Principles of Plant Genetics and Breeding

268 CHAPTER 15
that the agencies typically evaluate for every product that falls under their purview. All of the agencies evaluate a detailed
molecular and genetic characterization of the product to obtain information about the identity of the GE plant and confirm
that the inserted gene is functioning as intended. In addition, the FDA and USDA-APHIS examine plant composition to gauge
unintended, pleiotropic changes due to transgenesis, and the FDA and EPA evaluate the allergenicity potential of the expressed
protein(s).
USDA-APHIS determines the potential of a GE plant to become an agricultural weed, or to cause damage to agriculture
through the introduction of a novel plant pathogen produced by the transgenic plant or a change in plant susceptibility to pests.
Under FIFRA the benefits of products as well as their potential risks are evaluated in the registration process, so the EPA assesses
the potential economic impact of the introduction of the PIP product, along with the environmental and human health benefits of
the altered pesticide-use regime. When appropriate, the EPA evaluates insect resistance management (IRM) plans proposed
by applicants to confirm that the plan will be sufficient to delay resistance development to Bt. Both EPA and USDA-APHIS are
concerned with the potential for gene flow to occur from the transgenic plant to wild relatives (Table 1). In the case of plants
engineered to produce a PIP, USDA-APHIS and EPA evaluate whether there will be toxicity to non-target organisms that might
come into contact with the crop or its residues.
Using science for risk management: the insect-resistance management example
In addition to the data provided to the agencies during their assessment of specific products during the approval process, both the
EPA and USDA fund active research programs to continue studying the environmental and human health impacts of GE crop
plants. An example of the use of science in determining regulatory policy is that of the EPA’s IRM plan for PIPs utilizing proteins
produced by the bacterium Bacillus thuringiensis (Bt), which are the most common PIPs engineered into plants.
Insect populations exposed to pesticides over a long enough timeframe
will develop resistance (Feyereisen 1995), so because preparations
of the bacteria that express Bt toxins are an important pest
management tool for the organic farming industry, concerns were
raised that the development of resistance to Bt would deprive the agricultural
community of a safe, environmentally friendly pesticide. Due
to the adverse health and environmental effects of having to use conventional
pesticides instead of Bt, the EPA has required a very stringent
IRM plan for the use of Bt-PIP-containing crops to delay resistance
development, in contrast to almost all other pesticides.
After studying insect-resistance models and experimental data, the
80 : 20 external
refuge
(a)
80 : 20 external
refuge
(b)
50 : 50 refuge in
cotton growing areas
95 : 5 embedded
refuge
Figure 1 Examples of refuge strategies for Bt
crops that are acceptable to the EPA: (a) corn
refuge requirements and (b) cotton refuge
requirements. Light areas represent fields of Bt
crops, which the dark areas represent non-Bt
refuges. In all cases, the refuge can be sprayed
with non-Bt pesticides if economically necessary.
EPA developed a program to delay resistance development based on a
“high dose/structured refuge” approach. This strategy relies on resistance
to Bt being a genetically recessive trait and the initial frequency of
the resistance allele being very low. When this is the case, refuges for
susceptible insects can be designed so that in principle any resistant
insects that arise in the population will almost certainly mate with a susceptible
individual so that the heterozygous offspring will be susceptible
to the PIP. The high dose requirement for PIP products necessitates
that the plant expresses a level of Bt protein at least 25-fold greater than
that needed to kill 99% of susceptible insects in laboratory assays. The
basic structured refuge requirements for Bt crops are satisfied in general
by planting 20% of the field as a contiguous non-Bt refuge that should
be located within 0.8 km of the Bt crop fields (Figure 1). However, if Bt
corn is planted in cotton-producing areas then the non-Bt refuge should
be 50% of the corn acreage because cotton pests could feed on both
cotton and corn and develop resistance more rapidly.
Monitoring for insect-resistance development has always been a
requirement by the EPA for registrants, who inform the EPA of the
results of their monitoring program on an annual basis, along with any
grower observations of increased crop damage by insects normally susceptible
to Bt toxins. In addition, academic researchers have performed
recent studies in Arizona, North Carolina, and Iowa to measure resistance
development in Bt corn and cotton fields over multiple growing
seasons (Tabashnik et al. 2003). In most cases, the initial frequency of
resistance alleles to particular GE Bt-containing plants in the target populations
was very low (