Insect Resistance to Bacillus thuringiensis (Bt) Transgenic Crops and Its Management 89
Insect Resistance Management (IRM)
Strategy
The ultimate goal of IRM programs for Bt
crops—as with IRM programs for any insect-
control technology—is to slow the rate at which
insect-resistance evolves. IRM programs cannot
be expected to prevent resistance, but they should
be designed to maximize the effective life of a
Bt crop. The economic benefits of this strategy
are obvious and prolonged product life increases
the likelihood that next-generation products can
be developed and commercialized in a timely
manner, creating a paradigm of continuous im-
provement in technologies rather than sequential
replacement to keep up with resistance (Head and
Greenplate 2012 ). Resistance management strat-
egies try to prevent or diminish the selection of
the rare individuals carrying resistance genes and
hence to keep the frequency of resistance genes
sufficiently low for insect control. Strategy de-
velopment generally relies heavily on theoretical
assumptions and on computer models simulating
insect population growth under various condi-
tions (Alstad and Andow 1994 ). Proposed strate-
gies include the use of multiple toxins (stacking
or pyramiding), crop rotation, high or ultrahigh
dosages, and spatial or temporal refugia (Tabash-
nik 1994; McGaughey 1992). It is expected that
each pest-crop complex may require a specific
implementation of certain resistance manage-
ment strategies that may have to address the use
of both B. thuringiensis sprays and transgenic
crops. Experience with transgenic crops express-
ing cry genes grown under different agronomic
conditions is essential to define the requirements
of resistance management. It is equally important
to design a resistance management strategy ac-
ceptable to everyone involved: technology sup-
pliers, seed companies, extension workers, crop
consultants, regulators, and, most of all, growers
Kennedy and Whalon ( 1995 ). To prevent the loss
of this valuable management tool, IRM guide-
lines have been established to delay or stop the
development of ECB resistance.
The refuge strategy, which is mandated in the
USA and elsewhere, is based on the idea that
most of the rare resistant pests surviving on Bt
crops will mate with abundant susceptible pests
from nearby refuges of host plants without Bt
toxins. If inheritance of resistance is recessive,
the hybrid progeny from such matings will die
on Bt crops, substantially slowing evolution of
resistance. This approach is sometimes called
the “high-dose refuge strategy” because it works
best of the dose of toxin ingested by insects on
Bt plants is high enough to kill all or nearly all
of the aforementioned hybrid progeny. In prin-
ciple, if high-dose is achieved, resistance can be
delayed by increasing refuge abundance which
lowers proportion of the population selected
for resistance to compensate for survival of hy-
brid progeny of Bt plants (Tabashnik and Gould
2004 ). Thus, the US environmental protection
agencies guidelines for high-dose specified that
Bt plants should kill at list 99.99 % of susceptible
insects in the field. As an example, Anderson and
Hellmich ( 2005 ) described the refuge strategy of
corn in the Site-Specific Management Guideline
that Refuge plants, or nontransgenic corn, are
an important component of IRM. The purpose
of planting a refuge is to dilute resistance genes
by supplying an abundance of susceptible ECB
moths that can mate with the rare resistant moths
that have survived exposure to Bt corn (Fig. 3 ).
Offspring from these matings are likely to be sus-
ceptible to Bt corn.
Refuge strategy: reduce chances that resistant
moths mate with each other by providing large
numbers of susceptible moths from the refuge,
non-Bt corn. Offsprings of these moths are sus-Site-Specific Management Guideline: Anderson and Richard
Resistaat Moths Suseeptible MothsFig. 3 Refuge strategy