* Department of Zoology and Genetics, Iowa State University, Ames,
IA 50011; Edited by M. R. Berenbaum, University of Illinois at
Urbana-Champaign, Urbana, IL, and approved August 17, 2001 (received for review June 7, 2001)
The density of corn pollen on leaves of milkweed plants inside and
outside of cornfields was measured in several studies from different
localities. The purpose was to obtain a representative picture of
naturally occurring pollen densities to provide a perspective for
laboratory and field studies of monarch larvae feeding on milkweed
leaves with Bt corn pollen. Pollen density was highest (average 170.6 grains per cm2) inside the cornfield and was progressively
lower from the field edge outward, falling to 14.2 grains per
cm2 at 2 m. Inside the cornfield, and for each
distance from the field edge, a frequency distribution is presented
showing the proportion of leaf samples with different pollen densities.
Inside cornfields, 95% of leaf samples had pollen densities below 600 grains per cm2 and the highest pollen density observed was
1400 grains per cm2, which occurred in a study with a
rainless anthesis period. All other studies had rainfall events during
the anthesis period. A single rain event can remove 54-86% of the
pollen on leaves. Leaves on the upper portion of milkweed plants,
where young monarch larvae tend to feed, had only 30-50% of the
pollen density levels of middle leaves.
In order to accurately
interpret results of studies that examine the effects of Bt corn pollen
on monarch butterfly larvae it is necessary to know the range and
distribution of naturally occurring pollen densities on milkweed
leaves. This provides a perspective on both laboratory and field
studies in which monarch larvae feed on milkweed leaves with Bt corn
pollen (1, 2). It lets us determine how frequently the pollen densities
observed in these studies would occur in nature. The studies reported
here contribute to the exposure characterization necessary for
assessing the risk of Bt corn pollen to monarch butterflies. In
particular, this paper describes the densities of corn pollen on
milkweed leaves during corn anthesis for a number of geographic
locations and under a variety of environmental conditions. We describe
the pollen densities (pollen grains per cm2) that
were found on leaves of milkweed plants within cornfields as well as
near cornfields because corn pollen is wind-dispersed at least 60 m (3) and possibly more than 200 m (4). These data are used in a
companion paper (5) on the results of laboratory studies on the
responses of monarch larvae fed milkweed leaves with different
densities of artificially applied Bt corn pollen. These data are also
used in a second companion paper (6) to provide a frame of reference
for the Bt pollen densities found in field trials of larvae feeding on
milkweed leaves. Finally, these data are used in a summary companion
paper (7) that provides a full risk assessment of monarchs and Bt corn
pollen. In addition to characterizing naturally occurring pollen
densities, we examined several factors that affect pollen deposition on
milkweed leaves, including position of a leaf on the plant and rainfall.
This article includes the results of several studies conducted at
different locations. The study locations, Iowa, Minnesota, Wisconsin,
Ontario (Canada), and Maryland, were chosen to cover representative
portions of the range of the Eastern population of monarch butterflies.
In general, the methods involved measurements of pollen from field corn
(mostly non-Bt) that had accumulated on leaves of naturally occurring
milkweed plants (Asclepias syriaca), or surrogates for
natural plants such as transplanted plants, potted plants, or cut plant
stems. Studies either measured pollen deposition on leaves of milkweed
plants inside a cornfield, outside a cornfield, or both. For milkweeds
along transects into and away from the field edge, the field edge was
defined as 0 m, negative numbers refer to the number of meters
within a field, and positive numbers refer to the number of meters away
from the field edge. Leaf samples were taken either during or at the
end of pollen anthesis, or both. The state of anthesis was determined
by examining a sample of tassels throughout the field and calculating
the proportion that was shedding pollen. Some studies measured the
ambient levels of pollen in the air at sampling locations by using
pollen deposition on sticky slides or sticky plates.
Studies of In-Field and Off-Field Pollen Deposition.
