* Department of Environmental Biology, University of Guelph, Guelph,
ON, Canada N1G 2W1; Edited by M. R. Berenbaum, University of Illinois at
Urbana-Champaign, Urbana, IL, and approved August 17, 2001 (received for review June 28, 2001)
A collaborative research effort by scientists in several states and
in Canada has produced information to develop a formal risk assessment
of the impact of Bt corn on monarch butterfly (Danaus plexippus) populations. Information was sought
on the acute toxic effects of Bt corn pollen and the
degree to which monarch larvae would be exposed to toxic amounts of
Bt pollen on its host plant, the common milkweed,
Asclepias syriaca, found in and around cornfields.
Expression of Cry proteins, the active toxicant found in
Bt corn tissues, differed among hybrids, and especially
so in the concentrations found in pollen of different events. In most
commercial hybrids, Bt expression in pollen is low, and
laboratory and field studies show no acute toxic effects at any pollen
density that would be encountered in the field. Other factors
mitigating exposure of larvae include the variable and limited overlap
between pollen shed and larval activity periods, the fact that only a
portion of the monarch population utilizes milkweed stands in and near
cornfields, and the current adoption rate of Bt corn at
19% of North American corn-growing areas. This 2-year study suggests
that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible.
Concern regarding nontarget
effects of transgenic crops containing transgenes from the organism
Bacillus thuringiensis (Bt) arose after the
publication by Losey et al.(1) on the potential risk of corn
pollen expressing lepidopteran-active Cry protein to the monarch
butterfly, Danaus plexippus L. However, the U.S. Environmental Protection Agency (EPA) concluded in an earlier report
that the potential impact of Bt corn pollen, which contains variable amounts of Cry protein, on sensitive larvae of Lepidoptera was
negligible because of factors that limit environmental exposure (2).
Clarification of the risk posed by Bt corn pollen to monarch butterflies can now be undertaken because of the data reported in this
issue of PNAS (3-6) that address exposure and toxic effects of
Bt corn pollen.
The research contributions reported here represent a collaborative
effort established to specifically address the question of risk
associated with Bt corn pollen to the monarch butterfly. In
December 1999, the EPA issued a data call-in requesting industry, researchers and all interested parties to submit information and comments by March 2001 for use in evaluation and potential
reregistration of corn hybrids containing Cry proteins
(http://www.epa.gov/pesticides/biopesticides/otherdocs/bt_dci.htm). The U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) sponsored a Monarch Research Workshop in February 2000 to
identify research priorities regarding Bt corn and monarch butterflies, to establish cooperation among researchers, and to respond
to the EPA request for data. A request for proposals based on workshop
priorities was announced in April, after which a steering committee,
including Adrianna Hewings (USDA-ARS), Eldon Ortman (Purdue
University), Mark Scriber (Michigan State University), Eric Sachs
(Monsanto), and Margaret Mellon (Union of Concerned Scientists)
selected projects to be funded. Funding came from a grant pool provided
by ARS and the Agricultural Biotechnology Stewardship Technical
Committee (7). The guiding principles for problem formulation followed
by the consortium were the elements of risk assessment that underlie
the approach by EPA to ecological risk assessment
(http://www.epa.gov/NCEA/ecorsk.htm).
Only three papers concerning the impact of Bt corn pollen on
nontarget Lepidoptera have been published (1, 8, 9), and they are
limited in their application to risk assessment (7). For example, the
dose of pollen was not specified in the exposure study by Losey
et al. (1), and the study by Jesse and Obrycki (8) used
pollen collection and handling techniques that probably resulted in
contamination from corn anthers or tassel fragments, which contain
significantly higher levels of Cry protein than the pollen (3).
Finally, neither study addressed the spatial or temporal potential for
exposure by monarch larvae to pollen in cornfields, thereby precluding
a risk assessment.
In this paper, we develop a weight-of-evidence approach to the risk of
exposure of monarch larvae to Bt corn pollen and the impact
of such exposure on populations of the monarch butterfly in eastern
North America by using recently published information based on
collaborative research by scientists in the U.S. and Canada (3-6). We
use an approach to risk assessment that has been performed for many
nontarget species in relation to pesticides (10-14), industrial
by-products (15, 16), and other potential toxicants found in the
environment (17). The approach to this process is consistent, well
documented, and standardized
(http://www.epa.gov/NCEA/ecorsk.htm). It requires consideration
of both the expression of toxicity and the likelihood of exposure to
the toxicant as the basic components for a risk assessment procedure.
