The phytophagous insect pests could be killed
by the Bt toxin proteins only when they ingested
a lethal dose of toxins. There must be a certain
minimum quantity of the Bt toxins in the plant
parts on which the insects feed, particularly during
the more vulnerable phases of the crop, to control
the pest. Differences in Bt toxin expression levels in
leaves of Bollgard, which contains only one protein
toxin, have been correlated to survival differences in
several lepidopteran insect pests, including the fall
armyworm, Spodoptera frugiperda (JE Smith), that are
intrinsically tolerant to Bt in the field and in the
laboratory.11 It was demonstrated that the quantity
of toxin decreased throughout the growing season in