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August 28, 2000


Questionable Conclusions From Latest Monarch Study: Review


Questionable Conclusions From Latest Monarch Study

By MARK SEARS, Ph.D., University of Guelph
ANTHONY SHELTON, Ph.D., Cornell University

A recent paper titled, "Field deposition of Bt transgenic corn pollen:
lethal effects on the monarch butterfly", by Laura Hansen and John Obrycki
from Iowa State University, was published online in the European journal
Oecologia. It has been described as the first field study which describes
the effect of certain Bt corn pollen deposits on milkweed leaves and their
effects on larvae of the monarch butterfly. This report has received
considerable media attention and we believe it is important to offer some

We believe careful examination of the methods, conclusions and discussion
of this paper are warranted, and that such examination raises serious
questions about what, if any, conclusions can be realistically drawn from
this study. As field biologists we realize the difficulty in conducting
quality field research relevant to real world situations, but we believe
this study does little to help understand potential risks of deploying Bt
plants in the field.

In order to assess the deposition of pollen in the field, the authors
placed potted milkweed plants in or at various distances from the edge of
corn fields during pollen shed. Samples of milkweed leaves were brought to
the laboratory and newly hatched larvae of Monarch butterflies were placed
on leaf disks cut from the leaves and fed for 48 hours. While we
appreciate the difficulty of setting up these tests, these method make the
assumptions that the placement of the potted plants simulates the natural
distribution of plants in and near corn fields and that adult monarchs
would find and lay their eggs on these plants. These assumptions, as
well as the assumption that Monarch populations occur at the same time as
pollen shed, go to the heart of the matter of whether Monarch larvae will
be exposed to lethal concentrations of natural depositions of Bt pollen.
Furthermore, we are aware of work indicating that Monarch butterflies
avoid laying their eggs on leaf tissue with high deposits of pollen. These
factors, along with several others, make us question the "field" aspect
of this paper.

In order to assess the dose of pollen required to cause mortality of newly
hatched monarch larvae, another set of experiments was conducted. Pollen
was collected from the field and applied to leaves in the laboratory at
three different doses (14, 135 and 1300 grains per square centimeter).
Small larvae were placed on disks and fed for 48 hrs. Mortality and
development and growth characteristics were assessed. The point is, only
the collection of leaf samples with pollen and the collection of the
pollen itself was carried out in the field.

Field trials are those that are conducted entirely under the conditions
prevalent in the field during the experimental period. Because of this,
the results will be affected by factors such as moisture on the leaf
surfaces, variable temperature and humidity, degradation of the pollen by
sunlight, moisture, microorganisms, rainfall, wind, natural dispersal and
behavior of monarch larvae, predation on monarch larvae, and a host of
other ecological factors, not to mention the adult Monarch choosing
whether or not to lay her eggs on a pollen-infested plant. Some or all of
these factors will have a direct result on the measurement of mortality
and may completely overshadow the effects of the Cry 1Ab toxin expressed
in the pollen. True field trials are necessary to understand the nature of
pollen deposition on milkweed plants and the possible effect on monarch
larvae or any other species of caterpillars.

Field trials are underway in an international comprehensive program
involving scientists from Maryland, Iowa, Nebraska, Ontario, Canada and
other places and the first year results have been discussed at several
scientific meetings. The data from the comprehensive effort provide
information on many of the important questions not addressed in this study
such as whether natural populations of monarch are actually exposed to
lethal effects of Bt corn pollen in the field. We believe great caution
should be exercised in assessing the Hansen and Obrycki paper until
publication of the scientific data from the international program's
efforts, which we hope will be done in a timely fashion. But for now we
must address our concerns with the present study.