Maryland, 1999.
In two eastern shore and four central Maryland counties, 1317 leaves
were removed from 572 naturally occurring milkweeds located within and
at various distances from 81 cornfields. Sampling was conducted during
14-24 July when the fields had reached full anthesis, during which
time no rainfall had occurred in the sampled areas. For larger plants
leaves from the upper, middle, and lower portions of the plant were
sampled, otherwise an upper leaf was taken. Leaves were placed in
Ziploc bags and transported to the laboratory. Pollen was removed by
rinsing each leaf with water containing surfactant while brushing the
surface with a small soft-bristled brush. The Ziploc bags were also
rinsed. In some cases the rinse solution was filtered to trap pollen
and the filter paper dried and folded. Filters or wash sample were sent
to the U.S. Department of Agriculture, Agricultural Research Service
Pollen Laboratory at College Station, TX, where the samples were
processed and counted for corn pollen content according to the methods
described by Erdtman (8) and Jones and Coppedge (9). After pollen
removal the leaf area of each sample was measured by a Li-Cor leaf area meter (Li-Cor, Lincoln, NE). For each leaf, pollen density was calculated as the pollen count divided by the leaf area.
Agricultural Sciences
Corn pollen deposition on milkweeds in and near cornfields
,
,
,, and U.S. Department of Agriculture, Agricultural
Research Service, Corn Insects and Plant Genetics Research Unit,
Department of Entomology, Iowa State University, Ames, IA 50011;
§ Department of Entomology, University of Maryland,
College Park, MD 20742; ¶ Department of Environmental
Biology, University of Guelph, Guelph, ON, Canada N1G 2W1;
Department of Entomology, University of Nebraska,
Lincoln, NE 68583; and ** U.S. Department of Agriculture,
Agricultural Research Service-Areawide Pest Management Research
Unit, 2771 F&B Road, College Station, TX 77843
Abstract
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
Introduction
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
Materials and Methods
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
10, 5, 3, 1, 1, 3, 5, and 10 m
from the field edge. Plants within the field were placed between rows
and an additional plant was placed in the row directly under corn
plants at the 5- and 10-m positions within 2 m of the
corresponding plant situated between rows. Corn rows were the standard
0.76 m wide.
1.5, 0, 1, and 5 m
from the field edge. The sticky plates were made from Petri dishes
coated with sticky material (Sticky Stuff, Olson Products, Medina, OH).
Plates were placed horizontally on top of a 1-m-high wooden stake. The
plants and plates were placed in the field several days before the
onset of anthesis. Plates were changed daily for 16 days after the
commencement of anthesis and leaves were collected from each field 6 and 11 days after the onset of anthesis. A leaf was cut from the upper, middle, and lower third of each plant. Each leaf was immediately sandwiched between two strips of contact paper (ConTact7 Brand, Decora
Manufacturing, North Ridgeville, OH) to avoid the loss of pollen from
its surface. Plates and leaves were brought back to the laboratory and
frozen at 20 and 5°C, respectively, until pollen could be counted.
To determine pollen density, the sticky plates were stained with acid
fuchsin and computer images of the stained pollen in five
1-cm2 areas were created by AIMS Lab (Fremont,
CA) GRABIT IIJ version 1.10 software and a Panasonic
(Secaucus, NJ) WV-D5100 system digital camera mounted on a dissecting
microscope. Computer images were analyzed with Scion Image (Frederick,
MD) BETA 4.0.2 software to determine the pollen
density. Pollen density on milkweed leaves was evaluated by pulling the
contact paper strips away from the leaf, staining them with acid
fuchsin, and counting the pollen in five randomly chosen
1-cm2 areas on the top and bottom of the strips
between the leaf midrib and the leaf margin. Any pollen remaining on
the leaves was also counted in five 1-cm2 areas
and added to the counts made on the contact paper.
Off-Field Deposition.
Iowa, 1999.
