Friday, November 8, 2013

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.

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


  1. The has been no monarch decline at Cape May, New Jersey

    1. Nice graph. It brings some comfort to see the numbers up to 2012. Sadly, the numbers this year are as low as those of 1992 and 2004. We'll see what the numbers are in Mexico this winter.