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About Monarchs and Pesticides

Center for Food Safety

We all know that Monsanto’s genetically engineered crops and the pesticides they promote can wreak havoc on our farms, food, and environment. But did you know they are also playing a big role in the quickly disappearing Monarchs?

Monarch butterflies are one of the most beautiful and iconic insects in the world, and they are in serious trouble.  The Monarch butterfly population in North America has been shrinking at an alarming rate[i], and in 2012 was at the lowest level since consistent record-keeping began in 1994.[ii] Why? In part because a significant portion of their breeding habitat is being destroyed by herbicides used on genetically engineered (GE) crops.[iii]

Roundup doesn’t kill Monarchs directly, but rather kills their primary food source. Milkweeds, or “Monarch flowers,” are critical to the Monarch’s survival because they are the only plants Monarch larvae will eat. But the most important for Monarchs— common milkweed—is being demolished largely by the increasingly rampant use of glyphosate used in conjunction with Monsanto’s Roundup Ready crops (glyphosate is the active ingredient in Roundup herbicide). In North America, successive generations of Monarchs lay their eggs on different kinds of milkweed plants across the U.S. and southern Canada during the spring and summer. After their winter migration, they return north early in the spring to emerging milkweeds to start the cycle again.  But fewer milkweeds support fewer Monarchs.


[i] Brower LP, Taylor OR, Williams EH, Slayback DA, Zubieta RR & RamíRez MI (2012) Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk? Insect Conservation and Diversity 5, 95–100; Brower LP, Taylor OR & Williams EH (2012) Response to Davis: choosing relevant evidence to assess monarch population trends. Insect Conservation and Diversity 5, 327–329.

[ii] Taylor C. (2013) Monarch Population Status. Monarch Watch Blog [online] URL: (accessed 9 June 2013); Rendón-Salinas E., Tavera-Alonso G. Monitoreo De La Superficie Forestal Ocupada Por Las Colonias De Hibernación De La Mariposa Monarca En Diciembre De 2012. [online] URL: (accessed 29 August 2013).

[iii] Pleasants JM & Oberhauser KS (2012) Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity 6, 135–144.