Pollen densities on leaves and sticky slides were measured at several
points along transects perpendicular to the four sides of seven
cornfields. The fields chosen for sampling were sufficiently isolated
beyond the pollen drift zone of other cornfields. Fields were 
2.3 ha
and were surrounded by either soybeans or grass.
In-Field Deposition.
Iowa, 2000a.
Because rainfall may remove pollen from milkweed leaves, pollen
deposition studies conducted during rainy periods may not indicate the
maximum pollen densities to which a larva may be exposed. To estimate
the maximum amount of pollen that could be deposited on milkweed
leaves, cumulative pollen deposition over an entire rain-free anthesis
period (10 days) was measured. Two boutonnieres and one slide were
placed at four sampling locations within a cornfield, 6 m in from
the field edge. The slide and a leaf from one of the boutonnieres were
collected every 2 days from each sampling location. Slides were
replaced every 2 days, whereas boutonnieres were replaced with fresh
ones only as needed (the leaves remained fresh for 4 days). For
slides the densities of pollen measured every 2 days were added
together to estimate the total density of pollen available during
anthesis. For leaves the densities measured in separate sampling
periods were added together.
8 m in from the field edge, 32 plants per field. These plants were
examined for pollen deposition at three different times. On 14 July
three of the fields that exhibited 75% anthesis were examined. On 24 July all four fields were examined, three of which had reached 100%
anthesis and one that was at 75% anthesis. On 3 August all four fields
were again examined; three were 10 days postanthesis and the other was
at 100% anthesis. For purposes of the analysis, data from fields were
combined to produce three categories: 75% anthesis, 100% anthesis,
and postanthesis (10 days). At each sampling time a leaf was removed
from the upper third of three randomly chosen plants from each of the
12 fields.
Iowa, 2000d.
Before anthesis potted plants were placed along a single transect
within a cornfield. A cluster of six plants was placed at 0, 3, and
25 m in from the field edge. One leaf was taken from the top,
middle, and bottom third of each plant on 14 August (5% anthesis), 18 August (30% anthesis), 21 August (50% anthesis), 24 August (75%
anthesis), and 28 and 30 August (100% anthesis).
Data Analysis.
Laboratory and field feeding trials to determine the effects of Bt corn
pollen typically use first instar larvae (5, 6). During a feeding bout
these larvae carve out a leaf circle of 0.25
cm2, and during a 4-day feeding trial consume
1 cm2 of leaf tissue. Therefore, we used each subsample
of pollen density on a leaf rather than a whole leaf average as
representative of the dose a first instar larva might receive during a
feeding bout. Subsample size varied from 0.25 cm2
(Iowa studies) to 0.34 cm2 (Maryland 2000) to 1 cm2 (Ontario 2000). For Maryland 1999 whole-leaf
averages were used because the pollen counting method did not use leaf
subsampling. Pollen densities were expressed on a per 1 cm2 basis. In most of the studies pollen density
on the leaf was measured from locations flanking the leaf midrib. In
two studies (Maryland 2000 and Iowa 2000d) deposition along the midrib
itself was also measured. Only pollen on the upper surface of leaves was counted in most studies. The Ontario 2000 study measured pollen on
the underside of the leaf and found that levels of pollen on the
underside were 4% of those on the upper surface. Therefore, ignoring
the underside will only slightly underestimate pollen densities.
| |
Results |
|---|
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
Pollen Deposition. In field. Mean in-field pollen densities on milkweed leaves for the different studies are shown in Table 1. Means varied among the individual studies because of degree of anthesis and rainfall during the anthesis period as discussed below. A composite frequency distribution was developed from the results of all these studies showing the proportion of leaf subsamples that fell within different pollen density categories (Table 2). Of the leaf subsamples, 99% had pollen densities below 900 grains per cm2 and 95% of the samples had pollen densities below 600 grains per cm2.
|
|
Factors Affecting Deposition.
Leaf characteristics.
