• by Dr. Sarah Oktay, Director, University of Massachusetts Nantucket Field Station •
The milkweed plants are everywhere, waiting to host one of their primary users, monarch butterflies. Hundreds of other insects occupy the plants using every inch of them for food and shelter. Two years ago I wrote about the sea of milkweeds (1) at the Nantucket Field Station and this year, just as many, if not more, have popped up. The milkweeds are the only host plant for the larva of the monarch and scientists believe that one reason for the continent-wide decline in monarch numbers is due to a loss of milkweed habitat in the U.S. Milkweed flowers bloom from June to August, and are visited by many species of moths, butterflies, bees, and other insects. Milkweed is a shelter and hiding place for other species as well. Yellow Jackets eat bees and flies which get trapped in the flowers, and crab spiders ambush visiting insects. Monarch butterflies, milkweed bugs, and milkweed leaf beetles only eat milkweed, and could not survive without it.
For years, September has been “monarch time” on Nantucket. The past two years we have not seen nearly as many monarch butterflies here which is a pattern that has been repeated across the country. Researchers tell us there are three many reasons for this: bad weather including drought and high temperatures and storms over the past 2-3 years have literally stopped these majestic creatures in their tracks; deforestation in Mexico has reduced the forest habitat they require for breeding and overwintering; and the use of pesticides, especially in the Midwest, has reduced the population of milkweeds (2-5).
The World Wildlife fund has been recording numbers of monarchs and the reduction in hibernation acreage for years. In 2013 scientists recorded the lowest number of monarchs arriving in Mexico in the twenty years that they had been observing them (2-5). Monarchs take two months to reach forests in Mexico as they migrate down through the US from Canada. The easiest way to count them is to record how many arrive to cover every tree in Mexico. The total hectares of occupied habitat can be easily measured and correlates exactly to the population because monarchs need to overwinter together to survive. That area has shrunk drastically from an average of 30 acres in the mid-90s to less than 10 acres today as measured by scientists working at Mexican butterfly sanctuaries (4).
Monarch butterflies (Danaus plexippus Linneus, 1758) are a milkweed butterfly (subfamily Danainae), in the family Nymphalidae. It is perhaps the best known of all North American butterflies. It is thought that the colonists were so impressed by this species that they named it “monarch” in honor of “King William, Prince of Orange, stateholder of Holland, and later King of England.” Both genus and species name is linked to the Greek mythology tale of Danaus. (6)
The monarch butterfly’s obsession with milkweed plants is a survival mechanism. Most milkweeds contain cardiac glycosides (7) which are chemicals that affect heart rhythms. The cardiac glycosides are stored in the bodies of both the caterpillar and adult in various sections and concentrations. These poisons are distasteful and emetic (causes vomiting) to birds and other vertebrate predators. After tasting a monarch, a predator will associate the bright warning colors of the adult or caterpillar with an unpleasant meal, and avoid monarchs in the future. The beautiful colors of both the larval and adult Monarch are a warning not to eat them; the term for that is aposematism or warning coloration. Certain predators have a high tolerance level to the chemical defenses produces by monarch butterflies. Species such as the black-beaked orioles and the black headed grosbeaks are common monarch predators that can tolerate an increased level of cardiac glycosides. The flower nectar and pollen does not have glycosides in it, so bees and nectar imbibing insects do not become poisonous. There are a few monarch butterfly imitators like the viceroy butterfly (Limenitis archippus) who employ a defense mechanism called Batesian mimicry, in which a potentially tasty species attempts to mimic an unpalatable species in an attempt to increase its survival rate (6).
Milkweeds use three primary defenses to limit damage caused by caterpillars: hairs on the leaves, cardenolide (bitter tasting) toxins which form those cardiac glycosides, and latex fluids. Monarch larvae and other caterpillars have developed feeding techniques to avoid the latex and cardenolides in the sap. Of course no one feels sorry for the poor milkweed being predated upon by caterpillars but they have developed some coping mechanisms including learning to grow faster. Data from a DNA study indicate more recently evolved milkweed species depend less on hairs and toxins (hairy toxins?) but grow faster than older species, potentially regrowing faster than caterpillars can consume them (8). This may help reconstitute milkweeds in areas in which overzealous highway crews have mowed down all the habitat needed for the monarchs as they fly south.
Monarch butterflies begin their annual migration from Canada and the United States to central Mexico in late summer with the peak in travelers for our area typically passing through in September around the 8th-20th (9). Their trip may be as long as 3,000 miles, an amazing feat considering their size and the weather they have to endure. No other butterfly manages to make such a long two-way trip. Other butterfly species may travel long distances one way, but the monarch will go south in the winter in order to survive over the winter months, traveling together en masse after making that trip north the previous spring. There are populations of monarchs in California, Florida, and Texas that don’t migrate, but the majority of monarchs do.