Hazard Identification.
Toxicity of purified Bt proteins to larval stages of
butterflies and moths is well known (18, 19). Studies conducted on the
use of Bt sprays in forests for gypsy moth control have
shown that Cry proteins can adversely affect nontarget Lepidoptera (20, 21). Field data from these studies indicated a temporary reduction in
lepidopteran populations during prolonged Bt use, although widespread irreversible harm was not apparent (22). Lepidopteran-active Bt protein expressed in pollen of Bt corn hybrids
may pose a risk to sensitive species, such as monarch butterflies, in
or near cornfields during anthesis (1, 8). Milkweeds,
Asclepias spp., and especially common milkweed,
Asclepias syriaca (L.), are the sole larval food source for
monarch butterfly larvae and are abundant throughout the corn-growing
regions of North America (23). As such, hazard from Bt corn
pollen deposited on milkweed leaves warrants consideration of its
ecological risk to monarch populations.
Conceptual Model.
Risk assessment requires knowledge of four essential components:
(i) hazard identification, (ii) nature of dose
response to a toxin, (iii) probability of exposure to an
effective dose, and (iv) characterization of risk (24).
Components of a risk assessment approach as applied to the case of
Bt corn and monarch butterfly are depicted in Fig.
1. Bt proteins expressed in
corn plant tissues can bring about specific reactions in the gut of
lepidopteran larvae (25), including nontarget larvae that consume
Bt corn pollen. The magnitude of the reaction will depend on
the type of protein produced by various transgenic events of hybrid
Bt corn, the amount of protein expressed in pollen grains
from different events, the amount of pollen consumed by larvae of
different developmental stages, and the susceptibility of larvae to the
Bt protein. That a hazard may exist was suggested by Losey
et al. (1). Characterization of toxic effects is necessary
to establish the first component of risk. Laboratory and field assays
of lethal and sublethal toxicity resulting from exposure to doses of
Bt pollen are required to establish toxicity thresholds for
comparison against the dose encountered within the environment. These
toxicity thresholds will vary based on expression levels for individual
Bt corn events in conjunction with environmental factors
determining ecological exposure.
Agricultural Sciences
Impact of Bt corn pollen on monarch butterfly
populations: A risk assessment
,
,
, and United States Department of
Agriculture-Agricultural Research Service, Corn Insects and Crop
Genetics Research Unit and Department of Entomology, Iowa State
University, Ames, IA 50011; § Department of Ecology and
Evolutionary Biology, University of Minnesota, St. Paul, MN 55108;
¶ Department of Zoology and Genetics, Iowa State
University, Ames, IA 50011; Department of Entomology,
University of Nebraska, Lincoln, NE 68583; and ** Department of
Entomology, University of Maryland, College Park, MD 20742
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
View larger version (38K):
[in a new window]
Fig. 1.
Conceptual model of components of risk assessment of the impact of
Bt corn pollen on populations of the monarch
butterfly.
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Results |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Characterization of Effects of Bt Corn Pollen. The Cry1A proteins expressed in most commercial Bt corn hybrids are toxic to the monarch butterfly (3). Mortality, expressed as LD50, was estimated at 3.3 ng of protein/ml diet, whereas growth inhibition (EC50) was estimated to be 0.76 ng/ml (2). However, the expression of Cry1Ab endotoxin within pollen of various events varies considerably depending on the promoter gene involved (26). Expression is greatest in event 176 Bt corn (1.1-7.1 µg/gm pollen) (http://www.inspection.gc.ca/english/plaveg/pbo/dd9609e.shtml, http://www.epa.gov/pesticides/factsheets/fs006458t.htm), a line that is being phased out through 2003. This event exceeds, by nearly two orders of magnitude, protein expression in events Bt11 and Mon810 (0.09 µg/gm pollen) (http://www.epa.gov/scipoly/sap/2000/october/), which is near the current level of detection by immunoassay.
