Insect Resistance to Bacillus thuringiensis (Bt) Transgenic Crops and Its Management 85
Resistance and Its Mechanism
Resistance is the phenotype of an individual
that gives the individual ability to survive on
the transgenic insecticidal plants from egg to
adult and produce viable offspring. Resistance is
caused by gene in the target insect that reduces
susceptibility to a toxin and is a trait of an in-
dividual. Nine different insect like diamondback
moth, pink bollworm, Helicoverpa armigera,
Heliothis virescens, Earias vittella, etc., have al-
ready shown resistance to Bt toxins worldwide.
The global status of various traits incorporated
in GM crops summarized in ISAAA Executive
Report, 2011, (Fig. 2 ); it is showing a logarithmic
increasing trend in the usage. However, insect re-
sistance gene incorporation among various crops
has declined while traits like herbicide tolerance
or mixture of herbicide tolerance and insect resis-
tance shows an increasing trend (James 2011b).Table 1 Response of DBM from parental susceptible (LAB-P) and resistant (NO-QA) strains to four toxins.
(Tabashnik et al. ( 1997 ))
Strain Toxin 10 mg/lit 100 mg/lit
n % mortality N % mortality
LAB-P Cry1Aa 86 98 197 99
Cry1Ab 118 97 61 100
Cry1Ac 50 94 109 100
Cry1F 91 100 220 97
NO-QA Cry1Aa 86 11 168 4
Cry1Ab 130 2 164 1
Cry1Ac 140 10 212 3
Cry1F 91 0 120 0
Fig. 2 Global area under GM crops by traits (mha)