A misleading statement by the authors and media implied that the results
implicate all Bt corn types (or events expressing Cry 1Ab toxin). In the
study, only pollen from Event 176 Bt corn showed any consistent lethal
effect. Event 176 Bt corn represented about 2% of the total Bt corn
acreage planted in North America in 1999 and probably is no more than 1%
of the acreage in 2000. The pollen from Event 176 is considerably more
toxic than pollen from Bt11 or MON810, the leading Bt corn products. It is
misleading to imply that toxicity associated with that product is relevant
to Bt hybrids planted by the vast majority of farmers.

The conclusions of the authors go far beyond the extent of the data
presented. They imply in their discussion that significant amounts of
pollen could be distributed within and up to 10 meters outside of
cornfields such that significant mortality to monarch larvae would occur.
Their own data do not support this speculation. Nowhere have they reported
on the density of milkweed plants in or around fields or in different
habitats in the area, nor have they provided any information on the
phenology of the monarch populations in relation to the pollen shed period
of the Bt corn hybrids. These are the points being addressed in the
international comprehensive program. Without this information, it is
improper to speculate on the risk associated with the Cry 1Ab toxin found
in some Bt corn hybrids.

A more detailed discussion of study findings and procedures, some of which
raise questions, follows:

1. Toxicity results are not consistent with those of other studies.
Other researchers have found that pollen from Event Bt11 (and other events
with similar expression levels such as MON 810), is not toxic to Monarch
larvae at a dose of 150 grains per square centimeter or less, yet the
Hansen and Obrycki study found mortality at 135 grains per square
centimeter. A possible reason for this may be that the pollen used in the
laboratory feeding studies was contaminated. The authors found that the
pollen they collected from Bt11 plants contained 4.3 times more toxin than
had been seen in previous replicated studies accepted by the Environmental
Protection Agency (0.39 micrograms per gram at Iowa State vs. 0.09
micrograms in EPA-accepted data). The authors acknowledge that the
collected pollen may have been contaminated with anthers, a part of the
corn plant that retains pollen. Likewise, the toxin concentration in
Event 176 was calculated to be several times less toxic than previous
replicated research had shown. In addition, the pollen from non-Bt plants
was found to contain trace levels of Bt. The authors acknowledge that it
may have been contaminated during collection. These incongruities raise
questions about the purity of the tested material.

2. The sample size of larvae was small. With most biological assays of
this nature, usually hundreds of larvae are evaluated to give a realistic
sampling. Only 35 larvae were exposed to leaf sections containing pollen
from Event 176 actually taken from the field. Of the 35 exposed larvae, 20
percent (7 larvae) died. Given the small sample population, other factors
could have contributed to mortality, such as handling and transport. In
fact, the authors acknowledge that there was no correlation between dose
and mortality. The leaf sections contained doses varying from 10 grains of
pollen per square centimeter up to 306 grains. One would expect that
insects exposed to the highest concentrations would have the highest
mortality rate. This apparently was not the case, leading to the
possibility that at least some mortality was linked to other factors. In
the experiment where larvae fed on leaves dosed with 3 levels of pollen,
the sample sizes were also small (10 and 16 larvae per treatment).

3. The concentrations that caused toxicity were not representative of
field concentrations they report. The larvae were exposed to three
selected concentrations - 1300 grains of pollen per square centimeter of
milkweed leaf surface, 135 grains per square centimeter, and 14 grains per
square centimeter. No adverse effect was seen at the lowest dose. While
significant mortality was seen at the middle dose and the highest doses,
the actual field sampling showed that these doses were rarely seen near or
even within the cornfields. The authors predict that concentrations of
1300 grains per square centimeter could be seen within fields, but their
survey showed a range of 0 to 152 grains within the field. The average
concentration of Bt11 within the cornfield was 74.2 grains in 1998 and 115
grains in 1999.

