86 L. Pathak et al.
Tabashnik and Liu ( 1997 ) studied the re-
sponse of diamondback moth larvae from paren-
tal susceptible and resistant strain (Table 1 ) to
four toxins and observed that at concentration of
10 and 100 mg/lit toxin killed 90–100 % larvae
of susceptible strain but only 0–1 1 % of resistant
counterpart. Gould et al. ( 1992 ) studied the gen-
eration verses resistance ratio (RR) between se-
lected and control strain of H. virescens against
Cry1Ac and reported that after ten generations
the ratio of LC 50 of selected strain to control was
10 and RR suddenly increased to 50 after 17th
generation of selection.
Tabashnik et al. ( 2008 ) submitted the report
on field-evolved resistance to the Bt toxin in
sprays and transgenic cotton which say that H.
zea had maximum RR (> 1000 fold) to Cry1Ac
while field population of diamondback moth
and cabbage looper showed the 36 RR in Hawaii
and 160 RR in British Columbia, respectively,
against Bt spray (Table 2 ). He also conducted a
laboratory diet bioassay on field population of
H. zea resistant to Cry2Ab and observed that the
number of resistant populations to the total popu-
lation was 0 of 8 in 2002, 1 of 25 in 2003, 1 of 24
in 2004, and 5 of 10 in 2005. Means proportion
of resistant population was significantly higher
in 2005 so, he concludes that continues use of
Bt toxin may develop resistance simultaneously
in insect species. Rajagopal et al. ( 2009 ) stud-
ied development of resistance by H. armigera
to Cry1Ac on an artificial diet and observed that
after ten generations the larvae were able to toler-
ate 72-fold more Cry1Ac than susceptible strain.
The resistance dose is reflected more in the LC 90
values than in the LC 50 values, suggesting that
the larvae become much more adopted to survive
an ever increasing dose of the selection pres-
sure. Yenagi et al. ( 2010 ) studied the resistance
development in H. armigera in response to Del-
fin for different Bt cotton ecotypes in northern
Karnataka, namely, Dharward, Haveri, Raichur,
Bijapur, and Belgaum (Table 3 ). The populations
from Raichur and Haveri were found tolerant
to Bt toxin. LC 50 values resulting from 0.149 to
0.828 mg/ml. The Dharward strain was the most
susceptible.
Belgaum and Bijapur population were similar
to each other at RF. This would suggest that under
field condition tolerant individual is likely to per-
sist and may subsequently contribute to the resis-
tant pool. Studies carried out by CICR (Kranthi
2012b) showed that there was a decline in the
proportion of susceptible populations. These re-
sults were obtained by studying LC 50 (median
lethal concentration) and IC 50 (median growth
inhibitory concentration) which were expressed
in terms of μg Cry1Ac/ml of diet (Table 4 ) and
further they compared RR value with reference
to susceptible strains.
However, the LC 50 values ranged from
0.02 to 0.54 μg Cry1Ac/ml of diet in 2002 and
0.246–5.10 μg Cry1Ac/ml of diet in 2011–2012.
The IC 50 values ranged from 0.003 to 0.034 μgTable 2 Field-evolved resistance to Bt toxins is prays and transgenic cotton. (Tabashnik et al. ( 2008 ))
Insect Bt spray or toxin Resistance ratio
10 > 100 > 1000 Maximum
P. xylostella Dipel 2 0 0 36
T. ni Dipel 23 2 0 160
H. zea Cry1Ac 54 14 2 > 1000
Table 3 Geographical variability in susceptibility of H. armigera to Bt toxin across northern Karnataka cotton eco-
system. (Yenagi et al. ( 2010 ))
Location LC 50 mg/ml 95 % FL RF
Lower Upper
Dharwad 0.149 0.042 0.220 1.00
Haveri 0.710 0.507 2.154 4.77
Raichur 0.828 0.570 1.021 5.66
Bijapur 0.186 0.090 0.159 1.25
Belgaum 0.174 0.047 0.259 1.17