Tracking this migratory butterfly to its winter home west of Mexico City.
By Jennifer McCutchen
In September of 2004, thousands of monarch butterflies journeyed through Texas on their annual autumn migration toward their winter home, now known as the Monarch Butterfly Biosphere Reserve — a stretch of mountain forests in central Mexico that consists of protected sanctuaries where the monarchs overwinter. Anecdotal butterfly sightings provided by observers to entomologists who follow the orange and black monarchs indicated a substantial decrease in the number of migrating butterflies. Whereas monarchs generally travel along the 300-mile-wide central flyway zone that stretches from Wichita Falls to Del Rio, the width of the areas that included significant monarch sightings was much narrower in 2004, says Mike Quinn, entomologist for the Texas Parks and Wildlife Department.
The monarchs usually arrive in Texas in late August and journey to one of approximately nine winter roosting sites in the Transvolcanic belt west of Mexico City. These colonies are dispersed along a range that stretches from the western face of the Nevado de Toluca volcano to the northeastern region of the state of Michoacan. Here, at altitudes between 9,000 and 12,000 feet, they roost together in vast colonies among the oyamel fir trees. By March, the monarchs head north again — scattering throughout various states and provinces east of the Rocky Mountains.
While researchers rely on migratory sighting reports to predict the monarchs’ annual population, these estimates are often imprecise as it can be difficult to accurately assess how many butterflies actually make the migration each year. One reason for this is due to the fact that monarchs have the capacity to fly at surprising heights — up to 6,200 feet — sometimes beyond the range of unaided human vision. When winds favor the southward trek, the butterflies coast the thermal currents upward on rising air, says entomologist Bill Calvert, one of the first scientists to study the Mexican monarch colonies after scientists first discovered them in the winter of 1975. “It is possible for them to pass right over your location so high that you will not see them.”
The apparent reduction in the fall migration observations can be partially attributed to the freeze in early February 2004 in Mexico. If the conditions are favorable during the spring and summer, monarchs are able to recover their population numbers by the next annual winter census. However, last year’s cool temperatures during the spring and summer were not conducive to the monarchs’ recovery, notes Quinn.
The use of herbicides that kill milkweed could be factoring into the population’s continued low numbers. Monarch caterpillars rely on milkweed as their sole source of food. The female monarch will lay her eggs on the plant, and the growing larvae, or caterpillars, eat the milkweed leaves. These leaves contain toxins that do not hurt the caterpillar but are poisonous to predators — thereby making the butterfly poisonous as well. Herbicide-tolerant crops, the product of genetic modification, are now widely planted in North America. The increased use of genetically engineered crops that kill the milkweed can ultimately generate detrimental effects on the monarch populations.
“Still, it’s hard to know what is really going on,” Quinn adds. “The next three years should be the most instructive.”
Researchers are currently surveying the sanctuaries to calculate a final count on this year’s monarch population. All indications are that the current monarch population has been significantly reduced. Monarchs generally occupy just over 22 of the 138,000 acres that Mexico has set aside for monarch reserves. Karen Oberhauser, a biology professor of the University of Minnesota who is participating in the surveys, observes that the Sierra Chincua colony is the smallest that she has ever seen it — adding that the Mexican biologists estimate that the diameter of the colony now measures only about 2 acres. While the El Rosario colony was larger, the Cerro Pelon and Herrada colonies were the smallest, she says.
Though all anecdotal evidence points toward a low monarch population, if weather conditions are favorable during the spring migrations, the butterflies could experience a full recovery, states Quinn. At press time, just before the March migration begins, Quinn comments that conditions seem encouraging.