Last Saturday’s South Fork Natural History Society paddle on Georgica Pond, timed to coincide with the peak fall migration of the monarch butterflies here (September 8 through 20), was a bust. Not a single monarch was seen.Conditions were not perfect for a big flight, with a cool southeasterly wind blowing all day and rain in the afternoon. But Sunday was a different story: sunny, warm and a light northerly wind. Still, stationed at East Hampton’s Main Beach all day, I counted fewer than 20 monarchs passing by—a number normally seen in a few minutes on a sunny mid-September day with a light north wind.
These were the adults of the third brood hatched here on Long Island this summer. The adults from the first two broods live a very short life, just long enough to mate and lay eggs on milkweed plants. The eggs hatch in less than a week, producing a tiny, dark caterpillar that spends 12 days feeding on milkweed leaves, transforming itself into a striking, fat, yellow-black-and-white, 3-inch-long caterpillar and storing lots of unpalatable toxins gleaned from its milkweed diet.
The transformation from caterpillar (larva) to chrysalis (pupa) does not involve the spinning of a cocoon. Rather, the caterpillar spins a tiny silk button on its chosen support and clings to this with the claws of its last two pairs of legs, head hanging down. Next, the black-white-and-yellow-banded larval skin splits behind the head and is pushed up toward the last pair of legs, exposing the smooth, light green-colored pupal skin. With the larval skin out of the way, a spiny black stem becomes visible and is thrust into the silk button, allowing the pupa to cast off the larval skin and the last pair of feet to which the skin is attached, both of which drop to the ground.
Over the next 10 days the chrysalis darkens, and parts of the developing adult butterfly become visible through the thin chrysalis wall. The timespan from egg to winged adult is roughly one month.
Adults that emerge from chrysalises in September will begin the long and incredible journey south to winter in Mexico. How they navigate to and find the Mexican wintering sites is not known. Monarchs from the South Fork make the 2,500-mile journey in two months, joining monarchs from as far north as southern Canada and Maine at the Mexico site in mid-November.
They are largely dormant through the winter months, relaying on stored fat for the few short movements they make. They depart the second week of March, flying north in search of milkweed plants on which the mated females will lay eggs.
The farthest north this first wave of migrants gets on the Eastern Seaboard is the Carolinas. After mating and laying eggs, the adults die. The next generation completes its metamorphoses in a month, and it is this second generation, hatched in the South, that reaches Long Island in spring, possibly as early as late April. These adults mate and lay eggs here, producing a third generation. Another two or three generations may be produced here over the course of the spring and summer.
The final crop of the year, adults that emerge from pupal sacs in late summer and early fall, may be three or four generations removed from the adults that migrated south the previous fall. It is this generation that will make the return trip to Mexico and complete the cycle.
The life cycle and migration patterns of the monarch butterfly are one of the most fascinating stories in natural history. Much of that story has only quite recently been discovered, and several aspects of their amazing journey remain a mystery.
This is the second consecutive summer that our local monarch population has registered extremely low numbers. How does this compare with the “big picture,” the North American population? Biologist Chip Taylor of Monarch Watch notes that monarchs have been declining for the last 10 years, with the overwintering population in Mexico hitting an all-time low last winter (2013-14).
What is causing the decline? Monarch Watch lists three main factors. First, the widespread use of herbicide-tolerant corn and soybeans in North America is impacting milkweed and important nectar plants normally found growing in and near the crop fields. Second, the ethanol mandate passed by Congress in 2007 led to the conversion of grasslands to corn and soybean cropland and eliminated milkweed and nectar plants. Third, three consecutive years of unfavorable weather during the summer breeding season impacted reproductive success.
Monarch Watch believes that the decline is largely attributed to habitat loss, and that we need a plan that restores at least one million to 1.5 million acres of milkweed and other monarch habitat per year in order to reverse the trend.
On August 26, 2014, the U.S. Fish and Wildlife Service received a petition requesting that the monarch butterfly be granted threatened status under the provisions of the Endangered Species Act. The petition can be viewed at xerces.org.