Monarch butterfly in trouble in Mexico

October 2011
Monarch butterfly
Picture from Wikimedia Commons

In 1999, American citizens were briefly alerted to encroaching GM crops, and to the possibility such crops might not be totally benign.

The first warning didn't concern any risks to human health, soil integrity, agricultural sustainability, environmental stability, nor economic and commercial interests. Ironically, it sounded an alarm that their favourite and prettiest butterfly might be under threat.

Monarch butterflies, fluttering the length of America from the Mexican to the Canadian borders have become iconic. So much so that the striking orange-and-black insects have been adopted as state symbols, and have formed the impetus for butterfly festivals. Indeed, perhaps the American sons of pioneers identify with the monarch's yearly migrations during which it travels thousands of miles in search of suitable food and shelter.

The study which caused the furore in 1999 investigated what happened when monarch caterpillars were fed on leaves of the milkweed plant dusted with pollen from GM Bt-toxin producing maize. It found that 44 percent of them died. Because milkweed forms the sole food of the monarch caterpillars and milkweed is commonly found in maize fields, this suggested GM crops might decimate America's number one icon.

Predictably, the biotech industry financed a study of its own to refute the finding, and claimed that the amount of Bt pollen to which monarch's would be exposed under real-life field conditions would be negligible and, therefore, unlikely to be lethal. The science which suggested harm was effectively dismissed as 'wrong', and the fuss died down.

However, like all things GM, the fuss has re-emerged.

A new study has suggested another threat to the monarch.

Monitoring of the numbers of butterflies over-wintering in Mexico year-on-year has revealed a steady decline over the last 17 years, apart from a slight increase in the 2010-2011 season.

The authors of the study have speculated that contributory factors are likely to be the recent extreme weather conditions, over-logging in the butterfly's over-wintering habitat in Mexico, and the reduction of milkweed prevalence due to the rising use of Roundup herbicide on GM Roundup Ready crops in the US.

To understand the significance of Roundup Ready crops, the precarious migratory life-cycle of the monarch must be appreciated. The adult butterflies live for only 4-5 weeks, during which time their only breeding habitat is the milkweed. Eggs must be laid on milkweed leaves to provide food for the caterpillars. No single individual makes the whole astonishing migration across the North American continent, which can be as far as 2,800 miles, but successive generations 'leapfrog' from milkweed to milkweed.

In autumn, a special generation is born which lives for 7-8 months, allowing them to escape the harsh winters of North America and hibernate in Mexico until the warmth of spring signals the start of a fresh migration to the breeding grounds.

Until the advent of Roundup Ready crops in 1996, milkweed was a common 'pest' in stands of corn and soya throughout the midwest. Now that 72% of corn and 94% of soya are GM and liberally sprayed with Roundup herbicide, the milkweed has been largely obliterated in this huge area. Studies in Iowa have indicated that milkweed losses stand at up to 90% over the period 1999-2009.

A study published in 2001 estimated the importance of 'weedy' cropland to the butterflies. It found that farms in Iowa, Wisconsin and Minnesota produced 73 to 78 times the number of monarchs as non-agricultural sites. Recent figures to be published shortly indicate that monarch egg production in such previously major breeding grounds has now declined significantly due to the loss of milkweed.

Needless to say, the bigger picture is far more complex. Monitoring of the numbers of monarchs making the return journey in Michigan and New Jersey (i.e. north and east of the midwest breeding grounds) did not show a decline. Another study (locations unspecified) suggested year-to-year fluctuations but no decline.

Yet the numbers of the butterflies in their warm Mexican haven are going steadily down.


What are we seeing here?
  1. The science tells us that GM crops have created 'gaps' in the once plentiful supply of milkweed in the midwest. The monarch however seems to be wise enough to re-route its migration to skirt areas of food-deprivation and favour areas (to the north and east) with the resources it needs. Following a changing milkweed path may well be why the insects developed the migratory habit in the first place.
  2. It seems that the harsh weather conditions didn't bother the monarchs in Michigan nor in New Jersey.
  3. While the Mexican government has been taking action to stem the logging activities and increase the size of nature reserves, the area colonised by the over-wintering monarchs has continued to decline.
So, it seems that there is enough milkweed surviving on the continent to support the insects, and that neither the weather nor the destruction of its southern habitat are key damaging factors.

Why then are there fewer of the butterflies in Mexico?

Remember the story of the caterpillars and the Bt-laden pollen in 1999?

In 2001 one scientific reviewer responded to the Bt controversy by pointing out that:
“The sublethal impacts on the monarch butterfly and other Lepidoptera are also still not completely understood at this time. For example, it is still not clear what consumption of Bt pollen at sublethal levels will do to susceptibility to natural enemies and disease, nor to interactions affecting migratory or reproductive success of the monarch butterfly populations. The results in one study suggest that natural enemies can be major mortality factors in and around the cornfields. Similarly, Bt sprays can slow larval growth and enhance the mortality to Lepidoptera caused by predatory and parasitic activity”.
This leaves another niggling doubt about the cause of the monarch's decline. During the years in which their food supply was decreasing, their exposure to an increasing cocktail of different Bt pollens has been increasing. Could chronic toxic effects which the short-lived adults can survive turn into something more sinister in the long-living over-wintering generation?

Who knows? But, two things are clear:

First, the use of death as the only measure of harm as promoted by Monsanto is misleading and dangerous.

Second, reductionist science which focuses on just one factor in isolation, be it food supply, individual toxins, habitat destruction, weather conditions, data from single locations, won't give us the answers we need about the dangers of GM. We need to establish multi-factorial stress tests in the lab and develop these into meaningful tests for use in the field. Science hasn't even reached the starting post on this and there are a lot of very important insects out there.

  • L. P. Brower et al., 2011, Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?, Insect Conservation and Diversity
  • A. K. Davis, 2011, Are migratory monarchs really declining in eastern North America: Examining evidence from two fall census programs, Insect Conservation and Diversity
  • J. Mark Scriber,2001, Bt or not Bt Is that the question?, PNAS 23.10.01
  • Dr. Evan Sirinathsinghji, Glyphosate and Monarch Butterfly Decline, Institute of Science in Society Report 19.09.11
  • Andrew Pollack, In Midwest, Flutters May Be Far Fewer, New York Times, 11.07.11

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