Laboratory bioassays of pollen fed to first instar monarchs for 4 days on leaf discs or whole detached leaves of common milkweed, A. syriaca, indicate that pollen from event 176 Bt corn causes mortality and sublethal effects, such as growth inhibition, at very low concentrations (3). Mortality was evident, even at low densities of pollen, but was variable because of the typical reaction by larvae to Bt proteins of feeding cessation followed by extended periods of time before death (3). Growth inhibition, a more sensitive measure of protein intoxication, could be detected at 5-10 grains/cm2. Pollen from all other events, including Mon810 and Bt11 corn hybrids as well as events not presently grown, such as Dbt418, Cbh351, and Tc1507 (expressing Cry1Ac, Cry9C, and Cry1F proteins, respectively) did not demonstrate lethal or sublethal effects, even at densities above 1,000 pollen grains/cm2 (3). These data were used to establish a no-observable-effect-level for growth inhibition of larvae for event 176 pollen and for Bt11 and Mon810 pollen. Five field bioassays were undertaken to determine the outcome of exposure of larvae under field conditions on milkweed plants growing or placed in the field. Field studies performed in Iowa, Maryland, New York, and Ontario incorporating natural levels of pollen from Bt corn plants (6) demonstrated no acute effects of Bt11 and Mon810 corn pollen on survival or growth of monarch larvae. However, impacts of event 176 pollen were observed. In Iowa, reduced weight gain was noted for larvae exposed to event 176 pollen on milkweeds within cornfields at a density of 23 pollen grains/cm2. Both survival and weight gain were affected in Maryland, where a series of assays using milkweed leaves collected from plants in an event 176 cornfield were carried out over the pollen-shed period. After 6 and 9 days of accumulation of pollen within the cornfield, pollen concentrations on milkweed leaves reached an average of 67 and 161 grains/cm2. Survival of first instars was reduced by 60 and 51%, respectively, compared with larvae exposed to milkweed leaves collected from outside Bt cornfields. Weight gain of survivors was reduced because of consumption of Bt pollen, but only significantly so after exposure to pollen accumulated over a 6-day period (6). In a separate field trial in Maryland, effects on survival and growth of first instar monarchs on leaves of milkweed within a field of a sweet corn hybrid expressing Bt11 endotoxin were evaluated and compared with the effects of residues after applications of a pyrethroid insecticide. Survival of larvae that fed on insecticide-treated milkweed leaves from within the cornfield was low (0-10%). Survival also was influenced significantly (65-79%) by insecticide that drifted onto milkweeds leaves 3 m outside the field. In contrast, survival of larvae exposed to leaves taken from within both Bt and non-Bt corn plots ranged from 80 to 93%, and there were no significant differences in larval survival between these two plots (6).Characterization of Exposure to Bt Corn Pollen. Exposure depends on (i) phenological overlap between monarch populations and corn anthesis, (ii) spatial overlap between milkweeds used by monarchs and cornfields, and (iii) pollen densities encountered on leaves of milkweed plants in and near cornfields.
Phenological Overlap. Pollen from corn plants within a particular field is shed over a period of 1-2 weeks between mid-July and mid-August during the season, whereas larvae develop over a more prolonged period. Potential for exposure of susceptible stages of monarch larvae to corn pollen depends on synchrony of their development with pollen shed of corn plants. Locations in four corn-growing regions were monitored for phenological development of monarch populations and anthesis (4). These locations were established in Iowa, Maryland, Minnesota/Wisconsin, and Ontario. Overlap of the more susceptible stages of monarchs, primarily first and second instars, with pollen shed was considered for purposes of risk assessment.
