Post on 01-Apr-2023
Abstract
This paper will focus upon the science behind the
requirements to plant refuges of non-Bt corn. Considering that
many of scientific papers on this subject present rather
complicated -and at times contradictory - mathematical
models, no effort will be made to assess these data using
quantitative means. The main pest considered here is the
European corn borer (Ostrinia nubilalis, ECB). Because Bt corn is
so important in the fight against this pest, considerable
attention has been devoted to developing strategies to prolong
the use Bt corn before this pest becomes resistant. The newly
developed “refuge in a bag” technology offers the best hope of
controlling the ECB long term. Physically delimited refuges
may be a transitional phase of Bt corn technology. This is
important because the current regulatory regime for refuges is
complicated and subject to abuse. Particularly in less
developed countries with limited infrastructure, Bt crop
technology must be straightforward to utilize in order to
remain effective on a world wide basis.
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Introduction
Bt corn is now viewed as critical for the effective
control of the European corn borer (Ostrinia nubilalis, abbreviated
here as ECB). The reasons this is the case are not hard to
discern. Field spraying has a number of drawbacks, including
incomplete coverage of plants, degredation in UV radiation,
heating, and drying. For boring insects in general, sprays
are notoriously ineffective. Indeed, several commentators
suggest that genetically modified (GM) crops have allowed the
management of pests on a scale unheard of since the
introduction of synthetic organic insecticides over 50 years
ago (Kennedy 2008). The development of ever more resistant
plants to insect pests is something of an arms race. Until the
introduction of techniques of genetic manipulation, the main
constraining factor was the small number of resistant genes
that could be used. Sources of resistance were limited to
plants that could be cross pollinated with the target plant,
which were plants of the same or closely related species. In
some cases it was also possible to use “bridge species,”
manipulating ploidy levels, or use techniques such as embryo
rescue to transfer resistance from more distantly related
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plants (Smith 2005). Such manipulation could yield a
successful new cultivar after years of research. Now it is
possible to insert genes from other sources into plants in a
fraction of the time. The best known GM plants are those that
have been augmented with cry genes from Bacillus thuringiensis (Bt),
particularly cotton and maize.
Literature Review
The use of GM crops has carried with it a host of
other issues. There are ethical concerns regarding
manipulating genes. A sizable and vocal segment of consumers
fear that the toxins will damage their health. There are
lingering doubts about the long term environmental impact GM
crops will have. This is particularly the case if there are
wild relatives of crop plants that could pick up the altered
genes. The risk of outcrossing to related non-crop species
was addressed in a USEPA study, who found that the risks were
minimal for maize, cotton and potato (USEPA 2000). Use of Bt
cotton was restricted or prohibited in some parts of Florida
and Hawaii where wild relatives were common. In the case of
maize the wild plants originate from Mexico, Central, and
South America. However, the USEPA study suggested that there
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should be an isolation distance between GM and non-transgenic
maize to address the problem of cross fertilization (Sanvido et
al. 2008).
At the other end of the spectrum, there is also concern
that target pest species will, over time, develop resistance
to GM crops. Despite the promise of synthetic organic
pesticides in the middle of the last century, it was clear
that early predictions were false. Insects became resistant
to toxins, and developing new chemical agents proved to be
both costly and time consuming. While the first Bt crops to be
marketed used a single toxin, later generations of GM crops
used multiple Bt toxins to expand the number of potential pests
that could be controlled as well as to delay any potential
resistance that may develop over time. To date the target
species are lepidopteran and coleopteran (Kennedy 2008). Bt
maize and cotton as of 2007 was grown in 22 countries, and
together some 42.1 million hectares were planted (James 2007).
Considering the scale, it is certain that pest species will
develop resistance over time unless measures are taken.
