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Marvelous Migration of the Monarch Butterfly

By Sue Slotterback

Spring and autumn have long been known as the prime time for birders in New Jersey, with Cape May County being known worldwide as one of the best places to witness the awesome migrations of thousands of birds. But birds are not the only animals that migrate through our “Garden State”. Insects are also among the long-distance travelers: buckeye and red admiral butterflies as well as green darner and saddlebags dragonflies also grace our air space, natural habitats, and our gardens.

By far the most widely studied insect migration is that of the Monarch butterfly. The Monarch holds the record for insect migration at over 2,000 miles – one way! At least some of them travel that far.  Most Monarchs may only migrate a couple hundred miles before they mate, lay eggs, and die. But for some the migration is certainly marvelous. It depends on the time of year.

Monarchs found west of the Rocky Mountains winter in Southern California and Baja, Mexico. Those found east of the Rockies winter in the Transvolcanic Mountains of central Mexico. Around the end of February, beginning of March, Monarchs rouse out of their winter shelter in the oyamel fir forests. They mate and migrate towards the United States arriving in Texas, some venturing as far as Louisiana, where they lay eggs and die.

The female Monarch butterfly will lay approximately 500 eggs, delicately placing each one on the underside of a milkweed leaf. In a week or so the egg will hatch and a tiny caterpillar emerges and its job is to eat and grow. The caterpillar eats its egg shell, munches on its milkweed leaf, grows a little, sheds its skin, becoming a second instar caterpillar, and then goes back to eating milkweed. This constant munching and growing continues five times over the next two weeks. (To put this in perspective, if a human baby ate as much as a caterpillar and grew as fast as a caterpillar, in two weeks the human baby would be as big as a school bus!)

At the fifth instar stage the caterpillar goes on a “walk-about” in search of a horizontal surface, such as a tree branch (or even the clapboard of your house!) on which to enter into the next life stage: the chrysalis. In ten days to two weeks, the butterfly ecloses from the chrysalis. This newly emerged adult is ready to mate¸ fly north, lay eggs, and dies.

And so it goes with each generation living just long enough to migrate a little farther north until they reach southern Canada, the northern most range of their host plant milkweed. By this time it is mid-August. The milkweed’s season is brief in Canada, the nights are getting cool, and the days are getting shorter. These are signals to the Monarchs to not mate and produce eggs, but to their energy into migrating – not farther north, but south – all the way to the oyamel-forested mountains of Mexico – over 2,000 miles away.

The journey alone will take them the better part of two months to complete. Once they arrive, in late October, early November, they will remain huddled by the hundreds of thousands in the fir trees until the end of February, beginning of March, just like their great, great grandparents did the year before. Yes, this autumn generation of migrating monarch butterflies will live for eight to nine months, surpassing their parents’, grandparents’, and great grandparents’ life span by more than seven months.

This year the number of migrating monarchs has not been particularly “marvelous”, in fact they’ve been   noticeably low, as well as predictably low. Several factors seem to have turned this year’s numbers into a “perfect storm” effect, but they are not out of the game yet.

It is important to understand that population levels are naturally cyclical and often fluctuate between years of high population levels and years of low population levels.  Scientists and citizen science monitoring groups believe there are both natural and anthropogenic factors that are contributing to the low number of monarchs observed this year.  A few of these contributing factors and more information about this cyclical trend are explained in more details on our website

First, for several years genetically modified crops (known as GMO crops) have been modified to reduce insect pest damage by the insertion of genes into crops which essentially act as insecticides. When the pollen of the GMO crops lands on milkweed which surrounds the edges of crop fields, the Monarch caterpillars eat it and they die. Strike one.

Second, the spreading use of newly genetically modified “Round Up Ready” crops is quickly becoming “strike two”. Round Up is a very popular and very effective herbicide.  Instead of using more traditional means of eradicating weed species among crops, farmers can now spray Round Up on their “Round Up Ready” GMO crops. This practice effectively gets rid of all plant species, including milkweed, leaving the GMO crops unharmed.

Weather events also throw a curve at species trying to hang on. Strike three is the severe drought occurring in the Mid-West which also affects the milkweed, as well as the nectar plants which are food for the butterflies, located in areas away from the farms. Strike four and five are Hurricane Irene and Super Storm Sandy which hit the East Coast during the Monarch’s autumn migration two years in a row.

On top of all of this is the illegal logging of the forests in Mexico. The winter roosting sites for the Monarch butterflies just happen to be in very rural and impoverished areas. Even though the roost sites are part of the Monarch Butterfly Biosphere Reserve in Michoacan, the people need wood for cooking and heating. While efforts and great strides have been made within the past decade to uplift the economy chiefly through tourism, the recent few years of unrest and cartel violence in Mexico has taken its toll.

Despite all of these “strikes” the game isn’t over yet. It’s important to note that all species populations naturally fluctuate from year to year. According to the Cape May Monarch Monitoring Project’s census data ( we have had population fluctuations between 8.9 and 359.8 monarchs per hour over the past twenty years with only six years having higher numbers than the twenty –year average of 89.9. The average for this year is 13 Monarchs per hour, which admittedly isn’t high, but the census for this year still has another three weeks to go.  Additionally there has been an increasing trend in monarch numbers in the last few years. So, in other words, given this year’s numbers, things really aren’t so bad, in fact there have been previous years with much lower numbers.

Two of the reasons cited for the increasing trend in Monarch census numbers along the East Coast, according to the scientists working with the Monarch Monitoring Project, are the lack of large commercial farms in the East and the increase in knowledge and understanding about the importance of using native plants as landscaping materials and the increase of backyard habitat gardening, which is meant to attract butterflies and other wildlife.

Backyard habitat gardening is a key factor in the survival of many species and especially now for the Monarch butterfly. Their critical need is not only the establishment of good native nectar sources such as coneflower, New England aster, and the golden rods, but an increase of their host plants as well.

For each butterfly and moth species there is a host plant family on which they will lay their eggs. The host plant contains the specific nutrition for that species and by laying the eggs directly on or near the host plant the butterfly, or moth, will insure their offspring have the particular food source they need. The Monarch butterfly’s host plant is milkweed. No milkweed, no Monarchs. Milkweed not only provides the caterpillars with the food they need to grow, but the plant also contains cardiac glycosides, toxins which are ingested by the caterpillar and used to keep most would-be predators from eating both the caterpillar and butterfly. There are over a hundred species of milkweed in the US. A dozen are native to New Jersey, with the three most common being butterflyweed (Asclepis tuberosa), swamp milkweed (Asclepias incarnata), and common milkweed (Asclepias syriaca).

By providing shelter, native nectar sources, as well as host plants everyone can take part in ensuring the continuation of “Marvelous Migration of the Monarchs”.