The presence of susceptible larvae at the time of corn anthesis varied considerably across the regions studied (4). In the more northern locations (Minnesota/Wisconsin and Ontario), about 40 and 62% of the larvae overlapped with pollen shed, respectively, whereas in areas further south (Iowa and Maryland), about 15 and 20% of the larval stages overlapped, respectively. Data from a computer simulation of monarch phenology and corn development support the general observation that overlap increases at higher latitudes across the Corn Belt (D. Calvin, Department of Entomology, Pennsylvania State University, University Park, PA, personal communication). Projected overlap from these simulations is likely overestimated because in the model, 30-year average temperature data were used, whereas our in-field measurements were made during 2000 in three to five specific fields at each of four locations.Spatial Overlap. Density of milkweed stands in cornfields compared with nonagricultural lands and data on the proportion of the landscape in corn and nonagricultural lands provided a basis on which to determine the proportion of the milkweed population that was in cornfields (4). In all locations, densities were higher in nonagricultural lands than in cornfields, but the range of difference was considerable. In Minnesota/Wisconsin and Iowa, the density of milkweed was approximately four to seven times greater in nonagricultural fields than in cornfields, whereas in Ontario, the density was up to 115 times greater. In areas where corn is more intensively cultivated, as in Iowa and southern Minnesota/Wisconsin, less nonagricultural land exists, and the overall proportion of milkweed on a landscape basis is higher in cornfields and other crop lands than in nonagricultural land. In regions of the corn-growing area where mixed habitats are more common, such as in Maryland and Ontario, milkweeds are more abundant in the nonagricultural landscape and provide proportionately greater habitat than those in cornfields (4).
The likelihood of monarch larvae feeding on milkweed plants in cornfields depends not only on what proportion of milkweeds are in cornfields but also on the relative usage by monarchs of milkweeds in cornfields relative to milkweeds in other habitats. Observations from the four regions studied indicated that monarch butterflies locate and lay eggs on milkweeds in corn despite the canopy of the crop obscuring the milkweeds. Sampling done in cornfields and in nonagricultural land in these areas suggests that egg densities per plant are higher in corn in Iowa and Minnesota/Wisconsin but are the same in cornfields and nonagricultural lands in Maryland and Ontario (4). In Iowa, egg densities were higher also in soybean fields than in nonagricultural areas.Pollen Densities Encountered. Dispersal of corn pollen was described by Raynor et al. (27), who demonstrated deposition of pollen as much as 60 m from field edges. Because of the rapid decline in concentration of pollen from the field margin outward, risk assessment concerns are focused on the concentration of pollen on milkweed leaves within the cornfield and those leaves found outside the field up to 5 m from the field edge. During the period of pollen shed, samples of pollen were collected on sticky trap surfaces and on milkweed leaves (5). Samples were taken at various distances within and beyond the margins of cornfields to estimate the concentration of pollen that could be encountered by monarch larvae. Data from three locations, Iowa, Maryland, and Ontario (5), demonstrated a 5-fold reduction in concentration of pollen from just within the edge of the cornfield to about 2-3 m distant. Within-field densities across the different studies averaged between 65 and 425 pollen grains/cm2 on milkweed leaves at the peak of corn anthesis, with an average of 171 grains/cm2. From these data, frequency histograms were derived to determine the likelihood of encounter by first or second instars of different concentrations of pollen within and outside cornfields. These frequency distributions can be used to determine the probability of a larva encountering a toxic pollen dose.
Risk Characterization. To determine risks to monarch larvae associated with Bt corn pollen, two components of greatest significance are: (i) the frequency with which effective environmental concentrations exceed the thresholds for mortality or sublethal effects, such as growth inhibition, of each Bt pollen type, and (ii) the proportion of monarch larval populations in eastern North America that are exposed to toxic levels of Bt pollen.
Probability of Toxicity. It is clear from both laboratory and field-based studies (3, 6) that pollen from the dominant commercial Bt corn hybrids (Mon810 and Bt11) does not express Cry1Ab protein to a level that will impact monarch populations to any significant degree. Hellmich et al. (3) suggested a conservative lowest-observable-effect-concentration (LOEC) be established for these hybrids at 1,000 pollen grains/cm2 of milkweed leaf surface on the basis of a combined analysis of laboratory bioassays exposing larvae to 1,000-1,600 of pollen grains/cm2. Growth inhibition was evident for larvae exposed to event 176 pollen at 5-10 grains/cm2, the lowest dose where activity was noted by Hellmich et al. (3), therefore the effective environmental concentrations for event 176 corn pollen will frequently exceed this threshold in fields where it is planted.
