Contents - Faperta

Development of Resistance to Transgenic Plants 373
were 16.9 to 39.9 ng cm 2 , and the LC 90 values were 40.3 to 157 ng cm 2 . The most sensitive
Southwestern corn borer colony was collected from the Mississippi delta exhibiting an
LC 50 value of 22.6 ng cm 2 , which displayed the widest LC 90 confi dence limits of 40.3 to 94.8
ng cm 2 . This baseline data can be used for monitoring of resistance to Cry9C in the future.
Baseline susceptibility studies have also been carried out for Cry1Ab to fall armyworm,
Spodoptera frugiperda ( J.E. Smith) (Lynch et al., 2003). Comparison of LC 50 values for various
colonies did not indicate an appreciable change in susceptibility of this insect from 1998
to 2000.
The LC 50 for diamondback moth, Plutella xylostella (L.), larvae from different locations in
India varied from 1.0 to 10.97 mg per liter. Nine generations of selection for resistance
resulted in an increase in LC 50 from 2.76 to 5.28 mg per liter (using a selection pressure of
6.4 mg per liter), indicating that there is a possibility of developing resistance to Bt in this
insect in India (Mohan and Gujar, 2000). The susceptibility to different toxin proteins of Bt
ranged from 0.14 to 3.74 μg mL 1 for Cry1Aa, 0.007 to 1.25 for Cry1Ab, 0.18 to 2.47 for Cry1Ac,
and 0.12 to 3.0 μg for Cry1C (Mohan and Gujar, 2002). There were small differences in
susceptibility of Sesamia nonagrioides (Lefebre) populations, but no differences were observed
in two populations of O. nubilalis in Spain. In another study, differences in fi ve populations
of O. nubilalis from the United States were fi ve-fold in susceptibility to Dipel, but the populations
responded quite rapidly for resistance to Bt, indicating that low levels of susceptibility
do not necessarily imply low potential to respond to selection pressure. Very high differences
in susceptibility to Bt products and Cry1Ab and Cry1Ac have been found in the case
of H. virescens and H. zea (Stone and Sims, 1993; Zhao et al., 1996; Wu, Gou, and Nan, 1999).
Effect of Host Plant on Insect Susceptibility to Cry Proteins
The susceptibility of neonates of H. armigera to B. thuringiensis ssp. kurstaki (HD-73)
ranged from twofold (LC50 84.5 to 164.2 μg per liter) for insects collected from chickpea
to about fi ve-fold (LC50 51.1 to 247.7 μg per liter) for insects collected from cotton (Gujar
et al., 2004) (Figure 12.2). The insects collected from pearl millet were twice as tolerant as
those collected from cotton and sunfl ower at Sirsa, Haryana, India. Host-specifi c colonies
raised from insects collected from cotton in Bhatinda, Punjab, India (by incorporating
cabbage, caulifl ower, chickpea, green pea, pearl millet, and pigeonpea into the artifi cial
diet) indicated that insects reared on cabbage-, caulifl ower-, and pearl millet-based diets
% mortality/100 ppm
80
70
60
50
40
30
20
10
0
Pigeonpea Lady's finger Tomato
Delhi Madurai Vijayawada
Population and Host crop
FIGURE 12.2 Variation in susceptibility of Helicoverpa armigera to Bacillus thuringiensis across host plants.
(From Gujar, G.T. et al., 2004. Entomologia Experimentalis et Applicata 113: 165–172. With permission.)