The chilly mountains of Michoacan, Mexico welcome two types of visitors. I am one of them, a tourist who went to see you, the other, the Monarch butterfly. I am a human of the variety known in many nations as “the ugly American.” You are the first butterfly I ever met, a black and orange beauty with a complex biology that our science barely understands. We mean you no harm but we may exterminate you, collateral damage in three assaults we inflict on nature. We really are sorry but I understand why you might think that we don’t care about you.
- We humans are killing you in your Mexican winter home as we cut down your forest.
- We are killing you along your migratory flyway by eradicating the only plant you can lay eggs upon.
- And in a passive assault, we brought an invasive species to this land that will steal the food of your young.
Good luck in surviving, but you are pretty much on your own. Again, sorry.
I am an unapologetic lover of the natural world, including the rugged El Rosario Reserve in Mexico, an Oyamel fir and pine tree forest. It is the biggest of the winter homes for the Monarch butterflies (danaus plexippus) that visit the US states east of the Rocky Mountains. Two hundred million Monarchs live in the area around the Reserve, a billion in a slightly larger area. In the cold night, they line tree branches like file cards in a drawer, wings neatly tucked upright, perched together to preserve and share their body heat. In some parts of the Reserve ten million butterflies live in a single acre, making tree branches sag under their weight.
As the morning sun warms them, they awaken, flutter off and enough sleepy-heads fall to the ground that they make a carpet of butterflies. But their winter roosts are shrinking as poor Mexicans harvest trees for fuel in this tiny forest, exposing the Monarchs to killing cold winds. Our tolerance of poverty in the developing world shrinks the Monarch population, and likewise, human and other life elsewhere on Earth.
Monarchs survive a seven month winter period where they do not breed, a diapause. As spring arrives they begin their northward migration. It takes another three generations during the course of a calendar year to complete the full 2500 mile trip into the US and Canada. The fourth generation hatches in September or October and reverses the migratory direction, going south to make the long trip home.
Once they leave Mexico, each of the first three generations make progress in the trip north, breeding and dying, leaving it up to the following generations to finish the trip. Each Monarch lives between three and eight weeks, eating, flying north and breeding before they die. The butterflies feed on nectar and lay their eggs exclusively on milkweeds (aesclepias). In a marvel of evolution, Monarch caterpillars have developed the ability to eat foliage that contains glycosides, chemicals that are toxic for most other insects and birds. The caterpillar incorporates the toxins into its body and both the caterpillar and the mature butterfly are largely avoided by predators.
The second assault on the Monarch comes from North American factory-farm monoculture during the Monarch’s voyage. Milkweeds were once common across the continent, but no more. Monsanto invented a powerful general herbicide called Roundup, the number one selling herbicide worldwide since 1980. By 1996 Monsanto began marketing its first batch of genetically modified crops (GMOs) that are immune to Roundup, including soybeans, corn and grains. Before the GMOs were developed, the farming industry exercised a small measure of caution to protect their own crops. With the development of “Roundup Ready” crops, farmers now repeatedly broadcast the killer chemical across thousands of square miles of Roundup Ready fields. In doing so, they have eradicated the milkweeds and other plants that once grew amongst and along the edges of small farms that previously were unable to handle heavy doses of Roundup applied indiscriminately. Pre-Roundup levels of bio-diversity supported the four generations that Monarchs need to make the full migration north and back home in one year.
Monarchs don’t have it easy once they arrive in the northeast. Nectar is available but nectar only supports adult Monarchs. They need more milkweed foliage for egg laying. Northern gardeners, farmers and large land managers are doing more than our share of eradicating milkweed and now there is a new threat, black swallow-wort (cynanchum louiseae). This insidious invasive species has its origins in southern Europe. It is in the same broad family as milkweed (asclepiadaceae) and it deceives Monarchs into laying their eggs on its leaves. The newly hatched Monarch caterpillars cannot eat the toxic swallow-wort foliage and they die before they pupate and emerge as butterflies.
The fourth and final generation of Monarchs, hatched in September and October, does not breed and die as quickly as the previous three generations. It makes the long trip home to Mexico and becomes the long-lived diapause generation that winters in El Rosario and similar forests. The mass migration is all the more amazing as each generation uses a photoreceptor protein in their antennae to orient itself to the magnetic poles of the Earth to guide it on the journey. They follow a route that is genetically hardwired into them, moving without guidance or instruction from any other Monarch.
This year I rarely saw Monarchs in my garden, despite twenty previous years of abundance. I have spent two decades turning my 8000 square foot yard into an environmental haven, with three different species of native milkweeds (asclepia tuberosa, incarnata and syriaca) and hundreds of other native plants rich in nectar. I built a garden around the plants that are the food and shelter for native birds, animals and insects. Monarchs were ubiquitous, but last year there were many fewer and this year almost none at all. Isolated islands like mine, rich in native species of plants and fauna and free of chemicals, can preserve some species but they cannot save migratory ones threatened elsewhere in their life cycles.
I will maintain my milkweeds because they are beautiful and continue to fill an important role in providing nectar to many other insects. I give milkweed to other gardeners, private and public. I will cut off and send to the incinerator the seed pods of the black swallow-wort where I see them in public, and kill the plant where I have permission to do so http://www.nps.gov/plants/alien/fact/cylo1.htm. I will avoid Roundup Ready farm-factory foods. I will talk and write about our reckless behavior that imperils so many species on this planet. Yet it is not enough.
We mean you no harm but we may exterminate you. Sorry.