The monarch flaps in my net as I reach in and carefully pull it out. My eight-year-old daughter peels an adhesive tag the size of my small fingernail from a sheet and gently sticks it on the middle of the butterfly’s wing. This monarch will now be known as CAK 700, the code on its new wing tag.
Exactly what do we know about New England’s monarch butterflies? For starters, they over-winter at just 13 known sites in the volcanic mountains of central Mexico. (A 1984 study found that there may have been as many as 60 over-wintering sites, but commercial logging has eliminated most of them.) A single site may contain up to four million monarchs per acre and cover 0.1 to 8 acres of forest.
The butterflies arrive from the north between November and late December and generally hang out on the trees, metabolizing fat reserves that they have built up during migration. Remarkably, they actually gain weight on migration and arrive on the wintering grounds with fat reserves for the winter, unlike songbirds, which require huge fat stores to burn during migration.
The butterflies in the over-wintering sites begin to disperse in March and early April, and they then migrate to the Gulf Coast of the southeastern U.S., where females arrive just as milkweed is sprouting. They lay eggs on the fresh plants and then die. One or two generations of monarch caterpillars feed on milkweed before it becomes too hot and dry for the milkweed to persist. The adult butterflies then continue the northward migration and arrive in New England at the end of May or early June, just as our milkweed is growing strong. The females lay eggs and die.
It takes about 30 days to go from a monarch egg to a monarch caterpillar. Beginning in mid-August, after several generations of monarchs have been born and died in New England, the decreasing daylight triggers a physiologic change in monarchs, causing them to migrate back to the wintering sites in Mexico and complete their yearly cycle. Incredibly, the adults that leave New England have never seen Mexico. Yet somehow they are guided back to these small sites.
The winter generation of butterflies lives up to eight months while the successive spring and summer generations are lucky to live a single month. It takes up to six generations of spring and summer monarchs to produce the final “super generation,” which migrates to Mexico in the fall and then back to the southern United States in the spring.
How do we know that New England monarchs actually make it to Mexico and are not enjoying the winter in Florida or the Caribbean where there are some resident, non-migratory monarchs? Like my daughter and I, volunteers across New England capture monarchs and place tags on their wings each fall. With over 10 million monarchs out there, the odds of a recapture are very poor. For example, in Vermont we have had only a few lucky folks. Despite my efforts, I am not one of them. There have been seven recaptures from (or from close to) Vermont, including one found in Mexico. The lucky tag was applied in Essex Junction on September 9, 1999, and the monarch was found in El Rosario, Mexico on March 1st, 2,320 miles away!
Hydrogen isotope analysis also shows that New England monarchs winter in Mexico. Rainwater contains slightly different amounts of these isotopes across North America, and this unique chemical signature is transferred from rainwater to milkweed to caterpillars to adult monarchs. By selecting 100 sites scattered across eastern North America, researchers have created a map displaying the chemical levels for each region.
I raised monarchs from laboratory stock on the Vermont Institute of Natural Science’s Bragdon Preserve in Woodstock for this project in 1997 to contribute a Vermont site to the project. Later in the year, researchers went to the 13 wintering sites in Mexico and gathered over 500 butterflies that had died of natural causes. They examined the levels of isotopes in these and found that 50 percent came from the Midwestern corn and soybean belt, and that a small percentage had indeed come from Vermont.
By now, the autumn monarch migration is in full swing. If you want to contribute more monarchs from your property for the flight south, consider cutting part of your milkweed patch by the end of July next year to produce succulent young milkweed shoots for the final generation. The females prefer to lay their eggs on these shoots rather than on fully grown milkweed that is in bloom.
My daughter checks to see if the tag has adhered to the monarch’s wing. I note the date, location, and tag code in my notebook. The monarch flaps out of my grip and climbs high into the clear sky and glides out of sight. Maybe CAK 700 will be our lucky tag. Buen viaje, monarca!