Science in Society Archive

Glyphosate & Monarch Butterfly Decline

Glyphosate destruction of monarch butterfly breeding grounds suspected Dr Eva Sirinathsinghji

Monarch butterfly migration abundance has been declining over the last 17 years; a new study finds [1]. Extreme weather conditions, over-logging of their migratory destination in Mexico and the herbicidal destruction of their breeding grounds in the US are to blame.

Monarch butterflies (Danaus plexippus), also known as milkweed butterflies, are famous for their spectacular migration from North America to Mexico over the wintering months. The adults live for only 4-5 weeks, but another wonder of these butterflies is that one special generation born in the autumn lives for 7-8 months enabling them to make this astonishing journey that can be as long as 2 800 miles.

Millions descend on central Mexican forests every year to escape the harsh winters of North America. They hibernate until the warmth of spring brings them back to life, when they make the journey back to their breeding habitat on milkweed plants in North America. No individual makes the whole return journey, but instead the short-lived offspring lay eggs along the way, until their descendents make it back home.

This spectacular phenomenon is now showing significant signs of decline in a study led by Isabel Ramirez at Universidad Nacional Autonoma. They analysed data of the total area occupied by the butterflies in hibernation over the last 17 years (published online by World Wildlife Fund-Mexico since 1994) using two different statistical methods, and both showed significant decreases. These are standard regression analyses for determining the correlation between two variables, in this case colony area and time. The linear model assumes simple linear relationships between the two, while the exponential model assumes exponential decreases in area over time (as populations tend to grow or decrease exponentially, this is a common method for analysing population numbers). Although the numbers vary from year to year, the highest area reported was in 1997, where they occupied 20.97 hectares. In 2010, the lowest area was recorded, at just 1.92 hectares. The 2010-11 season has shown a slight increase to 4.02 ha. The results are presented in Figure 1.

Figure 1  Decline of monarch butterfly over time. Bars represent the total size of colonies over 17 seasons from 1994-5 to 2010-11. The dashed line shows the average colony size over this period of 7.24 hectares. The lines show the total area calculated by the regression statistical analyses using the linear model (upper) and exponential model (lower).

In the summer months in North America, eggs are laid on milkweed plants. The larvae feed exclusively on milkweed leaves, making the abundance of monarch butterflies critically dependent on milkweed availability. The spread of GM crops and the concomitant use of herbicides however, are threatening the milkweeds survival and their numbers have been steadily decreasing. Milkweed commonly grows among maize and soya, of which 23 and 92 percent are currently glyphosate tolerant. Studies assessing milkweed populations in Iowa recorded a 90 [2] and 79 percent (unpublished) loss between 1999-2009 and 2000-2009 respectively. The authors go on to speculate that with such widespread glyphosate usage, milkweed may almost completely disappear from crop lands altogether.

Additional increases in biofuel production and development led the authors to estimate a total loss of milkweed habitat to be 56 million hectares, more than one fifth of the north eastern US breeding range. The study concludes that this huge decrease in breeding habitat must be significant in accounting for dwindling butterfly numbers.

This is not the first time that scientific studies have warned against the effects of GM crops and glyphosate herbicides on monarch butterfly numbers. Back in 1999, three years after the launch of GM Bt corn into the US market, a report in Nature documented that Bt corn pollen, when dusted over milkweed plants, reduced the survival of larvae by up to 44 percent [3]. Although performed under artificial conditions, it is a proof-of principle study showing the potential harm of such products on non-target invertebrates.

Biotech companies have argued that exposure to Bt corn pollen may be minimal and thus have negligible effects on butterfly numbers. It is much harder to argue against the harmful effects of the rapid disappearance of the only monarch butterfly larval food source.

The authors also blame extreme weather conditions, as well as illegal over-logging in Mexico as contributors to the declining population numbers. One study found a 44 percent decline in high quality forest area due to logging degradation from 1971 to 1999 across a 40,000 ha area [4]. Increasing the size of nature reserves by the Mexican government has stemmed the logging, but has not put an end to the practice completely.