Only a portion of the airborne pollen adheres to milkweed leaves. Three
of the seven fields in the Iowa 1999 study of off-field deposition had
sufficiently high levels of pollen to allow a comparison of the amount
of pollen in the air, as measured by amounts on sticky slides, and on
leaves at the same sampling position. The regressions between the
pollen density on a slide and on its neighboring leaf were significant
in all three of these fields (r2 = 0.37, n = 29, slope = 0.21;
r2 = 0.77, n = 22, slope = 0.62; r2 = 0.78, n = 31, slope = 0.43). The average slope was
0.42. In the Iowa 2000a study of in-field deposition the slope of the
regression between pollen densities on leaves and slides was 0.38 (r2 = 0.68, n =20). This
indicates that in general 40% of the ambient pollen at any location
adheres to milkweed leaves.
1.5 to 1 m were 35.8, 68.0, and 35.7 grains per cm2 for upper, middle, and lower
leaves, respectively. The Maryland 2000 study (Fig. 1b) also
found that middle leaves had significantly higher pollen levels than
upper or lower leaves [F(2, 540) = 19.91, P < 0.0001], but again this pattern changed
beyond 1 m. The mean pollen levels for all samples from 10 to
1 m were 37.3, 132.9, and 103.0 grains per
cm2 for upper, middle, and lower leaves,
respectively. The Iowa 2000c study, which only included plants inside a
cornfield, also found significant leaf position effects with middle
leaves having levels higher than upper leaves [mean pollen density:
upper 61.4, middle 121.2, and lower 119.8 grains per
cm2; F(2,
540) = 19.91, P < 0.0001].
|
10 and 5 m within the field). The mean pollen density within
rows (176.6 grains per cm2) was significantly
higher than the pollen density between rows [118.9 grains per
cm2 (F(1, 247) = 8.47, P = 0.0039)].
Wind direction.
For off-field deposition, wind direction plays a large role in the
deposition level at any sampling location. For example, for one of the
sites in the Iowa 1999 study the average pollen densities on leaves at
0, 1, 2, 4, and 8 m were 732.2, 312.8, 60.0, 30.2, and 1.2 grains
per cm2, respectively, for the downwind sides of
the field (north and east transects) and 12.6, 2.6, 3.0, 2.2, 0, and 0 grains per cm2, respectively, for the upwind
sides of the field (west and south transects).
Rain effects.
The effect of rain was examined by taking advantage of a rainfall event
that occurred during the Iowa 1999 study and one that occurred in an
Iowa study in 2001. The pollen density on one leaf of each plant before
a heavy rain was compared with the pollen density of the opposite leaf
at the same node after the rain (amount of precipitation: 1.9 cm in
1999 and 1.27 cm in 2001). In 1999 and 2001 there were significant
linear regression relationships between the before and after values
(1999: r2 = 0.70, P < 0.0001, n = 17; 2001:
r2 = 0.56, P < 0.008, n = 13). The slopes for the 1999 and 2001 data were
0.14 and 0.45, indicating that the pollen levels after the rainfall
event were only 14 and 45% of those before the rainfall. Thus, a
single rain event removed 86% of the pollen in one case, which
involved leaves outside a cornfield, and 54% in another case, which
involved leaves from within a cornfield.
The Iowa 2000a study was designed to estimate the maximum possible
pollen levels on leaves if no rainfall occurred. The amounts on leaves
sampled at different times during the entire anthesis period were
summed to determine maximum accumulation. The four sample values were
752, 1349, 1440, and 1449 grains per cm2. Amounts
as high as 1400 grains per cm2 were very rarely
found in other in-field studies (Table 2) and the average pollen
densities (Table 1) were much lower. This may be because all but one of
the studies on in-field deposition had rainfall events during the
sampling period (Table 1).
| |
Discussion |
|---|
|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
Mean in-field levels of pollen varied among studies (Table 1). This variation may be due to several factors. First, leaves were collected at various times during anthesis. One might expect that levels of pollen would be highest on leaves sampled at the end of anthesis, because these leaves would have the cumulative pollen deposition of the entire pollination period. In some studies, leaves were sampled during anthesis as well as at the end. However, it was not always the case that leaves sampled later had more pollen (Table 1). This indicates that there are processes that remove pollen from leaves.