As fall approaches, the monarchs are born that will make that long trip south, waiting to mate until the following spring. They store fat in their abdomens as an energy reserve for their long trip and to sustain themselves over the several months of hibernation-like inactivity in Mexico. When they first arrive at their winter locations in November, monarchs gather into clusters in the trees. By December and January, when the weather is at its coldest, the monarchs will be tightly packed into dense clusters of hundreds or even thousands of butterflies. By mid-February these clusters of butterflies begin to break up and the monarchs will begin to gather nectar. In the spring they will reproduce and their offspring will make the return trip to the north, completing the cycle which is in essence a massive multi- generational relay race. The original butterfly dies along the way, but the offspring it leaves behind continues on to the north where the cycle will start again in the fall.
Lincoln Brower, a professor of biology at Sweet Briar College has studied the monarch migrations for decades. He describes the conditions that have created this severe drop in population. A severe drought in Texas reduced the nectar available to monarchs on their trip south. Fewer monarchs would then survive over the winter because they were “too skinny” in other words, their fat reserves were inadequate. The habitat they find when they arrive in Mexico is disappearing due to illegal logging. There is no room at the inn, because there is no inn. Last but not least, a combination of drought and heat in the summer and early fall and cold fronts and storms in the spring thwart the migrating animals.
His research showed in one mountainous area in Mexico the populations have been declining for twenty years: “The colonies are spread across twelve mountain ranges in Mexico, and in the biggest year on record they covered 21 hectares. Two years ago, that was down to 2.89 hectares. Last year, it was down to about 1.19 hectares. That’s a highly significant trend.” (2). The World Wildlife Fund report showed that things were even worse with the habitat occupied by butterflies at an all-time low of at 0.67 hectares. In the Washington Post interview Brower commented that a cold front prevented remigration north and that last year an entire generation of monarchs was lost. Monarchs go through several generations during their round trip migration and one major hiccup can put a significant dent in the population. There is not much that can be done about the weather and it is too early to tell if climate change is effecting the Mexican forests, but we can do a lot about herbicides, especially “Round-Up” (glyphosate) related ones that severely threaten monarch and many other native insects including bees. The herbicide use itself is not the only culprit, it creates a cascade effect of glyphosate resistant super weeds, and has also led to the creation of genetically modified crops to make them resistant to glyphosate. ”These crops are planted in the grassland ecosystems of the United States, where the monarchs do most of their breeding. And normally in that area there are milkweed growing all over the place on the agricultural fields and the edges of fields and the sides of roads. There are 108 species of milkweed in the United States — the whole monarch migration evolved in relation to evolution of this milkweed flora.
Anyway, where they use these herbicides, it kills all emergent seedlings and all the emergent perennial plants. A paper last year by John Pleasants of the University of Iowa and Karen Oberhauser of the University of Minnesota estimated that 60 percent of milkweed has been eliminated from the grassland ecosystem. We’re not just talking about one species, we’re talking about the entire native flora being eliminated.” (Brower, 2, and 3-5).
Solutions for both the Midwest and our east coast migration zones include not mowing roadsides and leaving wildflower zones for monarchs to feast on as they move south. For fields treated with herbicide, leaving a wide “pesticide free” buffer zones helps to reduce the amount of “friendly fire” incidents.
To see these beautiful butterflies, check out Smooth Hummocks area owned by the Land Bank and Nantucket Conservation Foundation off Madaket Road near the water fountain and Barrett Farm road turnoff (10) or anywhere large open meadows and sand plain grasslands are covered with goldenrod and asters in bloom. According to the Massachusetts Audubon website find a butterfly website we should keep our eyes peeled over the next few weeks. “Aggregations are most regularly seen along the coast as well as in association with other leading lines such as river valleys and ridgelines. While each Monarch is an independent traveler, congregations are regularly observed especially at the coast and on cold nights at communal roosts. Migrants display highly directional flight when winds are favorable. Under adverse conditions, such as strong southwesterly winds, the migrants may often be found nectaring or just “hunkered down” awaiting a change in the weather.” (11) If you get a chance to work with Emily Molden of the Nantucket Land Council on monarch tagging I highly recommend it, assuming we have monarchs to tag (6 and 12). Let’s hope for smooth sailing and perfect weather for our spectacular migrating guests.
References used for article; all accessed/retrieved September 7th, 2014.
- Ramanujan, Krishna (Winter 2008). “Discoveries: Milkweed evolves to shrug off predation”. Northern Woodlands (Center for Northern Woodlands Education) 15 (4): 56.