4. Larvae were exposed to very small sections of leaf and had no choice of
diet. The authors collected milkweed leaves from the field, and then they
cut out small disks (0.79 square centimeters). The disks were placed on a
moistened filter paper in a petri dish. One neonate (first instar) larva
was placed on the top of each disk for 48 hours. This is a very artificial
environment. In the wild, larvae would most likely hatch on the underneath
side of a leaf, sheltered from any pollen that may have collected on the
upper surface. They also would be free to move across the entire leaf
surface (about 82 square centimeters per side) or even to move to other
leaves or adjoining plants. They would not have to eat pollen. Other
research has shown that larvae tend to avoid pollen of any type. Other
observations have shown that pollen on milkweed leaves congregates in vein
depressions and folds, leaving most of the leaf surface unexposed.

5. Older larvae that were exposed to concentrations in the laboratory had
much higher survivorship than neonate larvae. Two studies were conducted;
one used larvae less than 12 hours old and the other used larvae 12 to 36
hours old. At the rarely seen dose of 135 grains per square centimeter,
only 30 percent of young larvae exposed to Event 176 survived, but 63
percent of older larvae survived. For Bt11, the survival was 40 percent
for young and 75 percent for older. This is a significant finding, because
Monarchs tend to lay their eggs on the underside of leaves, sheltered from
pollen. The neonate larvae would most likely spend their first 12 hours
feeding on the underside of the leaf.

6. Survival rates at various doses raise questions. The study showed a 40
percent survival rate for larvae exposed to Bt11 at 1,300 grains as well
as 135 grains. It is very unusual that a dose 10 times lower would produce
the same effect. It is also interesting that larvae exposed to 1,300
grains of non-Bt pollen had a 40 percent survival rate, the same as those
exposed to 1,300 grains of Bt11.

7. Larvae that survived exposure developed into normal adults. There was
no difference between total development times for exposed and unexposed
larvae. Pupal weights, adult dry weights, lipid weights and forewing
lengths were also similar among exposed and unexposed insects.

8. We also would like comment on what we think is an important point in
the paper but which has not received adequate comment, i.e. what are the
risks and benefits of Bt corn compared to alternative methods of control
of the European corn borer? We believe this is the crux of the argument
about the future of Bt crops, and question a major point made in the last
paragraph of the paper. The authors are trying to make the point that the
comparison of the safety of transgenic crops like Bt corn may not be
"appropriate if previous insecticide use against the target pest was low".
Their claim that in 1995 only "2.2% of the corn in Iowa was treated with
broad-spectrum insecticides for control of the European corn borer" is a
bit of a misleading snap shot. Marlin Rice, an experienced corn
entomologist who is the same Department as the authors, was recently
quoted in the press as stating from a recent survey of corn growers that
"27 percent of (corn growers) wanted (Bt corn) to eliminate the need for
insecticides to control the pest (corn borers)". In addition, foliar
sprays of insecticides do not provide maximum control since timing is
crucial and because it is difficult to get the insecticide to the target
pest, especially when it is applied by air. Such concerns are not issues
in Bt corn. For this reason, the comparison the authors try to make is
also misleading.

The examples given above illustrate that the study does not duplicate a
field environment, and they raise questions about whether the tested
material was more toxic than pollen that would occur in the field. The
lack of a dose response is particularly troubling, as is the small sample
size. The study does confirm what has been seen in various field studies -
that pollen density drops off rapidly at the edge of a cornfield.

The results of these experiments do not represent the potential impact of
Bt pollen from all events, as the authors suggest, across the extensive
range of the monarch butterfly in North America. We await the results of
the comprehensive international study which includes studies conducted
over a two year period (1999-2000) in the field and whose preliminary
results, already presented at several scientific meetings, provide a much
more complete and balanced assessment which indicates that Bt corn does
not pose a significant threat to Monarch populations as implied in this
study or as portrayed in some of the press.

Mark Sears, Ph.D. ` Anthony Shelton, Ph.D.
Professor of Entomology Professor of Entomology
University of Guelph NYSAES, Cornell University
Guelph, Ontario, Canada Geneva, NY 14456
MSEARS@evbhort.uoguelph.ca ams5@cornell.edu
519-824-4120 315 787 2352