Probabilities of toxicity for events 176, Bt11, and Mon810 pollen are depicted in Fig. 2 as a dose-effect relationship for exposure of larvae to pollen plotted on log-probability scales following methods accepted by the EPA (http://www.epa.gov/NCEA/ecorisk.htm). Growth inhibition of first instar monarchs in response to increasing concentrations of event 176 pollen, as reported by Hellmich et al. (3), is illustrated, with a no-observable-effect-level at 5-10 pollen grains/cm2. In comparison, a hypothetical response curve for Bt11 and Mon810 pollen is depicted by using the same slope parameter for the event 176 response (the Cry1Ab protein is identical in each event) and with a LOEC established, for sake of argument, as a range between 1,000 and 4,000 grains/cm2. Pollen deposition on milkweed leaves during 1999-2000 (5) is represented on a separate scale in a cumulative frequency occurrence curve.
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Probability of Exposure. Milkweeds exist in cornfields across most of the North American corn-growing regions, and monarch butterflies use this resource as a host for their offspring during the period of pollen shed (4). Quantification of the proportion of monarch populations in each region potentially exposed to Bt corn pollen is difficult to ascertain. Our data, collected over a single season at four locations throughout the corn-producing area of North America, illustrate the various factors influencing potential exposure to Bt corn pollen (4). These data are insufficient to provide a definitive estimate of exposure in most cases, but a bounding estimate is possible. Wassenaar and Hobson (28), by using isotope analysis of overwintering monarchs in Mexico, estimated that 50% of the monarch population originates within all or part of 15 states and one province that represent the central core of the North American Corn Belt (Table 1). More than 93% of North American corn is grown in this area, which extends from eastern Kansas/Nebraska to western New York. By using USDA statistics, we estimate that about 28% of crop and pasture land within this area, which together constitute the monarch breeding habitat, consists of corn (Table 1). Adoption of Bt corn across this area encompassing 50% of the monarch-breeding habitat was about 19% of the corn crop in 2000 (29).
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[ 1 ] |
50% of the eastern North
American monarch population arising from the portion of the Corn Belt,
as indicated by Wassenaar and Hobson (28), provides a broad view of
potential exposure (Table 2). Our
estimates of overlap of the pollen-shed period in each location with
the presence of monarch larvae are based partly on the projections of
the simulation model described previously and partly on our own
observations. In this instance, our estimate for the exposure of
monarchs in the Corn Belt states and Ontario is 1.6%. Because monarchs
in the Corn Belt represent 50% of the total monarch population, the exposure for the entire monarch population would be no greater than
0.8%.
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[ 2 ] |
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[ 3 ] |
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[ 4 ] |
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[ 5 ] |
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Discussion |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Previous reports (1, 8) indicating the hazard of Bt corn pollen to monarch butterfly are inadequate to assess risk, because assigning risk can be accomplished only when the likelihood of toxic response can be properly expressed and the likelihood of exposure is estimated through appropriate observations. We have used a comprehensive set of new data and a formalized approach to risk assessment that integrates aspects of exposure to characterize the risk posed to monarchs from Bt corn pollen. Characterization of acute toxic effects alone indicates that the potential for hazard to monarchs is currently restricted to event 176 hybrids, which express Cry1Ab protein in pollen at a level sufficient to show measurable effects. Event 176 hybrids have always had a minor presence in the corn market and current plantings, which comprise <2% of corn acres, are rapidly declining.
Other events either express negligible Cry1Ab protein in corn pollen (Mon810 and Bt11) or express Cry protein of significantly less toxicity to monarch (Dbt418, Cbh351, and Tc1507 expressing Cry1Ac, Cry9c, and Cry1F proteins, respectively). These corn hybrids have little or no effect on monarch populations, although sublethal effects due to chronic exposure to Bt pollen over the entire larval growth of monarchs has not been accounted for in these studies. Should chronic effects he documented, the impact on monarch populations will remain low or negligible, because overall exposure of monarch larvae to Bt pollen is low.
Monarch populations share their habitat with corn ecosystems to a degree previously undocumented (4). Despite this conclusion, the portion of the monarch population that is potentially exposed to toxic levels of Bt corn pollen is negligible and declining as planting of event 176 hybrids is phased out through 2003. The exposure portion (Pc) of the risk (R) equation described above is low, and the toxicity portion (Pt) of this equation for the dominant corn hybrids is negligible, therefore the impact of Bt corn on monarch populations should remain low.
