Contents - Faperta

390 Biotechnological Approaches for Pest Management and Ecological Sustainability
Planting Refuge Crops
A refuge composed of nontransgenic plants of the same crop or other susceptible hosts of
the target pest can be used to produce insects that have not been exposed to or selected for
resistance to the transgene product. The insects produced on the nontransgenic crop will
dilute any resistant genes in the gene pool of the target insect exposed to the transgenic
crop (Figure 12.4). This approach can be used to suppress or delay the development of an
insect population capable of surviving and multiplying on the transgenic crops. For the
refuge strategy to be effective, the insects must emerge from the refuge crop at the same
time as those on the transgenic crop so as to dilute the production of insects homozygous
for the gene(s) conferring resistance to the transgene. Farmers growing Bt transgenic cotton
in the United States and Australia are adopting this method. In India, the farmers grow
cotton along with sorghum, pigeonpea, sunfl ower, or tomato in the same season, and chickpea
towards the end of the cotton crop. All these crops are alternate hosts for H. armigera.
Pigeonpea and chickpea are capable of supporting substantial populations of this pest, and
thus serve as a natural refuge. In such situations, there may be no need to undertake planting
of refuge crops if transgenic cotton occupies less than 75% of the cropped area.
Refuge Types
The Environment Protection Agency (EPA, 2001a,b) has advocated three types of refuge:
(1) 95:5 external structured unsprayed refuge in which 5% of the area is planted as refuge
within 0.5 km of the Bt cotton fi eld; (2) 80:20 external sprayed refuge in which 20% of the
area is sown as border rows of nontransgenic cotton fi eld, which can be treated with insecticides,
excluding foliar sprays of Bt; and (3) 95:5 embedded refuge in which 5% of the area
is used as a contiguous block of nontransgenic cotton, which can be treated with any insecticide,
excluding foliar sprays of Bt. As a novelty, a community refuge program based on
the fi rst two strategies has also been advocated. The Australian Cotton Cooperative
Growers Association (2001) has recommended that the Ingard cotton area should not
exceed more than 30% of the total cotton area in Australia.
Patchworks of treated and untreated fi elds can delay the evolution of pesticide resistance,
but the untreated refuge fi elds are likely to sustain heavy damage (Alstad and Andow,
1995). A strategy that exploits corn borer preferences and movements can eliminate this
problem. Shelton et al. (2000) used Bt-transgenic broccoli plants and the diamondback
Bt-Cotton Refuge
FIGURE 12.4 Planting refuge crops for delaying development of resistance to transgene in insects.