Insect Resistance to Bacillus thuringiensis (Bt) Transgenic Crops and Its Management 87
Cry1Ac/ml of diet in 2002 and 0.036–0.363 μg
Cry1Ac/ml of diet in 2011–2012. Ninety percent
of the populations showed typical susceptible re-
sponse, RRs of 31-fold were recorded in one or
two locations during 2008–2009 to 2010–2011
and 51-fold in one location during 2011–2012.
Apart from that, Bt cotton was found effec-
tive in bollworms in some districts where the RRs
were 51-fold. Thus, it appears that the data did not
indicate levels of resistance in the populations that
may be adequate for significant survival of the
populations under field conditions in any of the
populations tested. However, the data indicated
that there was a clear decrease in the proportion of
susceptible populations. Bt produces δ- endotoxin
which comes in contact with insect gut enzymes;
gets solublized and is converted from protoxin to
active toxin to which insect is susceptible. If there
is any change in configuration the gut enzyme/s
may be by mutation or selection, the protoxin
does not get converted to active toxin and in
turn it is unable to bind to the receptors on brush
border membrane which result into inactivation
of toxin and insect becomes resistant. Tabashnik
et al. ( 1994 ) studied cross resistance to Cry1B,
Cry1F between resistant and susceptible strain
of DBM. Resistant strain of Cry1Ab and Cry1F
showed RROF 750 and 240, respectively, at the
same concentration. Moar et al. ( 1995 ) studied
the cross-resistance of CryIC resistant S. exigua
to other Cry proteins and observed that at 22nd
generation of resistant strain (Table 5 ) showed
highest RR of 93 against Cry1Ab followed by an
RR of 73 against CryIIA at 34th generation.Biochemical and Genetic Basis of
Resistance
Evolution of resistance to Bt toxin has necessitat-
ed proper understanding of biochemistry of tox-
in-insect interaction. Some biochemical mecha-
nisms responsible for resistance development
are altered proteolytic processing, modificationTable 4 Resistance monitoring to Cry1Ac toxin of Bt cotton on cotton bollworm populations collected from various
locations in India. (Kranthi (2012b))
Year Sites Highest IC 50 Resistance ratio Highest LC 50 Resistance ratio
1999–2000 10 0.034 2 0.67 7
2002–2003 45 0.043 2 0.54 5
2003–2004 20 0.023 1 0.38 4
2004–2005 21 0.104 5 0.74 7
2005–2006 39 0.166 9 0.72 7
2006–2007 27 0.195 10 0.79 8
2007–2008 49 0.201 11 1.15 12
2008–2009 26 0.58 31 3.12 31
2009–2010 31 0.59 31 3.14 31
2010–2011 27 0.24 13 3.26 33
2011–2012 17 0.36 19 5.10 51
Table 5 Cross-resistance of CryIC resistant S. exigua to other Cry proteins. (Moar et al. ( 1995 ))
Generation Toxin n LC50 (μg of protein/g diet) RR
20 Cry IAb 92 Nd 20
Susceptible 86 25.3
22 Cry IAb 96 5866 93
Susceptible 96 63.2
34 Cry IIA 258 10,731 73
Susceptible 282 147
34 Cry IH 53 80 12
Susceptible 409 6.6
All proteins used were toxins except Cry IIA (protoxin)
Nd not determined