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Posted: March 2014

Where Have All the Monarchs Gone?
Craig Holdrege

Genetically modified (GM) crops that are resistant to the herbicide glyphosate do not die when sprayed with this plant poison. Weeds do (at least until they evolve resistance; see Service 2013). GM glyphosate-resistant crops were first grown in 1996 and were increasingly adopted in subsequent years so that in 2013 they made up 93% of the soybeans, 85% of the corn, and 82% of the cotton in the U.S. (Fernandez-Cornejo 2013). About 163 million acres (the combined area of California and Oregon) of these glyphosate-resistant crops were grown in the U.S., mainly in the Midwest, in 2013. To keep any other plants from growing in the fields, each acre is sprayed annually with an average of one-and-a-half to two pounds of glyphosate.

One of the weeds in crop fields that Midwestern farmers are happy to see disappear is the common milkweed. As a result of the extensive spraying of glyphosate, its populations have been dramatically reduced since the advent of herbicide-resistant GM crops. One study estimates an 81% decline in milkweeds in Midwestern agricultural fields between 1999 and 2010, a 31% decline in non-agricultural habitats such as roadsides, and an overall decline of 58% (Pleasants and Oberhauser 2012; see also Hartzler, 2010). This may be good news for biotech farmers, but it is not necessarily good news for the larger natural world that is bound up with the life of common milkweed.

This brings us to the monarch butterfly. Its larvae feed on milkweed leaves. After metamorphosis, and later in the season, the adult butterflies that live in the Midwest and eastern U.S. make their long migration journey to a remarkably small area in the forests of central Mexico, where they overwinter. It is estimated that 92% of the monarchs wintering in Mexico fed on common milkweed when they were larvae, and that over half of that winter population originated in the Midwest (see Pleasants and Oberhauser 2012). Since monarch larvae are milkweed specialists, and they lay nearly four times more eggs on plants in agricultural fields than on plants in other habitats, it would not be surprising if this substantial loss of food plants were to result in a substantial reduction in the number of Monarch butterflies. And this is the case.

While it is impossible to count the exact numbers of Monarch butterflies, it is possible to discern whether the populations grow or shrink. When overwintering in Mexico, the butterflies gather in colonies of millions in the trees. They remain in quite circumscribed areas. The extent of these areas has been surveyed since 1993 (Rendon-Salinas and Tavera-Alonso, 2014). There has been a drastic decline in the areas inhabited by the overwintering monarchs, which dropped to 0.67 hectares (1.66 acres) in 2013. The average size of the area colonized by the monarchs during the past 21 years is about 6.2 hectares (15 acres); the largest area of 18.19 hectares (45 acres) was recorded in 1996. While there have been substantial year-to-year variations, the overwintering area has been decreasing during the past decade (an average of about 3.5 hectares between 2004 and 2013) and by far the smallest areas—reflecting reduced numbers of butterflies—were recorded in 2012 and 2013. Aerial surveys in the region have not discovered any new sites where the butterflies gather (Slayback and Brower 2007). Also, the shrinking area is not correlated with increased butterfly density; the opposite is the case: recent direct observations in Mexico indicate that the colonies are not as dense as they were previously (Ernest Williams, personal communication).

While no researchers believe that the reduction in common milkweed populations in the Midwest is the sole reason for the radical drop in the Monarch butterfly overwintering population, it seems clear that an unintended effect of the widespread planting of GM herbicide resistant crops has been to contribute to the drastic reduction in Monarch butterflies (Brower et al. 2011). Other factors are loss of overwintering habitat due to deforestation, and extreme weather conditions, such as freezing temperatures, that can decimate Monarch populations (Vidal et al. 2013; Brower et al. 2011).


Brower, L. P. et al. (2011). “Decline of Monarch Butterflies Overwintering in Mexico: Is the Migratory Phenomenon at Risk?” Insect Conservation and Diversity. doi:10.1111/j.1752-4598.2011.00142.x

Fernandez-Cornejo, J. (2013). “Recent Trends in GE Adoption,” USDA – Economic Research Service. Availaible online:

Hartzler, R. G. (2010). “Reduction in Common Milkweed (Asclepias syriaca) Occurence in Iowa Cropland from 1999 to 2009. Crop Protection doi:/10.1016/j.cropro.2010.07.018"

Pleasants, J. M. and Oberhauser, K. S. (2012. “Milkweed Loss in Agricultural fields Because of Herbicide Use: Effect on the Monarch Butterfly Population,” Insect Conservation and Diversity. doi:10.1111/j.1752-4598.2012.00196.x

Rendón-Salinas, E. and Tavera-Alonso, G. (2014). “Forest Surface Occupied by Monarch Butterfly Hibernation Colonies in December 2013,” Available online:

Service, Robert. (2013). “What Happens When Weed Killers Stop Killing?” Science vol. 341 (Sept. 20), p. 1329.

Slayback, D.A., & L.P. Brower (2007). “Further Aerial Surveys Confirm the Extreme Localization of Overwintering Monarch Butterfly Colonies in Mexico,” American Entomologist vol. 53, pp. 146-9.

Vidal, O. et al. (2013). “Trends in Deforestation and Forest Degradation after a Decade of Monitoring in the Monarch Butterfly Biosphere Reserve in Mexico,” Conservation Biology. doi:10.1111/cobi.12138

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