Analysis/Discussion
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While toxins from other organisms besides B. thuringiensis are
under investigation or development, there is something of a
bottleneck in developing new toxins. This is because the
genetics of B. thuringiensis is well understood. In contrast it
will take time and money to develop toxins from other
organisms that can be expressed in plants. The risk involving
relying upon Bt toxins alone is expressed by Gould (1998):
“Most Bt strains produce a number of related toxins, each coded
for by a single gene. Each toxin has a very specific target
site within the insect. Both the classical most-plant
resistance and pesticide resistance literature indicate that
toxic factors that impinge on a single, specific target site
may offer less of an evolutionary barrier than toxins with
multiple effects.” However, there is some good news in that
Heckel (1994) notes that protcolysis of a toxic fragment in
the midgut of the insect involves a gain of function, and is
therefore not likely to be inherited recessively.
It is essential to delay or prevent the development of
resistance in pest populations. The two main mechanisms that
scientific papers address are high levels of toxins produced
by the plants, and the establishment of refuges. This is
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often abbreviated as the HDR (High Dose Refuge) system. In
the case of Bt toxin, this means that the plants must contain
enough toxin so that only resistant insects can survive.
Indeed, the target is that all insects will perish. It was
suggested that toxin levels must be 25 times the dose required
to kill all susceptible larvae (FIFRA, 1998). Bt corn would be
resistant to insect attack and be of direct benefit to
farmers. In contrast refuges could be described as
sacrificial plantings that maintain a supply of toxin
susceptible insects that could mate with resistant individuals
from the Bt crops. The refuge should ideally contain a rough
parity of individuals so that heterozygous offspring are
produced, leaving the next generation vulnerable to the Bt
crop. The USEPA has required plans for insect resistance
management (IRM) in order to register Bt crops for sale. These
plans change over time as more is learned about the problem.
Broadly there are three different kinds of resistance a
plant can have towards potential pests: antibiosis,
antixenosis, and tolerance. Antibiotic resistance interferes
with a pest’s metabolism. In extreme cases, the plant can
reduce target insect numbers so low that a viable population
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cannot be established. Most if not all transgenic insect
resistance is conferred by antibiosis. Antixenotic resistance
makes a plant unsuitable for feeding and/or oviposition. It
may be chemical (smell for example) or physical (hairs are one
example). This type of resistance still leaves live insects
that may attack other crops. It would also be quite difficult
to execute genetically. Tolerance refers to making the plant
more robust so that it can withstand attack. This raises the
economic injury level. While this has been a focus of much
“traditional” selective breeding, to date there are no GM
examples of this type of resistance. As is the case for Bt
crops, plants that are tolerant to insect pests exert
selection pressure for adaptation even when the pest is as
sub-economic levels.
According to Tyutyunov (et al. 2006) three main
considerations of insect resistance to Bt crops are:
1. Insects are known to become resistant to insecticides,
particularly in cases where it is applied regularly in
high doses.
2. The short life cycle of many pest species speeds up the
development of Bt susceptible insects. In the case of the
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European corn borer, four generations per year can be
achieved in optimal conditions.
3. Some pest species have been known to develop resistance
under laboratory conditions.
The theoretical basis for dealing with Bt crop resistance
was formulated decades before as the result of research with
the spread of resistance to insecticides. Comins (1977) was
the first to address, on a theoretical basis, how random
genetic exchange between two insect populations (one subject
to insecticides and one insecticide free) can retard the
spread of resistance. The critical factor of course is that
resistance must be a recessive trait. In time – well after
1977 - the terminology would become standardised. For
instance, the non-treated plots would be known as refuges.
The exact time it would take resistance to become prevalent in
a population would also be influenced by the exact dominance
level of the trait, the initial frequency of the dominance
allele, the effectiveness of the insecticide, and the extent
of insect migration. Because all these factors are difficult
to control in field experiments, mathematical models have been
the preferred method of study. All models will assume that
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the initial frequency of the resistance allele is low and that
only rare homozygous individuals survive. Those with
heterozygous resistance alleles must be at least partly
susceptible to the toxin. Finally, the refuges must be in a
location that encourages mating between individuals from the
two different populations (Tyutyunov et al. 2007).
ECB populations have been extensively studied, and from
several studies it appears that the Bt resistant alleles are
relatively scarce in populations in the US. However, genetic
studies from the upper Midwestern United States (Krumm et al.
