http://www.saynotogmos.org/ud2011/fp-co ... r_2011.pdf
The PDF has the complete article.
Decline of monarch butterﬂies overwintering in Mexico: is the migratory phenomenon at risk?
1. During the 2009–2010 overwintering season and following a 15-year downward trend, the total area in Mexico occupied by the eastern North American population of overwintering monarch butterﬂies reached an all-time low. Despite an increase, it remained low in 2010–2011.
2. Although the data set is small, the decline in abundance is statistically signiﬁ-
cant using both linear and exponential regression models.
3. Three factors appear to have contributed to reduce monarch abundance: degradation of the forest in the overwintering areas; the loss of breeding habitat in the United States due to the expansion of GM herbicide-resistant crops, with consequent loss of milkweed host plants, as well as continued land development; and severe weather.
4. This decline calls into question the long-term survival of the monarchs’ migratory phenomenon.