Evidence supporting this risk conclusion has been collected over a wide geographic area and under a variety of conditions in both laboratory and field settings (3-6). Findings from studies done in multiple locations were consistent, even though methods differed from one study to another. This approach to risk characterization is consistent with accepted risk assessment procedures and shares many similarities with previous assessments over a wide range of situations describing potential risk associated with a described hazard. It is imperative that future conclusions concerning the environmental or nontarget impacts of transgenic crops be based on appropriate methods of investigation and sound risk-assessment procedures.
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Acknowledgements |
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We thank Jeffrey Wolt, Keith Solomon, and Anthony Shelton for their input and critical comments and suggestions during the development of this paper. This research was supported by a pooled grant provided by USDA-ARS and the Agricultural Biotechnology Stewardship Technical Committee (ABSTC), and by funding from the Canadian Food Inspection Agency (CFIA), Environment Canada, and the Ontario Ministry of Agriculture, Food and Rural Affairs, the Maryland Agricultural Experiment Station, and the Leopold Center for Sustainable Agriculture, Ames, IA. Members of ABSTC are Aventis CropScience USA LP, Dow AgroSciences LLC, E. I. du Pont de Nemours and Company, Monsanto Company, and Syngenta Seeds, Inc.
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Abbreviations |
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Bt, Bacillus thuringiensis; USDA-ARS, U.S. Department of Agriculture Agricultural Research Service; EPA, U.S. Environmental Protection Agency; LOEC, lowest-observable-effect-concentration.
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Footnotes |
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To whom reprint requests should be addressed. E-mail:
msears{at}evb.uoguelph.ca.
This paper was submitted directly (Track II) to the PNAS office.
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References |
|---|
|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
| 1. |
Losey, J. E.
, Rayor, L. S.
& Carter, M. E.
(1999)
Nature (London)
399,
214 |
| 2. | U.S. Environmental Protection Agency. (1995) Publ. No. EPA731-F-95-004 (U.S. Govt. Printing Office, Washington, DC). |
| 3. | Hellmich, R. L., Siegfried, B. D., Sears, M. K., Stanley-Horn, D. E., Daniels, M. J., Mattila, H. R., Spencer, T., Bidne, K. G. & Lewis, L. (2001) Proc. Natl. Acad. Sci. USA, in press. |
| 4. | Oberhauser, K. S., Prysby, M., Mattila, H. R., Stanley-Horn, D. E., Sears, M. K., Dively, G., Olson, E., Pleasants, J. M., Lam, W.-K. F. & Hellmich, R. L. (2001) Proc. Natl. Acad. Sci. USA, in press. |
| 5. | Pleasants, J. M., Hellmich, R. L., Dively, G., Sears, M. K., Stanley-Horn, D. E., Mattila, H. R., Foster, J. E., Clark, P. L. & Jones, G. D. (2001) Proc. Natl. Acad. Sci. USA, in press. |
| 6. | Stanley-Horn, D. E., Dively, G. P., Hellmich, R. L., Mattila, H. R., Sears, M. K., Rose, R., Jesse, L. C. H., Losey, J. F., Obrycki, J. J. & Lewis, L. (2001) Proc. Natl. Acad. Sci. USA, in press. |
| 7. | Hellmich, R. L. & Siegfried, B. D. (2001) in Genetically Modified Organisms in Agriculture-Economics and Politics, ed. Nelson, G. C. (Academic, London), pp. 283-289. |
| 8. |
Jesse, L. C. H.
& Obrycki, J. J.
(2000)
Oecologia
125,
241-248 |
| 9. |
Wraight, C. L.
, Zangerl, A. R.
, Carroll, M. J.
& Berenbaum, M. R.
(2000)
Proc. Natl. Acad. Sci. USA
97,
7700-7703 |
| 10. |
Giddings, J. M.
, Hall, L. W., Jr.
& Solomon, K. R.
(2000)
Risk Anal.
2,
545-572 |
| 11. |
Giesy, J. P.
, Solomon, K. R.
, Coats, J. R.
, Dixon, K. R.
, Giddings, J. M
& Kenega, E. E.
(1999)
Rev. Environ. Contam. Toxicol.
