Insect Resistance to Bacillus thuringiensis (Bt) Transgenic Crops and Its Management 91
interplay among Cry toxins, their bacterial hosts,
their target organisms, and the ecosystems they
share will allow for the long-term, effective use
of Cry toxins for pest management.
Evolution of resistance in target pests to trans-
genic insecticidal crops is a significant environ-
mental risk that could affect multiple stakeholders
including those outside of agriculture. Reduction
in binding is a major mechanism of resistance in
all cases of field-evolved resistance to Bt products
or Cry proteins in P. xylostella, except for resis-
tance against Cry1Ca. In Pink bollworm and other
pests, resistance could evolve via selection for
alleles that are not recessive, have lower fitness
cost or completely overcome disadvantage on Bt
plants. A number of studies have reported the abil-
ity of insects resistant to Cry1Ac to acquire cross-
resistance to structurally similar insecticidal pro-
teins such as Cry1Aa, Cry1Ab, Cry1C, etc. Of all
of the various strategies and tactics considered for
IRM, the high-dose/refuge strategy is by far the
most widely considered and used. Refuges can
delay pest resistance to Bt crops, especially when
resistance is recessive and refuges are abundant.
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