Contents - Faperta

Development of Resistance to Transgenic Plants 381
• Use of temporal refugia (rotation with a non-Bt
insect host crop) led to a decline in
the frequency of the resistance allele.
• Higher rates of movements increased the rate at which the frequency of resistance
increased.
• The frequency of the resistance allele increased when the refugia were sprayed
with insecticide.
The frequency of resistant phenotypes can be estimated as the ratio of density of larvae
per plant in Bt crops to the density of larvae per plant in an adjacent non-Bt crop (Andow
and Hutchinson, 1998). Functional dominance of resistance alleles and the initial frequency
of those alleles have a major impact on evolution of resistance (Storer et al., 2003). The
survival of susceptible insects on the transgenic crops and the population dynamics of the
insect, driven by winter survival and reproductive rates, were also important. In addition,
agricultural practices, including the proportion of the area planted with maize, and the
larval threshold for insecticide sprays affected the resistance-allele frequency.
Initial frequency of resistance genes is the key determinant for predicting evolution of
resistance in an insect population. However, only a few estimates are available for resistance
gene frequencies under fi eld conditions. Andow and Alstad (1998) proposed a procedure
to estimate resistance gene frequencies in fi eld populations based on F 2 screening.
It does not require a laboratory resistant population, and is more sensitive than discriminating
dose assay for detection of recessive traits. This method estimated no homozygotes
in O. nubilalis, and the frequency of Cry1Ab resistance alleles ranged from 0.013 to 0.0039
in the United States (Andow et al., 2000). Resistance gene frequency in H. virescens has been
estimated to be 1.5 10 3 in the United States (Gould et al., 1995), 4 10 3 for low-level
resistance in H. armigera, and 10 3 for high levels of resistance in P. xylostella in Australia.
However, high levels of resistance alleles have been estimated for P. xylostella in Hawaii
(0.12) and Arizona (0.16) (Tabashnik et al., 1997a, 2000b). Gould et al. (1997) reported 10 3
frequency of resistant alleles in natural populations of H. virescens. Akhurst, James, and
Bird (2000) also estimated the frequency of resistance alleles at 10 3 in H. armigera, which
was supposed to be three times more common than normally assumed (10 6 ) for resistant
genes in the fi eld population. At this frequency, resistance is likely to be a signifi cant problem
in the fi eld in less than 16 generations (4 to 5 years), if Bt resistance management strategies
are not implemented. Estimated frequency of a recessive allele conferring resistance
to Bt toxin Cry1Ac in P. gossypiella was 0.16 (Tabashnik et al., 2003). Unexpectedly, the estimated
resistance allele frequency did not increase from 1997 to 1999 and Bt cotton remained
extremely effective against pink bollworm. The fi nding that 21% of the individuals from a
susceptible strain were heterozygous for the multiple-toxin resistance gene indicated that
the resistance allele frequency was 10 times higher than the most widely cited estimate of
the upper limit for the initial frequency of resistance alleles in susceptible populations.
These fi ndings suggest that insect pests may evolve resistance to some groups of toxins
much faster than previously expected.
Frequency of resistance in O. nubilalis to Bt maize was 3.9 10 3 in an Iowa population
(Andow et al., 2000), indicating that the refuge plus high-dose strategy may be effective
for managing resistance in Bt maize. Partial resistance to Cry1Ab toxin was found to be
common. The 95% CI for the frequency of partial resistance were 8.2 10 4 to 9.4 10 4 for
the Iowa population. Variable costs of the method were $14.90 per isofemale line, which
was a reduction of 25% compared with the initial estimate. Bourguet et al. (2003) screened
1200 isofemale lines of O. nubilalis, and no alleles conferring resistance to Bt maize