160,
1-129 |
| 12. |
Solomon, K. R.
, Baker, D. B.
, Richards, P.
, Dixon, K. R.
, Klaine, S. J.
, La Point, T. W.
, Kendall, R. J.
, Weisskopf, C. P.
, Giddings, J. M.
, Giesy, J. P.
,
et al.
(1996)
Environ. Toxicol. Chem.
15,
31-76 |
| 13. |
Solomon, K. R.
, Giesey, J. P.
, Kendall, R. J.
, Best, L. B.
, Coats, J. R.
, Dixon, K. R.
, Hooper, M. J.
, Kenaga, E. E.
& McMurry, S. T.
(2001)
Environ. Toxicol. Chem.
7,
497-632 |
| 14. |
Geisy, J. P.
, Dobson, S.
& Solomon, K. R.
(2000)
Rev. Environ. Contam. Toxicol.
167,
35-120 |
| 15. |
Klaine, S. J.
, Cobb, G. P.
, Dickerson, R. L.
, Dixon, K. R.
, Kendall, R. J.
, Smith, E. E.
& Solomon, K. R.
(1996)
Environ. Toxicol. Chem.
15,
21-30 |
| 16. |
Hall, L. W., Jr.
, Giddings, J. M.
, Solomon, K. R.
& Balcomb, R.
(1999)
Crit. Rev. Toxicol.
29,
367-437 |
| 17. |
Kendall, R. J.
, Lacher, T., Jr.
, Bunck, E. C.
, Daniel, F. B.
, Driver, C.
, Glue, G. E.
, Leighton, F.
, Stansley, W.
, Watanabe, P. G.
& Whitworth, M.
(1996)
Environ. Toxicol. Chem.
15,
4-20 |
| 18. | Kreig, A. & Langerbruch, G. A. (1981) in Microbial Control of Pests and Plant Diseases, ed. Burges, H. D. (Academic, New York), pp. 837-896. |
| 19. |
Peacock, J. W.
, Schweitzer, D. F.
, Dale, F.
, Carter, J. L.
& Dubois, N. R.
(1998)
Environ. Entomol.
27,
450-457 |
| 20. |
Miller, J. C.
(1990)
Am. Entomol.
36,
135-139 |
| 21. |
Johnson, K. S.
, Scriber, J. M.
, Nitao, J. K.
& Smitley, D. R.
(1995)
Environ. Entomol.
24,
288-297 |
| 22. | Hall, S. P. , Sullivan, J. B. & Schweitzer, D. F. (1999) USDA Bull. No. FHTET-98-16 (USDA, Washington, DC). |
| 23. | Malcolm, S. B. , Cockrell, B. J. & Brower, L. P. (1993) in Biology and Conservation of the Monarch Butterfly, eds. Malcolm, S. B. & Brower, L. P. (Natural History Museum of Los Angeles County, Los Angeles, CA), pp. 253-267. |
| 24. | National Research Council. (1983) Risk Assessment in the Federal Government: Managing the Process (Natl. Acad. Press, Washington, DC). |
| 25. |
Koziel, M. G.
, Beland, G. L.
, Bowman, C.
, Carozzi, N. B.
, Crenshaw, R.
, Crossland, L.
, Dawson, J.
, Desai, N.
, Hill, M.
, Kadwell, S.
,
et al.
(1993)
Biotechnology
11,
194-200 |
| 26. |
Christensen, A. H.
, Sharrock, R. A.
& Quail, P. H.
(1992)
Plant Mol. Biol.
18,
675-689 |
| 27. |
Raynor, G. S.
, Ogden, E. C.
& Hayes, J. V.
(1972)
Agron. J.
64,
420-427 |
| 28. |
Wassenaar, L. I.
& Hobson, K. A.
(1998)
Proc. Natl. Acad. Sci. USA
95,
15436-15439 |
| 29. | USDA-National Agricultural Statistics Service. (2000) Census of Agriculture (USDA-NASS, Washington, DC), Vol. 1, Part 57. |
| 30. | USDA, Natural Resources and Conservation Service & Iowa State University, Statistics Laboratory. (1997) Summary Report, National Resources Inventory (USDA, Washington, DC). |
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