Posted: March 2014
Where Have All the Monarchs Gone?
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
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
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
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
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.
Rendón-Salinas, E. and Tavera-Alonso, G. (2014). “Forest Surface Occupied
by Monarch Butterfly Hibernation Colonies in December 2013,” Available
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.