Tuesday, May 3, 2011

Decline of Monarchs

Here are two studies concerning the effect of herbicide resistant crops on the Monarch butterfly

Is the migratory phenomenon at risk?
By Lincoln P. Brower etal
Insect Conservation and Diversity
(2011) doi: 10.1111/j.1752-4598.2011.00142.x


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 significant 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.

Read the Brower study

Reduction in common milkweed (Asclepias syriaca) occurrence in Iowa cropland from 1999 to 2009
By Robert G. Hartzler
Crop Protection
(2010) 1542e1544


The role of common milkweed in the lifecycle of the monarch butterfly has increased interest in the presence of this weed in the north central United States. An initial survey conducted in 1999 found that low densities of common milkweed occurred in approximately 50% of Iowa corn and soybean fields. In 2009, common milkweed was present in only 8% of surveyed fields, and the area within infested fields occupied by common milkweed was reduced by approximately 90% compared to 1999. The widespread adoption of glyphosate resistant corn and soybean cultivars and the reliance on post-emergence applications of glyphosate for weed control in crop fields likely has contributed to the decline in common milkweed in agricultural fields.

Read the Hartzler study

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