The most important process that removes pollen from leaves is rainfall. In a comparison of leaves before and after a single rain event, 54-86% of pollen was removed. One or more periods of rainfall occurred in all but one of the studies reported (Table 1). The highest in-field pollen levels were found in the Iowa 2000b study from samples collected at the end of anthesis during which there had been just one light rain event. These were still well below the 1400 grains per cm2 found in the Iowa 2000a study, where samples were collected at the end of a rainless anthesis period. This may indicate the upper limit of pollen density.
Factors other than rain could also remove pollen from leaves. The surface of a milkweed leaf has a low density of hairs. Thus, it is probable that pollen grains do not adhere well to the leaf surface and move around on the leaf in response to gravity, wind, or leaf movement, causing pollen to fall off.
Some of the observed in-field pollen density differences may be
due to differences in the position of the leaves sampled. In the three
studies in which leaf position effects were examined, pollen densities
on upper leaves were 30-50% of the densities on middle leaves. The
studies differed in how pollen densities on lower leaves compared with
middle leaves; pollen densities on lower leaves were 50-100% of
middle leaf densities. Jesse and Obrycki (2) noted a similar pattern
for upper, middle, and lower leaves. The reason that upper leaves had
less pollen could be the effect of rainfall. Rain is more likely to
wash pollen from upper leaves than middle or lower leaves that are
protected by the leaves above them. In an Iowa 2001 study the pollen
density level for upper leaves was 90% of the level for middle leaves before a rain event, but only 65% afterward (J.P., unpublished data).
Lower leaves may have less pollen than middle leaves because pollen
deposition on lower leaves is blocked by the leaves above them. Leaf
orientation also could contribute to the differences among leaf
positions in pollen density. In the Ontario 2000 study upper leaves
were found to have a more upright orientation (leaf angle = 32.6o, where 0o is
horizontal), whereas middle leaves tended to be horizontal (leaf angle
is 2.3o) and lower leaves tended to be slightly
declined (leaf angle is 24.2o). Pollen is less
likely to be removed from horizontal leaves by rain, wind, or shaking,
and it was the middle, more horizontal leaves that had the highest
pollen densities. Leaf area also may play a role. Upper leaves tend to
be smaller than middle and lower leaves [top = 50.1 cm2, middle = 90.8 cm2, and bottom = 73.1 cm2 (Maryland 1999)]. In the Iowa 1999, 2000a,
and 2000b studies boutonnieres were used as surrogates for natural
plants. In side-by-side comparisons it was found that boutonnieres had
50% more pollen than upper leaves of potted plants (amounts
deposited on leaves of potted plants were regressed against amounts on
adjacent boutonniere leaves; slope = 1.52, r2 = 0.84, n = 17).
Leaf angle and leaf area may explain these differences. Boutonniere
leaves are larger and more horizontal than the upper leaves of potted
plants, so they are more like middle leaves on natural plants in terms
of their pollen capture.
The position of a milkweed plant relative to the corn canopy also can affect deposition levels. Milkweed plants within rows had more pollen than plants between rows. This could also be an effect of rain; leaves on plants within rows may have less pollen removed by rain because of sheltering by the corn canopy.
