1 INTRODUCTION
The first-generation transgenic cotton (Gossypium
hirsutum L.), Bollgard, which contains a single
toxin protein gene cry1Ac from Bacillus thuringiensis
var. kurstaki Berliner, was approved by the United
States Environmental Protection Agency (USEPA)
for commercial growing in the United States in
1996. Since then, Bt transgenic cotton varieties (Bt
cotton) have been widely and effectively used against
many primary lepidopteran insect pests of cotton,
such as Heliothis virescens (Fabricius), Helicoverpa zea
(Boddie) and Pectinophora gossypiella (Saunders).1–3
Consequently, the acreage of Bt cotton has increased
rapidly around the world in recent years because it
is highly effective against its target insect pests and
benign to the environment.4 Second-generation Bt
cotton, Bollgard II (Monsanto Co., St Louis, MO),
which contains two Bt genes, cry1Ac and cry2Ab, or
cry1Ac and cry1F, was approved for commercial use
by the USEPA in 2002 and 2004 respectively.