Professor Mark Sears harshly critical of recent Iowa State Bt corn & monarchs study.

Paul Cherubini cherubini at
Sun Oct 8 16:25:55 EDT 2000

Copied from Issue #15 Crop Pest Ontario, Aug 25, 2000

Comments On Recent Reports Dealing With Bt Corn and
                         The Monarch Butterfly
                  Mark K. Sears, Professor and Chair
Department of Environmental Biology - University of Guelph

A recent scientific paper entitled, "Field deposition of Bt transgenic 
corn pollen: lethal effects on the monarch butterfly, which describes 
the effect of certain Bt corn pollen deposits on milkweed leaves and its
effects on larvae of the monarch butterfly has been published online by
the journal Oecologia (August 19- See paper abstract at end of this 
article). A CBS news report on the evening of August 21 followed up
on some of the issues presented in the paper. I feel it is important to
review these two presentations and comment on the nature of the 
information presented.

The paper, by Laura Hansen and John Obrycki from Iowa State 
University, does not constitute field research as claimed by the CBS
news report. Leaf samples of milkweed were collected from
locations in or at the edge of corn fields where pollen had been deposited 
naturally on the foliage. 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. The number of pollen grains that had
been deposited on each leaf disk were counted before being fed to 
individual larvae. The larvae were allowed to feed on the leaf disks for 
2 days (48 hrs), at which time mortality of the larvae was assessed. In
other experiments, pollen was collected from the field, applied to 
leaves at three different doses (14, 135 and 1300 grains/cm2), small 
larvae fed for 48 hrs and mortality and development and growth 
characteristics assessed. The studies themselves were conducted under 
laboratory conditions, where temperature, lighting and humidity were 
maintained to prescribed conditions. The point is, only the collection of
leaf samples with pollen and pollen itself was carried out in the field 
while assessments of their effects were conducted under controlled
laboratory conditions.

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 predation, moisture on the
leaf surfaces, variable temperature and humidity, degradation of the
pollen by sunlight, moisture, microorganisms, rainfall, etc., effects 
of wind and natural dispersal of the monarch larvae. 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 completely understand the
nature of pollen deposition on milkweed plants and the possible effect 
on monarch larvae or any other species of caterpillars.

A consistent and misleading statement by the authors and the CBS report 
indicate that the results implicate all Bt corn types (or events expressing 
Cry 1Ab toxin). The widespread assumption that all Bt corn hybrids
express the Cry toxin to the same degree in all tissues is incorrect and 
leads to erroneous conclusions about risk to monarch populations. 
The conclusions of the authors go far beyond the extent of the data presented.
In their study, only pollen from corn hybrids created from event 176 Bt corn 
showed any consistent lethal effect. The effects from consumption of pollen 
from Bt 11 hybrids (entirely measured in the laboratory and with extremely
small sample sizes, 10 and 16 larvae per treatment) were inconsistent and, 
in the case of the highest dose tested, not different from the control group. 
No sub-lethal effects were noted for any of the treatments in the small sample 
of insects reared to adulthood. In order to put these results in perspective, a
wider view of the use of Bt corn technology must be taken. 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 authors imply in their discussion that significant amounts of pollen 
could be distributed within and up to 10m ouside of corn fields such that
significant mortality to monarch larvae would occur. Their own data do not 
support this speculation. Only in one instance did they record more than 135 
grains/cm2 of Bt 176 pollen (the lowest toxic dose recorded), and that was 
from within the field. 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. Without this
 information, it is entirely improper to speculate on the risk associated 
with the Cry 1Ab toxin found in some Bt corn hybrids.

Therefore, the results of these experiments cannot possibly 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. In fact, the
combined impacts of destruction of overwintering habitat of the butterfly in 
Mexico, herbicide use and mowing that eliminates milkweed stands in and 
around field crops, and even mortality to migrating monarch butterflies by
collisions with vehicles are far more likely to impact on monarch populations
than the tiny fraction of Bt corn acreage that may deposit sufficient pollen to 
the extent that some larvae will be effected. In fact, the latest reports from 
Monarch Watch, an organization that has documented the changes in monarch
butterfly populations in North America for a number of years, have indicated
that populations of this insect are on the increase in the past few years. This
in the face of speculation that monarch populations are under severe threat 
due to the increased acreage of Bt corn as this new technology becomes more
widely accepted.

More information about the Leps-l mailing list