One additional factor that could account for some of the in-field pollen density differences among studies is how deep inside the field the sampling was done. Sampling distances from the field edge ranged from 1.5 m (Ontario 2000), 6 m (Iowa 2000a), 8 m (Iowa 2000c), 3 and 25 m (Iowa 2000d), and 1, 3, 5, and 10 m (Maryland 2000) to more than 100 m (Iowa 2000b). In the Iowa 2000d study there was significantly higher pollen density at 25 m inside the field than at 3 m [pollen density = 147.5 (25 m) and 55.5 (3 m); F1, 140 = 59.26, P < 0.0001]. In the Maryland 2000 study there was a trend of increasing pollen density with greater distance inside the field (Fig. 1b), but the differences were not significant. Therefore, samples that are taken close to the field edge may experience an edge effect and underestimate pollen densities deeper in the field.
Other factors that could explain differences among studies in pollen deposition are corn cultivar differences in total pollen production, environmental effects on total pollen production, and environmental influences on temporal release of pollen.
Mean off-field pollen densities on milkweed leaves were much lower than the mean in-field density (Table 2). Mean pollen density at the field edge was 37% of the density inside the field and pollen densities declined by about half with each successive distance category away from the field edge. Jesse and Obrycki (2) found a similar result with a smaller data set. The limited dispersal of corn pollen away from a cornfield is due to the fact that a corn pollen grain is 90-100 µm in diameter, one of the largest wind-dispersed pollen grains (3). Consequently, a pollen grain in the air has a greater tendency to settle out than to move upward and outward (3). Variation in pollen densities at a particular distance from the field edge can be attributed to such factors as whether it is the upwind or downwind side of the field, the rainfall history during the deposition period, and the time during anthesis when the sample was made.
Significance for Monarch Larvae. The main purpose of these studies was to characterize the range and distribution of pollen densities, and thus potentially Bt corn pollen densities, to which monarch larvae could be exposed. Although our data were collected in somewhat different ways, the variation introduced by methodological differences is not large compared with the naturally occurring variation in deposition caused by such things as rainfall. Thus, we feel that we have produced a fairly representative picture of naturally occurring pollen densities. In general, the exposure to monarch larvae would be highest inside the cornfield; pollen density drops off very steeply away from a cornfield and only plants on the downwind side of the field receive any appreciable pollen.
Using the data in the present study, published studies on monarch larvae feeding on leaves with Bt corn pollen can be examined to determine how commonly the densities observed in those studies occur in nature. In the study by Losey et al. (1) pollen was applied to milkweed leaves, but no evaluation is possible because the pollen density was not quantified. In the field portion of the study by Jesse and Obrycki (2) their observed pollen densities within a cornfield (overall average 121.6 grains per cm2) were somewhat below the average of 170.6 grains per cm2 found in our study, but the authors note that there were three or eight rainfall events that occurred during anthesis. Their study was much more limited in scope (n = 270, one locality) than ours (n = 1450, three localities). In the laboratory portion of their study the three densities of pollen applied to leaves (14, 135, and 1300 grains per cm2) were within the natural range we found and correspond to low, medium, and very high values. A further discussion of their paper can be found in Hellmich et al. (5). The pollen exposure period for monarch larvae will include the anthesis period, usually 7-10 days, but perhaps beyond because pollen can persist on leaves after the anthesis period, as was the case for Iowa 2000c (Table 1). The effect on monarch larvae of postanthesis Bt pollen will depend on the rate at which the Bt toxin breaks down in pollen over time, something that is currently being evaluated. Three factors will tend to reduce the exposure risks to Bt corn pollen. The most important of these is rainfall, which removes pollen from leaves. Because of rainfall, it is unlikely that pollen levels on leaves will