2008) suggest that the highest gene flow exist in southern
samples, and the lowest in northern samples. This is clearly
linked with temperature and number of generations per year.
However, there is gene flow between northern and southern
groups. According to Krumm (et al. 2008) the most significant
finding was that there was very little genetic variability
between groups. This suggests that there is a relatively high
degree of gene flow. This demonstrates that for the ECB, there
is no “island phenomenon” of limited breeding pools of
individuals isolated from one another. Showers (et al. 2001)
tracked the movement of the ECB via mark and recapture
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studies. This study found 37–52% of males recaptured flew 800
m or more, 8–11% flew 3.2 km or more, and some captures of
male and female moths occurred at distances greater than 40
km. Over 99.6% of released moths were never recaptured. The
latter point may indicate that these individuals travelled
further than the geographic extent of the trapping program.
Taking the airflow across the region into account, according
to Showers (2001) and others it is likely that the moths can
disperse at a rate of at least 32 km a year. These studies
suggest that for many purposes the ECB can be considered a
single population, and that resistance could spread quickly
once established.
The EPA has devised mandatory requirements - as of the
growing season in 2000 - that commercial users of Bt corn must
follow. Agreements must be signed and growers must renew the
agreements annually. There are three broad categories of
requirement:
1. At least 20% of the area that is under Bt corn cultivation
must be matched by a corresponding refuge of non-Bt corn.
This rule still applies to most Bt crops for both corn
borers and corn rootworms, but there are now some
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exceptions. This “typical” refuge must take a specific or
structured shape, and be planted in strips in the Bt corn
field (at least 4 but 6 recommended), as a block, or as
separate fields. In counties that grow cotton as well,
the refuge must be at least 50% o the area of the Bt corn
crop. This is because there is the potential for
interaction between Bt corn and Bt cotton. The distance
the refuge must be from the Bt crop depends upon the pest.
It must be from ¼-1/2 mile if the Bt corn is designed to
control corn borers, and must be adjacent to the Bt corn
if the variety is for corn rootworm control (USEPA 2006).
2. For particular Bt crops, a structured refuge need only be
5% of the area. This is the case for the SmartStax and
Intrasect crops. The reasoning is that because these
crops use multiple independent toxins to control pests,
it is less likely that insects will develop resistance.
However, the refuges must be from ¼ to ½ mile of the Bt
corn if it is designed to control borers (Intrasect) and
adjacent to the GM corn if it is designed to control
rootworm, as in the case of SmartStax (Bessin 2010). The
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separate field system for corn borers is acceptable
because the moths are very mobile.
3. In the case of AcreMax1 and AcreMax RW, the EPA states
that Bt seed can be blended with 10% non-Bt seed. No
supplementary refuge is needed for AcreMax RW (for
rootworms). With Acremax1 a 20% structured refuge is
required if borers are the pest to be controlled, but in
this case AcreMax RW can serve as the refuge for the
AcreMax 1 fields. This technology uses multiple toxins
for each targeted pest species, and is not the same as
simply mixing the standard Bt seed with non-GM seed.
Attempts to produce mixtures oneself, with no EPA
approval may in fact increase the risk of insects
developing resistance.
Many papers that discuss mathematical models of the
development of resistance in the ECB discuss the sizes ranges
of refuges. The mandatory guidelines introduced by the EPA in
2000 were a departure from previous years, as before that date
if refuges were treated with pesticides they were required to
be bigger. It was thought that because a treated refuge
produced fewer insects they would have to be larger. After
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2000 it was recognized that farmers would not know in advance
if the ECB infestation went beyond the economic threshold. In
addition, insecticide treatments for the ECB are known not to
be particularly effective (Hurley et al. 2002). At the same
time Hurley (et al. 2002) along with most other papers stress
that the 20% refuge cannot be appreciated as an ideal, and
that if increased resistance is desired a larger refuge would
be warranted.
However, all the authors were keenly aware that the
larger the refuge, and the less economical it was, the less
likely it would be for the guidelines to be followed. As
stated in Hurley (et al. 2002): “Whether treated refuge should
be allowed in regions with historically high frequencies of
pesticide use depends on the primary objectives of the policy.
