Contents - Faperta

372 Biotechnological Approaches for Pest Management and Ecological Sustainability
%mortality 100 ppm (cotton)
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Delhi Hyderabad Mansa Guntur Bapatla
Population
FIGURE 12.1 Variation in susceptibility of Helicoverpa armigera to Bacillus thuringiensis at different locations in
India. (From Gujar, G.T. et al., 2000. Current Science 78: 995–1001. With permission.)
have shown more than 16-fold resistance to Bt toxins (Karim and Riazuddin, 1999). Preadaptation
of insects to various temperature regimes, differences in genetic make-up of
populations, and use of xenobiotics might be responsible for variations in susceptibility of
insects to B. thuringiensis (Broza, 1986; Chandrashekar et al., 2005).
Extensive investigations on baseline susceptibility of H. armigera to Bt have been carried
out in China (Zhao et al., 1996). Shen et al. (1998) reported that populations of H. armigera
from Yanggu (Shadong), Handan (Hebei), Xinxian (Henan), Xiaoxian (Anhui), and
Fengxian ( Jiangsu) were resistant to Bt as compared to the susceptible strain. Populations of
H. armigera from Yanggu and Xinxian were also resistant to transgenic cotton. This variation
in insect susceptibility was attributed to large-scale use of B. thuringiensis var. kurstaki,
and signifi cant cultivation of transgenic cotton in China. Wu, Guo, and Nan (1999) reported
100-fold variation in susceptibility of H. armigera from fi ve regions in China to Cry1Ac.
Further studies on monitoring of insect susceptibility to Cry1Ac were carried out for the
populations sampled from B. thuringiensis transgenic cotton fi elds by Wu, Guo, and Gao
(2002), who found fi ve-fold variation in IC 50 (concentration producing 50% inhibition of
larval development of third-instars). The IC 50 ranged from 0.020 to 0.105 μg 1 mL, 0.016 to
0.099 μg 1 mL, and 0.016 to 0.080 μg 1 mL for 1998, 1999, and 2000 insect populations,
respectively.
Holloway and Dang (2000) did not observe any differences in susceptibility of H. armigera
and Helicoverpa punctigera (Wallengren) in cotton, in contrast to sweetcorn, using a discriminating
concentration (10.02%) of MVP II formulation of Cry1Ac in Australia. Helicoverpa
armigera is more tolerant to Bt toxins than H. punctigera (Liao, Heckel, and Akhurst, 2002).
Only Cry1Ab, Cry1Ac, Cry2Aa, Cry2Ab, and Vip 3 killed H. armigera at dosages that could
be considered to be effective for controlling this pest. There were no differences in the
relative toxicity of Cry1Fa and Cry1Ac for H. punctigera, but Cry1Fa showed little toxicity to
H. armigera. The differences in susceptibility to Cry1Ac and Cry2Aa were signifi cant.
The LC 50 of Cry1Ac to Earias vittella (F.) ranges from 0.006 to 0.105 μg g 1 of diet, representing
a 17.5-fold variation in susceptibility from 27 locations in India (Kranthi et al.,
2004). Variation in a majority of sites was around 10%. The strains collected from South
India showed greater variation in susceptibility to Bt toxins. Reed and Halliday (2001)
established Cry9C baseline susceptibility for fi eld-collected populations of European
corn borer, O. nubilalis and Southwestern corn borer, Diatraea grandiosella (Dyar). For the
European corn borer, LC 50 values ranged from 13.2 to 65.1 ng cm 2 , and the LC 90 values
from 46.5 to 214 ng cm 2 . The LC 50 values for neonate larvae of Southwestern corn borer