The Monarch's Breadbasket

Monarch on milkweed © Beatriz Moisset

For years the so called Corn Belt has been our breadbasket, as well as that of the monarch butterfly. Despite farmers' efforts to remove all weeds, common milkweeds proved well adapted to corn fields and prospered year after year, encouraged, rather than hindered by annual plowing. Other milkweeds, with different habitat demands lost ground to the point of becoming endangered. But common milkweed (Asclepias syriaca) and its rich accompanying fauna, including the monarch butterfly, continued to do well.

Monarch caterpillars need milkweed,

which used to grow in relative abundance

in corn fields of the Midwest

© Beatriz Moisset

Recently this changed with the advent of genetic modifications that make corn and other crops resistant to weed killers called glyphosates, mainly Roundup. Nowadays, approximately 90% of corn and soy seeds are genetically modified. Now farmers can use Roundup freely on these resistant crops. This radical change in farming practices is having unpredictable impacts in ecosystems. One effect of herbicides used in this manner is that it finally became possible to wipe out populations of common milkweed that previously had managed to prosper in cultivated fields and along field edges. What is good for the farmers may prove devastating for the monarchs.

According to some recent studies, most of the monarchs in Canada and the East Coast (fourth and fifth generations) are descended from the ones born in the Corn Belt (second and third generations). It seems that the weakest link in the chain is the Midwest where herbicide-resistant crops plus herbicides are decimating the common milkweed. Trying to strengthen the other links may be futile.

Monarchs used to be numerous in Cape May, NJ,

during fall migration, but no more despite abundant nectar

© Beatriz Moisset

Our milkweeds and nectar plants, here in the East, are almost devoid of monarch butterflies. The same thing applies to the oyamel forests of Mexico where overwintering monarchs used only a small fraction of the available habitat last year (2012).

Common milkweed patch in Southeastern Pennsylvania

Very few monarch caterpillars this year, 2013

© Beatriz Moisset

 If we want to save the monarchs, we need to save their breadbasket by stopping the use of herbicides in our own breadbasket.

References

Benbrook, C.  Impacts of genetically engineered crops on pesticide use in the U.S. -- the first sixteen years. Environmental Sciences Europe (2012)

Brower, L.P. Understanding and Misunderstanding the Migration of the Monarch Butterfly (Nymphalidae) in North America: 1857-1995. Journal of Lepidopterists' Society. First observations page 306. Mexico, pages 312-213

Flockhart, D. et al. Tracking multi-generational colonization of the breeding grounds by monarch butterflies in eastern North America. Proceedings of the Royal Society. 

Pleasants, J., Oberhauser, K. Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity (2012)

When did "Common Milkweed" Become Common?

Monarch on common milkweed. © Beatriz Moisset. 2010

The prairies of this continent used to be rich in biodiversity before they were plowed under and turned into cropland. We can only guess at the structure of those lost plant communities by studying the remaining plots of prairie. A couple dozen species of milkweed (Asclepias) prospered in the Midwest two centuries ago, each adapted to its own habitat. Certain species preferred high moisture, others, drier spots. Some showed a preference for coarse, loose, damp, or undisturbed soils; still others did well on almost any soil type. Some needed more sunlight than others. A few survived drought or fire better than others.

Among this variety of habitat preferences, common milkweed, Asclepias syriaca, a "weedy" or pioneer species, does best in disturbed areas, patches that, for a variety of reasons, have lost their established vegetation. In a few years, it is displaced by other species when the ecological succession continues. Two hundred years ago, disturbed spots must have been rather uncommon. Everything changed when the prairie became farmland and was annually plowed. Many species of these plants lost ground under the new treatment. However, plowing created ideal conditions for the common milkweed. It can grow between rows of plantings and along edges, and its deep rhizomes, underground stems, allow it to survive year after year despite plowing. Is this when common milkweed earned its name?

For many years, farming practices in the Midwest continued to disrupt the land creating the conditions that this plant prefers. Farmers dislike this plant for its tenacity. It is said that farmers rejoice at the sight of big fat yellow and black caterpillars devouring the cursed weed. By now, you probably guessed that those caterpillars are monarchs.

USDA map of corn production

Monarch butterflies feed on most types of Asclepias, but Asclepias syriaca is by far their main food source. Perhaps, when the prairie became our breadbasket, it also became the monarch's breadbasket, who took advantage of the spreading weed.