build up to 1400 grains per cm2, our estimate of the maximum possible pollen density. In fact, the average pollen density inside fields for the studies reported here, most of which experienced rainy periods during anthesis, was much lower at 170.6 grains per cm2. A second factor that limits exposure to first instars, the larvae most vulnerable to Bt toxins, is that they tend to feed primarily on upper leaves. The majority of monarch eggs are laid on upper leaves (13) and 55% of first instars were found on upper leaves, compared with 31% on middle leaves and 13% on lower leaves (J.P. and W. K. Lam, unpublished data; n = 159). Our data show that upper leaves have only 30-50% of the pollen density of middle leaves. Third, young larvae (instars 1-3) do not tend to feed on the leaf midrib. Pollen densities were 1.5-1.9 times higher along the leaf midrib. This would, however, mean a higher exposure for 4th and 5th instars. Whether the levels of exposure inside a cornfield or within 2 m of the field edge could have any negative effects on monarch larvae depends on the expression level of endotoxin in the pollen and the pollen density threshold above which there are fitness or mortality consequences. Companion papers on laboratory and field bioassay studies (5, 6) address toxicity issues and the summary paper (7) evaluates what proportion of naturally occurring pollen densities would exceed the toxicity threshold. Determining the potential negative impact on the monarch population of Bt corn pollen at the densities we observed requires information on the threshold pollen density above which there are fitness or mortality consequences and the probability of larvae feeding on milkweeds growing in and near Bt corn fields. Companion papers provide information on toxicity (5, 6) and exposure probabilities (14), and a summary paper (7) combines this information with pollen density data to produce a full risk assessment.| |
Acknowledgements |
|---|
Field and laboratory assistance was provided by Pat Beaupre, Laura Timms, Bryan Muscat, Matt van Ast, and Chad Harvey (Ontario); Randy Ritland, Rachel Pleasants, and Stacy van Loon (Iowa); Jeff Miner, Greg Hess, Mike Embrey, Jessica Hopper, Annie Donnelly, Eric Olson, Mike Raupp, and Terry Patton (Maryland). Barbara Pleasants provided beneficial comments. This research was supported by a pooled grant provided by the U.S. Department of Agriculture, Agricultural Research Service, and the Agricultural Biotechnology Stewardship Technical Committee (ABSTC). Members of the ABSTC are Aventis CropScience USA LP, Dow AgroSciences LLC, E. I. du Pont de Nemours and Company, Monsanto Company, and Syngenta Seeds, Inc. Additional funding came from the Canadian Food Inspection Agency, and Environment Canada.
| |
Footnotes |
|---|
To whom reprint requests should be addressed. E-mail:
jpleasan{at}iastate.edu.
This paper was submitted directly (Track II) to the PNAS office.
| |
References |
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|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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K. S. Oberhauser, M. D. Prysby, H. R. Mattila, D. E. Stanley-Horn, M. K. Sears, G. Dively, E. Olson, J. M. Pleasants, W.-K. F. Lam, and R. L. Hellmich Temporal and spatial overlap between monarch larvae and corn pollen PNAS, October 9, 2001; 98(21): 11913 - 11918. [Abstract] [Full Text] [PDF]
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D. E. Stanley-Horn, G. P. Dively, R. L. Hellmich, H. R. Mattila, M. K. Sears, R. Rose, L. C. H. Jesse, J. E. Losey, J. J. Obrycki, and L. Lewis Assessing the impact of Cry1Ab-expressing corn pollen on monarch butterfly larvae in field studies PNAS, October 9, 2001; 98(21): 11931 - 11936. [Abstract] [Full Text] [PDF]
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R. L. Hellmich, B. D. Siegfried, M. K. Sears, D. E. Stanley-Horn, M. J. Daniels, H. R. Mattila, T. Spencer, K. G. Bidne, and L. C. Lewis Monarch larvae sensitivity to Bacillus thuringiensis- purified proteins and pollen PNAS, October 9, 2001; 98(21): 11925 - 11930. [Abstract] [Full Text] [PDF]
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M. K. Sears, R. L. Hellmich, D. E. Stanley-Horn, K. S. Oberhauser, J. M. Pleasants, H. R. Mattila, B. D. Siegfried, and G. P. Dively Impact of Bt corn pollen on monarch butterfly populations: A risk assessment PNAS, October 9, 2001; 98(21): 11937 - 11942. [Abstract] [Full Text] [PDF]
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