This study adds further evidence to the harmful effects of glyphosate to the ecosystem and the natural diversity of insects and plants alike (see [5] Scientists Reveal Glyphosate Poisons Crops and Soil. GM meltdown continues, SiS 47, [6] Glyphosate Tolerant Crops Bring Diseases and Death, SiS 47), and its devastating health impacts (see [7] EU Regulators and Monsanto Exposed for Hiding Glyphosate Toxicity, SiS 51). It is the most popular herbicide in the US, and as a consequence it will have far reaching effects on the environment.

A study released by the U.S. Geological Society last month assessed the presence of glyphosate in air and rain. Detection ranged from 60 to 100 percent, highlighting how exposed the environment and people of the US are to this herbicide [8]. Its use needs to be urgently curbed to prevent any further damage to health and the environment.

Article first published 19/09/11


  1. Brower LP, Taylor OR, Williams EH, Slayback DA,  Zubieta RR, Ramírez MI. Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk? Insect Conservation and Diversity 2011, doi: 10.1111/j.1752-4598.2011.00142.x
  2. Hartzler, R.G. 2010. Reduction in common milkweed (Asclepias syriaca) occurrence in Iowa cropland from 1999 to 2009. Crop Protection. 29, 1542–1544.
  3. Losey JE, Rayor LS, Carter ME. Transgenic pollen harms monarch larvae. Nature 1999, 399, 214.
  4. Brower LP, Castilleja G, Peralta A, Lopez-Garcia J, Bojorquez-Tapia L, Diaz S, Melgarejo D,  Missrie M. Quantitative changes in forest quality in a principal overwintering area of the monarch butterfly in Mexico:  1971 to 1999. Conservation Biology 2002, 16, 346–359.
  5. Ho MW. Scientists reveal glyphosate poisons crops & soil. GM meltdown continues. Science in Society 47, 10-11, 2010.
  6. Ho MW and Cherry B. Glyphosate tolerant crops bring diseases and death. Science in Society 47.12-15, 2010.
  7. Sirinathsinghji E and Ho MW. EU Regulators and Monsanto Exposed for Hiding Glyphosate Toxicity. Science and Society 51, 46-48, 2011.
  8. “USGS: Glyphosate pollutes air, rain and rivers in US”, The Peoples Voice, September 12th, 2011

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There are 3 comments on this article so far. Add your comment above.

Rory Short Comment left 20th September 2011 02:02:09
It is in a sense pointless opposing each of the aberrations that emerges because until we change our basic orientation toward what lies outside of ourselves to one of cooperation and away from one of exploitation, conditions such as those described above will continue to proliferate until the consequences of our own destructiveness destroys us too.

Todd Millions Comment left 22nd September 2011 17:05:38
I live on a major monarch path,with pastures containing choice feed-showy milkweed.I've not seen a monarch in 3years and the milk weeds are showing stress and poor reproduction.Granted the clime of the last two years has being poor for both.Some analouge species from high land nortern states do seem to be filling this pollenatiors gap-but not as fully,they simply don't reproduce quickly enough.

Chef John Rasmussen Comment left 1st November 2012 01:01:50
My wife and I served as Monarch butterfly counters at Point Pelee Provincial Park in Ontario, Canada. I recall in the mid 1980's when Monarch migration peaked, trees filled with Monarchs rest overnight. In early morning sunlight Monarchs open & close wings until a magical moment when colorful Monarchs fly in mass to cross open water of Lake Erie. It looks like the "leaves of the trees" turn orange and fly away leaving an almost leafless tree. It is very sad that chemical companies justify toxic chemicals as harmless when in fact toxic chemicals used on corn and soybean can kill majestic Monarch butterfly by destroying milkweed habitat, the main Monarch food source.