monarchs fly north, but this trip is a relay race rather than a marathon, with successive generations each taking a leg.
Over the last 20 years, the eastern monarch population has declined an estimated 80 percent. One theory for the decline is the advent of herbicide-tolerant (aka “Roundup-Ready”) corn and soybeans in 1996, which purged the monarch’s primary host plant, common milkweed (Asclepias syriaca), from Midwestern fields. One study estimated that the Midwest lost more than 860 million milkweeds between 1999 and 2014, mostly in agricultural fields.
Surprisingly, some studies have found that monarch egg survival rates are higher on milkweed in row crops than in prairie areas. Andrew Myers, a Ph.D. candidate in the Landis Lab at Michigan State University (MSU), says it is this “intriguing mystery” that led to a study published last week in Environmental Entomology.
Myers, along with Christie Bahlai, Ph.D., of Kent State University and MSU’s Douglas Landis, Ph.D., set out to learn if this habitat difference was because monarchs laid more eggs in row crops or because predation rates were lower there—or both. They had some unexpected results.
A Two-Phase Experiment
The field work took place in 2016 and 2017 at the MSU Kellogg Biological Station and was broken into two parts: habitat preference for egg-laying and predation rates in each habitat. Both phases used potted milkweeds placed in four different habitat blocks: corn, soy, bare ground, and prairie.
In 2016, monarchs laid the most eggs on milkweed in corn, followed by prairie and bare ground. In 2017, the butterflies favored prairie milkweed while the bare ground and corn habitats tied for second. Perhaps the biggest surprise was that, for both years, soybean came in last, a different result from previous studies.
For the predation study, the researchers glued eggs (with that schoolhouse favorite, Elmer’s glue) onto the milkweeds. For the first 24 hours, the team checked the eggs every two to three hours, counting and identifying predators throughout the night. Myers notes, “The prairie plots were crawling with both arthropod herbivores and predators that I rarely saw during the day,” and he was surprised at “how effective predator and parasitoid arthropods were at regulating herbivore populations. … [It] really increased my awareness and appreciation for the biological control services that insects provide.”
- Both predation rates and egg-laying preferences varied between habitats, but those variations weren’t consistent between years or month-to-month.
- Corn was the best habitat when both egg-laying preference and typical predation rates are combined, but Myers notes that corn has other risk factors, such as pesticides.
- Several factors may contribute to the inconsistent habitat results, including the weather, which differed significantly between the two years.
- Since most of the predation occurred in the first 24 hours, weekly egg surveys are insufficient to distinguish egg-laying habitat preference from losses due to predation.
Regarding what people should know about monarchs in general or the paper specifically, Myers says, “Monarchs are fascinating and beautiful and valuable in their own right, but I think that it’s helpful to view them as a symbol for insect declines more broadly. Recent research is pointing to decreases in insect abundance as a more widespread phenomenon. This trend should really force us to question what’s going wrong with the way we’re approaching agriculture and our stewardship of nature more generally.”
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Email: firstname.lastname@example.org.
Editor’s Note: This post was updated on May 15 to more accurately describe the migratory destinations of the eastern and western populations of monarch butterflies.