How the position of milkweed influences the laying and survival of monarch eggs

By Paige Embry

A monarch butterfly (Danaus plexippus) weighs about as much as a trombone, but in late summer, these tiny animals begin a migration that can cover up to 4,800 kilometers (3,000 miles). The eastern population is heading to Mexico and the west to the California coast. After being huddled during the winter, monarchs fly north, but this trip is a relay race rather than a marathon, with successive generations each taking a leg.

Over the past 20 years, the population of Eastern Monarchs has decreased by about 80%. One of the downward theories is the advent of herbicide-tolerant corn and soybeans (aka "Roundup-Ready") in 1996, which purged the monarch's main plant, the milkweed (Asclepias syriaca), Midwestern fields. One study estimated that the Midwest had lost more than 860 million milkweeds between 1999 and 2014, mostly in agricultural fields.

Surprisingly, some studies have found that survival rates of monarch eggs are higher for milkweed in row crops than in grasslands. Andrew Myers, a Ph.D. candidate at the Landis Lab at Michigan State University (MSU), explains that it is this "intriguing mystery" that led to a study published last week in Environmental Entomology.

Myers, with Christie Bahlai, Ph.D., of Kent State University, and Douglas Landis, Ph.D. of the MSU, investigated whether this difference in habitat was explained by the fact that monarchs spawned more eggs in row crops or predation rates were lower. -or both. They had unexpected results.

A two-phase experience

Fieldwork was conducted in 2016 and 2017 at the MSU Kellogg Biological Station and was divided 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, soybean, bare soil and meadows.

In 2016, monarchs laid the most eggs on milkweed, followed by meadows and bare soil. In 2017, butterflies favored Prairie Milkweed, while bare soil and maize habitats were tied for second place. Perhaps the biggest surprise is that, for both years, soybeans came last, a different result from previous studies.

For the predation study, researchers stuck eggs (with this school favorite, Elmer's glue) on milkweeds. During the first 24 hours, the team examined the eggs every two or three hours, counting and identifying predators throughout the night. Myers notes that "Prairie plots swarmed with both arthropod herbivores and predators that I rarely saw during the day," and he was surprised at "the effectiveness of predatory and parasitoid arthropods for regulating populations. d & # 39; herbivores. … [It] really increased my awareness and appreciation for the biological control services provided by insects. "

To take away

• Predation rates and nesting preferences vary from one habitat to another, but these variations are not consistent from year to year or from month to month.
• Corn was the best habitat when oviposition preference and typical predation rates are combined, but Myers notes that corn has other risk factors, such as pesticides.
• Several factors can contribute to inconsistent housing outcomes, including weather, which differed significantly between the two years.
• Since most predation occurred in the first 24 hours, weekly egg surveys do not distinguish nesting habitat preferences from predation losses.

As for what people should know about monarchs in general or the newspaper in particular, Mr. Myers said, "Monarchs are fascinating, beautiful and precious, but I think it's useful to think of them as a symbol of the more general decline of insects. Recent research indicates that the decrease in insect abundance is a more widespread phenomenon. This trend should really force us to question what is wrong with the way we approach agriculture and our management of nature in general. "

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: Endangered Pollinators in North America and the Struggle to Save Them. Email: [email protected].

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