Refuge treatments should not be allowed without higher refuge
requirements if the primary goal is to limit the risk of
resistance. However, if the primary goal is to reduce
conventional pesticide use or improve agricultural production,
then allowing treatments using economic thresholds with
current refuge requirements should be sufficient.” According
to Tyutyunov (et al. 2007) a 20% refuge system with even rare
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applications of insecticide would promote insect resistance.
In contrast, a 30% (or larger) refuge system would essentially
negate the development of resistance.
Because the guidelines on planting refuges is so
complex, the National Corn Growers Association has established
on online calculator to assist planters in making decisions.
It does not replace or supplement manufacturers IRM
information. Instead it seeks to present in a more easy to
understand manner the rules (NCGA 2010). Because the site can
be updated according to the latest science, it is flexible in
ways that a printed source cannot be. However, users in
developing countries may not have access to the internet.
Hopefully the online calculator will increase compliance, but
in such a complicated regulatory landscape this is difficult.
Even seed companies can make mistakes. In 2010 Monsato agreed
to pay a $2.5 million fine for failing to notify purchasers
that their Bt cotton was not approved for planting in certain
parts of Texas with large amounts of corn under cultivation.
The notices did not appear on seed bags from 2002-2007, and
resulted in the largest civil administrative penalty imposed
under the Federal Insecticide, Fungicide, and Rodenticide Act
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(Kaskey 2010a). No information was given as to assess the
nature of the fine in relation to the profits made from
selling Bt seed to areas where it should not have been planted
for during the period in question. This example raises a
disturbing possibility that compliance may be thwarted in an
effort to maximize profits.
A particular brand of Bt seed will only gain popularity if
the product is superior to other Bt varieties on the market.
While there has been a rush to new seed that offered increased
yields, there are some indications that some new technologies
may not offer enough. An example is SmartStax produced by
Monsato. The concept is that more than one toxin is used to
suppress a pest, so that if one toxin is not successful then
another will work. Because of increased effectiveness, a
smaller refuge is required. The Monsato company stock price
fell 40% in 2010 and had the largest drop the biggest decline
on Sept 28 2010 in the Standard & Poor’s 500 Index because of
worries about SmartStax seeds. After 10% of the corn crop was
gathered, according to analysts, the new technology was
yielding 3 to 5 percent less than seeds with two or three
added traits (Kaskey 2010b). This is a serious problem, as it
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appears that the various toxic traits were expressed at the
expense of yield. This is clearly not acceptable from the
farmer’s viewpoint.
Refuges should be timed so that there are plants at the
same stage of development as the Bt corn crop. For example,
flying insects would select fields on the basis of the
developmental stage of the plants there. Within the refuge a
variety of techniques can be used to minimise pest damage.
This can include a spraying programme (according to economic
thresholds), early harvest, or weed control. The mixed refuge
technologies - so called “refuge in a bag” - such as AcreMax1
and AcreMax RW, mean that there is no difficulty in
maintaining a structured refuge. It also ensures that
compliance with requirements is straightforward. There is no
need for a structured refuge, except for corn borers and other
lepidopteran pests. The end result is that AcreMax1 is a two
“refuge in a bag” solution. The reason is that corn borers can
shift from plant to plant during their life cycle. The result
is that they could by a combination of feeding on Bt and non-Bt
plants receive a non-lethal dose of toxin and therefore
develop resistance. Press releases (from Dow in particular)
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during late 2010 state that data about new refuge in a bag
technologies that target above and below ground pests have
been submitted to EPA approval in late 2009. Pending
approval, the seeds should be made available for planting in
2012. According to statements made by Dow, a major factor in
the development of the refuge in a bag concept is the
uncertainty that surrounds compliance with current refuge laws
(Mattke 2010). It would appear from developments in
technology that the entire concept of a separate refuge will
be discarded in time.
