ChangingCottonLandscapeNeilForrester

whatever natural refuges are available will dilute any resistant moths. This may or may not be
the case as the refuges have to be: 1) sufficient in moth production capacity to dilute resistant
moths from any Bt crops; 2) close enough to the Bt crops to ensure cross mating; and 3)
producing moths at the same time as the Bt crops, also to ensure cross mating. These
conditions may not necessarily be met in all cases, so resistance risk will be generally greater in
these scenarios. The resistance risk will be greatly enhanced if these countries then deploy the
same Bt toxins as found in Bt cotton (such as Cry 1Ab/c), in these other deemed refuge crops
which would then not be diluting resistance but simply adding to it. This could be happening
soon in many developing countries with plans to incorporate Cry 1Ab/c Bt toxin into current
bollworm conventional refuge crops such as corn, chick peas, sorghum etc.
The efficacy of the various Bt technologies is also an important factor in designing or evaluating
the requirement for refuge size. Efficacy is defined as the ability for a Bt cotton plant to kill
heterozygous larvae in the field. Heterozygous bollworm larvae carry only one of the two
potential resistance alleles and are the commonest individuals found in the early stages of
resistance development. These are the individuals that are targeted for mating with susceptible
refuge moths so that resistance alleles can be kept heterozygous (that is single) rather than
homozygous (that is carrying two copies and thus being more resistant and usually harder to
control). The following figure shows Roush’s model for the varying estimated kill levels of
Bollgard 1 and Bollgard 2 compared to a relative poor performing Bt technology. In this case, we
are looking for the necessary refuge sizes for each of these Bt technologies to keep resistance
under control for say 20 generations, (equals 7-10 years). This is where the thin horizontal line
at 20 generations in the figure below, meets the three vertical red lines representing the varying
levels of field efficacy.
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