The impact of transgenic crops on the environment has stirred
much interest in some circles. Thus the report by Losey et
that Monarch larvae were poisoned by pollen from Bt corn was
guaranteed to spark debate.
|The Monarch larva is the caterpillar life
stage of one of North America's most colorful and familiar
natives, the Monarch butterfly. The larvae feed only on
the leaves of milkweed plants, which are commonly found
in both natural habitats and cultivated fields. Because
their diet does not include corn plants, Monarchs are
non-target organisms in the scheme of insect pest control
in a corn field. Yet as members of the insect order Lepidoptera
(butterflies and moths), they are susceptible to poisons
that affect Lepidopteran pests. One such Lepidopteran
pest, the European corn borer, is the main target of the
Bt Cry1 toxins in transgenic corn varieties.
Monarch butterfly on alfalfa flower
Photo: Marlin E. Rice
Does the effort to kill European corn borers imperil Monarch
butterflies as well?
||Losey and two fellow researchers suggested
that it might. In laboratory tests, they dusted Bt corn
pollen onto milkweed leaves and then gave the leaves to
Monarch caterpillars to eat. Additional caterpillars were
fed on leaves dusted with conventional corn pollen or
leaves without pollen. The caterpillars eating leaves
dusted with Bt pollen ate less and grew more slowly. Over
four days, nearly half of the caterpillars on Bt-dusted
leaves died while no caterpillars died in the other two
groups. Five replications of the treatments provided enough
data to indicate that the results were statistically significant.
Larva of a Monarch butterfly
feeding on a milkweed leaf
Losey and his co-workers reported their findings to the weekly
scientific journal Nature, and a brief story with graphs showing
the reduced feeding and the number of deaths was published
in May 1999. The reaction was immediate and extreme. Opponents
of transgenic crops hailed the report as proof that Bt corn
was an environmental nightmare, while supporters of transgenic
crops criticized the research methods and dismissed the study
as "preliminary" and "flawed." Agencies of the U.S. and Canadian
governments, state agencies, growers groups, and the big agrobiotechnology
companies--Aventis, Dow, du Pont, Monsanto, and Syngenta--jointly
funded a set of studies in Canada and the United States in
the summer of 2000 to explore the main issues:
- Does Bt corn pollen kill Monarch caterpillars in laboratory
tests and/or in field tests?
- How likely is it that Monarch caterpillars will be exposed
to corn pollen under natural conditions?
But this transgene is used in a very small (and decreasing)
portion of the corn acreage in North America, less than 2
percent by one estimate. The transgenic components in the
corn varieties most commonly grown in North America, MON 810
and Bt 11, do not produce much of the Bt toxic protein in
pollen. Larvae can eat large amounts of this pollen, more
than 1,600 grains of pollen per square centimeter of leaf
surface, without showing ill effects.
Table 1 shows the amounts
of pollen from Bt 176, Bt 11, MON810, and several other Bt
corn sources that were tested on butterfly larvae.
Bt 176 corn is more toxic to butterfly larvae than other
versions of Bt corn because the promoter used in Bt 176 is
very effective in causing Bt protein to be produced in the
corn pollen. Table 2 shows
how Bt 76 differs from Bt 11 and MON810.
Because the vast majority of the Bt corn now grown in the
U.S. does not produce pollen that kills Monarch butterflies,
experts conclude that the risk to Monarch populations from
Bt corn is low.
Other factors contributing to the low risk estimate:
- Corn pollen is shed for only about two weeks of the summer,
whereas Monarchs produce several generations of offspring
over the course of a summer, so some generations of caterpillars
will not be exposed to pollen.
- Rain washes some of the pollen off the leaves, so caterpillars
may be exposed to much less than the potential maximum amount
of pollen, depending on the frequency and severity of rain
during the period of pollen shed.
- Most of the pollen produced in a corn field falls within
the field or nearby. Concentrations of pollen fall rapidly
as you move away from the corn field, so caterpillars feeding
on milkweed plants at a short distance from corn fields
are unlikely to encounter any pollen.
3 shows the concentrations of pollen deposited on leaves
at various distances from corn fields. The highest amount
of pollen measured within a corn field in these studies is
harmless to Monarch larvae if the corn type is Bt 11 or MON810.
Factors contributing to an increased risk for caterpillars:
- In U.S. study areas, Monarch butterflies seemed to show
a preference for laying their eggs on milkweed plants inside
corn fields (Oberhauser
et al., 2001), where the caterpillars might be exposed
to heavy doses of pollen if they are feeding during the
two week period of pollen release. However, in the Canadian
study area, Monarch seemed to prefer natural habitats over
- Survival of young caterpillars was higher in corn fields
than in other habitats.
Thus, it seems likely that most of the Monarch butterflies
seen in the Midwest begin their lives in corn fields (Oberhauser
et al., 2001).
Scientists continue to discuss the issues raised by Bt corn.
A good exchange of letters on this topic is available in the
November 2001 issue of the journal BioScience. The citation
is BioScience 51(11):900-906.
The USDA has a question-and-answer page on Monarchs and Bt
corn at http://www.ars.usda.gov/is/br/btcorn
and a discussion of the controversy at http://www.ars.usda.gov/sites/monarch.
For a review of the controversy surrounding Monarchs and
Bt corn see
Page last updated : March 11, 2004
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