Conclusions
From the literature it is clear that all mathematical
models are based upon assumptions regarding the origin and
spread of resistance. No matter what parameters are given, a
larger refuge with little or no pesticide use is ideal. The
recommended proportion of resistant individuals to non-
resistant ones as found by FIFRA is 500:1 (FIFRA 1998).
However, there is a considerable amount of debate about this
issue. There are also serious questions regarding the precise
arrangement of the refuges, which clearly is dictated by the
behaviour of various insect species. In the case of the ECB,
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a mobile species, there is perhaps more limited debate about
the distances refuges should be from the Bt corn.
Clearly the refuge requirements are not based primarily
upon science, and consider the crop yield of the refuge area
in economic terms as an important criterion. Indeed money it
is no doubt a critical consideration in the developed world,
and may be even more important in the developing world. With
a reduced infrastructure, without the ease of spot checking
for compliance and the imposition of penalties, it appears
that guidelines would not be uniformly followed. In high pest
areas, particularly considering the lack of effective sprays
against the ECB, there are strong economic incentives for not
planting refuges.
However, there is every reason to be optimistic. Although
it is clear that once established resistance in the ECB would
spread rapidly, in the field not a single homozygous ECB has
been found. However, given that Bt corn has only been
commercially available since 1996, there are good reasons not
to be complacent. Insect resistance to Bt maize has been
produced in the laboratory. In the end the questions of
refuges may be moot. It is in this light that the new “refuge
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in a bag” technology should be appreciated. It offers the
hope of making compliance a matter of routine.
Clearly a number of areas need further research, these
include:
1. Large scale use of Bt corn may have long term impact upon
pest populations, particularly the ECB. Studies in the
field to monitor pest levels are essential.
2. Refuge in the bag technology for the ECB is not yet for
sale, so studies that address farmer compliance with
refuge requirements are needed. While these seem to have
taken place on a limited scale it may be that certain
areas may follow particular trends. Refuge compliance in
developing countries may be particularly important. The
cosmopolitan nature of pests may make the development of
resistance a world wide issue.
3. Following from point 2, Bt corn might be designed for use
against one pest, the ECB for example, and be planted in
a country where other insect species are the target. A
pest species that might not be considered in the US may
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become a serious worldwide pest after developing
resistance in another country.
4. While there have been some studies that have identified Bt
resistance in some insect species in the laboratory (for
example Tabashnik et al. 2003) much more needs to be done
with a range of insects, particularly the ECB. The
prevalence and nature of Bt resistance in natural
populations is essential, and may take years of study to
unravel.
5. The impact of Bt corn on IPM may be significant. For
instance, a range of natural enemies that would otherwise
be sensitive to pesticides may be used in Bt corn and may
also be used in refuges. The reduction of pesticides can
also have negative effects on secondary pests - allowing
their populations to rise - that would need to be
controlled as well.
6. There have been several papers that have addressed the
impact Bt maize pollen may have non-target species, such
as butterflies and bees. These are serious issues to
address in that a disruption of the environment could
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have significant economic impact (perhaps particularly in
the case of bees).
7. The spread of Bt genes into wild maize is not an issue
that is important in the USA, as there are no wild
populations of the plant here. However, in areas where
the Bt crop are grown and wild populations exist (south of
the US border) there may be a nascent problem. The
spread of Bt genes to a wild population could affect the
development of resistance in insects.
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Literature Cited
Bessin, R., 2010. BT corn refuges for corn borer management, University of Kentucky College of Agriculture, Entofact – 128,http://www.ca.uky.edu/entomology/entfacts/ef128.asp (last accessed 11/11/10).
Comins, H. N.,1977. The development of insecticide resistance in the presence of migration. Journal of Theoretical Biology 64, pp. 177-197.
FIFRA 1998. Scientific Advisory Panel, Subpanel on Bacillus thuringiensis (Bt) Plant-Pesticide and Resistance Management FinalReport, http://www.mindfully.org/GE/FIFRA-SAP-Bt.htm (last accessed 11/11/10).
Gould, F., 1998. Sustainability of transgenic insecticidal cultivars: integrating pest genetics and ecology. Annual Reviewof Entomology 43, pp. 701-26.
