Investigating the long-term effects of Bt maize

Monarch butterflies: A threat to individual caterpillars, but not to the population as a whole

Renewed discussion of Bt maize and monarch butterflies: A recently published study has revealed that, in the long-term, pollen from Bt maize can damage the larvae of one of the USA’s most popular butterflies, the monarch. The conclusions drawn from this study could hardly be more different: the authors of the study see no threat to the monarch population, since only a small proportion of these butterflies come into contact with Bt maize. Greenpeace, on the other hand, is calling for a ban on the cultivation of Bt maize in Europe, since there are no comparable long-term studies involving native butterfly species.

The monarch butterfly is protected in the USA. It can be found in many regions in the summer, especially the Mid West. In Autumn it migrates to Mexico. Its offspring head north again the following spring.
(Photo: Monarch butterfly on milkweed).

The monarch caterpillars feed exclusively on milkweed, which grows in fields, meadows and waysides – and also in or on the edge of Bt maize fields.

It is not the first study to look at the potential risks posed by the cultivation of Bt maize to the monarch butterfly Daunus plexippus, which has protected status in the USA. The debate began five years ago, when a group led by John E. Losey from Cornell University (New York) showed in laboratory trials that the larvae of monarch butterflies can be harmed as a result of being fed Bt maize pollen. This finding provoked intense debate and media interest.

This was then followed by a two-year study involving the universities of Guelph, Iowa State, Minnesota, Nebraska and Maryland and also the US Ministry of Agriculture. This study looked at the extent to which monarchs do in fact come into contact with pollen from Bt maize in natural conditions and the likelihood of the butterflies actually ingesting Bt pollen in quantities which would be toxic to them. It was found that at most 0.012 percent of all monarch caterpillars in the USA are acutely threatened by Bt maize varieties currently grown commercially. The researchers rated this finding as a negligible risk.

However, these and other studies have focussed only on the consequences of butterfly larvae being exposed to Bt maize pollen for four to five days at the most. Under field conditions a small number of monarch butterflies (about 0.8 percent) are actually exposed to Bt pollen for twelve days or longer. The American Environmental Protection Agency (EPA) therefore recommended further investigations into the possible long-term effects.

Exposure to Bt maize pollen over several days

Exposure to Bt maize pollen over several days. The results from five of these studies have now been published. Over the two-year period monarch larvae both in the laboratory and in the Bt maize fields were continually fed with plant leaves dusted with Bt maize pollen until they developed into adult butterflies. The amount of maize pollen on the leaves corresponded to the typical concentration found in maize fields during flowering.

According to the research, prolonged consumption of Bt maize pollen has a harmful effect on the caterpillars and on their development. The number of larvae which developed into butterflies either in a Bt maize field or under conditions simulated in the laboratory fell by 23.7 percent. The developmental time increased on average by 1.8 days and the weight of the adult butterflies was approximately 5.5 percent lower than the control insects, which did not come into contact with Bt maize pollen.

But the harmful effects pose a threat only to those monarchs which ingest Bt pollen over a long period of time. The effect of Bt maize on the total monarch population cannot be determined without first considering just how many butterflies are exposed to Bt maize in the Corn Belt, the largest area of maize cultivation in the USA. Researchers have worked out two scenarios, based on the assumption that 2.4 percent of the monarch butterflies in the 16 states of the Corn Belt are exposed to Bt maize pollen.

  • Scenario 1: Since Bt maize increases the mortality rate of the directly affected monarch butterflies by 23.7 percent, it follows that the total population in this region will decrease by 0.6 percent.
  • Scenario 2: It is assumed that the increased developmental time and the reduced weight of the adult butterflies, which were also measured, will have an additional negative impact on the fitness of the butterflies and so further increase their mortality rate. Based on this – admittedly theoretical – assumption, all butterflies which come into contact with Bt maize pollen will be harmed to such an extent that they will either die or fail to reproduce. As a result 2.4 percent rather than 0.6 percent of monarch butterflies in the Corn Belt would be affected.

The authors point out that about half of the monarch population live in the US Corn Belt. Very little Bt maize is grown outside this region. Furthermore, the flowering period and the appearance of the larvae in this area do not coincide. So the calculated negligible harm would only be expected in the Corn Belt. The report states that Bt maize cultivation in the USA poses no risk to the monarch population. According to the authors, Bt maize reduces the amount of insecticide used to control the European corn borer. Pyrethroids, which are widely used, would kill monarch caterpillars on their forage crops within a few hours.

Bt maize and butterflies: a topic for safety research. In Germany there have so far been no detailed long-term investigations of this type into the potential risks to butterfly species from Bt maize. However, several projects are being funded in the field of biosafety research. According to the findings so far, there are no indications that pollen from Bt maize (MON810) has a harmful effect on native butterflies. The use of insecticides, however, does reduce the numbers of caterpillars on the fodder crops.