In the meantime, all along the Eastern United States, forests were being cut down. A tree-covered area is not the right habitat for any types of milkweeds. When forests were cleared, the conditions favored by common milkweed emerged. Populations of monarchs must have grown along with the expansion of this plant.

If all this is true, and much of it may be just speculation, then these butterflies must have benefited from these man-made changes. We must ask ourselves: what were the monarch's populations like before the expansion of common milkweed numbers? Were they as abundant before the early 1800s as in recent times? What were the population sizes in their overwintering sites in Mexico? We became aware of the monarch butterfly migration in the past one hundred years. The whole story of their incredible trip to Mexico became known only in recent years. Curiously, no references to monarchs in Mexico can be found until 1890. By then, the transformation of the Midwest with its expansion of common milkweed was well underway. Were their numbers in Mexico so low before this time to escape notice?

Present day discussions about monarchs and their preferred food plant, common milkweed, seem to accept the numbers reported around the 1950s as the norm. Those were the highest numbers ever recorded, and it is assumed that these had remained the same for a long time, perhaps from the days when glaciers receded, tens of thousands of years ago. However, we must consider the possibility that milkweeds and monarchs were never as abundant as in the twentieth century; that this is largely a man-made phenomenon. The numbers of both, plant and butterfly have been going down steadily in recent years. Is the new normal similar to the old normal of hundreds of years ago?

Monarch caterpillar on common milkweed. © Beatriz Moisset. 2010

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Monarch Butterfly, a Case of Mistaken Identity

Male monarch butterfly

© Beatriz Moisset

 A few days ago, I arrived at my favorite native flower garden at Churchville Nature Center with my camera, looking for pollinators as usual. A mother, also with a camera hanging from her neck, was visiting the garden with two children.

Black swallowtail caterpillar on fennel

© Beatriz Moisset

Milkweed tussock moth caterpillar

on common milkweed

© Beatriz Moisset

She pointed at a fuzzy, hairy, colorful caterpillar eating milkweed leaves. "Look at the monarch butterfly caterpillar," she said to the kids.

I couldn't stay quiet, "It is a tussock moth caterpillar," I explained.

"Oh, yes," she saved face by adding, "it will turn into a large moth, the tussock moth."

Later, she moved to a fennel plant and announced: "this is the plant that monarch caterpillars need for food."

The real thing, monarch caterpillar on common milkweed

© Beatriz Moisset

Should I have corrected her again? I would have to explain that monarchs feed only on milkweeds. Black swallowtails are the ones that need fennel. I simply walked away shaking my head.

Monarchs, monarchs, monarchs! I thought. How often people mistake anything else for monarchs?

Tiger swallowtail

© Beatriz Moisset

Just a few days earlier, a gentleman photographer at Pennypack Nature Center was busy snapping shots of something I couldn't see at the time. "There is a monarch right there." He pointed at a gorgeous large black-and-yellow striped butterfly with trailing tails on its wings, nothing like the orange and black monarch.

"It is a tiger swallowtail," I told him.

Fritillary butterfly on butterfly weed

© Beatriz Moisset

Then, I remembered all the times when people look at my framed photo of a fritillary butterfly and ask me if, or even tell me that, it is a monarch. All this sounds like Elvis Presley's sightings.

From pandas to dolphins to monarch butterflies: some animals become iconic. The monarch butterfly is, perhaps, the most iconic of all insects. Entomologist Marlin Rice (Iowa State University) referred to the monarch as the "Bambi of the insect world." This spectacular butterfly acquired its fame, in part due to a movement devoted to preserve the monarch and its remarkable migration. The movement is sponsored by websites such as Monarch Watch, the Monarch MonitoringProject, and a few others listed in the Walter H. Sakai's website. They are all worthy causes that engage the public; these projects can use the income generated by all the publicity.

I applaud the efforts of these organizations. At the same time, I wonder: A poster child is valuable if it raises awareness of a broader issue. The ecological importance of the monarch butterfly is that it is one of many species at risk because of human-caused habitat degradation. Other species may not be as appealing as the poster child, but they deserve our attention, too.

Red admiral, sometimes mistaken for a monarch

© Beatriz Moisset

The real thing, monarch butterfly on common milkweed

© Beatriz Moisset

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