Heckel, D.G., 1994. The complex genetic basis of resistance to Bacillus thurigiensis in insects. Biocontrol Science and Technology 4, pp. 405-417.
Hurley, T.M., S. Secchi, B.A. Babcock, and R.M. Hellmich, 2002. Managing the risk of European Corn Borer Resistance to Bt Corn. Environmetal and Resource Economics 22, pp. 537-558.
James, C., 2007. Status of Commercialized Biotech/GM Crops: 2007. ISAAABrief No. 37, International Service for Acquisition of Agri-Biotech Applications, Ithaca, NY, 143 pp.
Kaskey, J., 2010 a. Monsato to pay $2.5 million fine on incorrect labels. Bloomberg July 8 2010, http://www.businessweek.com/news/2010-07-08/monsanto-to-pay-2-5-million-fine-on-incorrect-labels.html (last accessed 11/11/10).
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Kasky, J., 2010b. Monsato plunges on SmartStax corn yield. Bloomberg Sept 28 2010, http://www.businessweek.com/news/2010-09-28/monsanto-plunges-on-smartstax-corn-yield-concerns.html (last accessed 11/11/10).
Kennedy, G.C., 2008. Integration of insect-resistant genetically modified crops within IPM programs, in Romeis, J.,A.M. and G.G. Kennedy, G.G. eds. Integration of Insect-Resistant Genetically Modified Crops Within IPM Programs. Springer Science, New York, pp. 1-26.
Krumm, J.T., T.E. Hunt, S.R. Skoda, G.L. Hein, D.J. Lee, P.L. Clark, and J.E. Foster, 2008. Genetic variability of the European corn borer, Ostrinia nubilalis, suggests gene flow between populations in the Midwestern United States. Journal of Insect Science 8:72 available online at: insectscience.org/8.72
Mattke, C. 2010. Dow AgroSciences showcases new refuge in a bag concept. Dow Press Release Aug 31 2010, http://www.dowagro.com/newsroom/corporatenews/2010/20100831b.htm (last accessed 11/11/10).
NCGA, National Corn Growers Association IRM Refuge Calculator http://www.irmcalculator.com/irmcalculator/irmcalculator/index.html (last accessed 11/11/10).
Sanvido, O., F. Widmer, M. Winzeler, B. Streit, E. Szerencsits, and F. Bigler, 2008. Definition and feasibility of isolation distances for transgenic maize cultivation. Transgenic Research 17, pp. 317-335.
Showers W.B., R.L. Hellmich, M.E. Derrick-Robinson, and W.H. Hendrix III, 2001. Aggregation and dispersal behavior of marked and released European corn borer (Lepidoptera: Crambidae) adults. Environmental Entomology 30, pp. 700-710.
Smith, C.M., 2005. Plant resistance to Arthropods: Molecular and Conventional Approaches, Springer Science & Business Media, Dordrecht, The Netherlands, 423 pp.
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Tabashnik, B.E., Y. Carriere, T.J. Dennehy, S. Morin, M.S. Sisterson, R.T. Roush, A.M. Shelton and J.-Z Zhao, 2003. Insect resistance to transgenic crops: Lessons from the laboratory and field. Journal of Economic Entomology 96:4, pp. 1031-1038.
Tyutyunov, Yu V., E.A. Zhadanovskaya, R. Arditi, and A.B. Medvinsky, 2007. A spatial model of the development of pest resistance to a transgenic insecticidal crop: European Corn Borer on Bt Maize. Complex Systems Biophysics 52:1 95-113.
USEPA United States Environmental Protection Agency, 2000. Biopesticides Registration Document: Preliminary Risks and Benefits Sections; Bacillus thuringiensis Plant-Pesticides. US Environmental Protection Agency, Office of Pesticide Programs,Biopesticides Pollution Prevention Division, Washington, DC.
USEPA, 2006. Regulating Pesticides http://www.epa.gov/oppbppd1/biopesticides/pips/bt_corn_refuge_2006.htm (last accessed 11/11/10).
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