Contents - Faperta

374 Biotechnological Approaches for Pest Management and Ecological Sustainability
were more susceptible than those reared on chickpea-, green pea-, and pigeonpea-based
diets, suggesting that the host plant exercises a considerable infl uence on insect susceptibility
to Bt toxins.
Detection, Development, and Monitoring of Resistance
Detection of Resistance
The probability of detecting one or more rare resistant larvae depends on sample size, the
density of larvae on nontransformed plants, and assumed frequency of resistant phenotypes
in a given population. Probability of detection increases with an increase in sample
size, background density, or the frequency of resistant individuals (Venette, Hutchison,
and Andow, 2000). Following binomial probability theory, if a frequency of 10 3 to 10 4 is
expected for the resistance alleles, 10 3 to 10 4 samples must be collected to have 95% probability
of locating one or more resistant larvae. Estimates of the phenotypic frequency of
resistance from an in-fi eld screen can be useful for estimating initial frequency of resistant
alleles.
Deeba et al. (2003) developed an insect growth inhibition assay to monitor development
of resistance to Bt toxins in spotted bollworm, E. vittella. A discrimination-dose-assay
based on 160 g of seeds of Bt cotton with the cryAc gene in 1.3 liters of artifi cial diet has
been proposed to monitor H. armigera resistance to Bt (Kranthi et al., 2005a). Cotton leaf
feeding assay has been also used to assess the toxicity of Cry1Aa, Cry1Ab, and Cry1Ac
to the semilooper, Anomis fl ava (F.) (Kranthi and Kranthi, 2000). The LC 50 values of
Cry1Ac, Cry1Ab, and CryA1a for neonate larvae were 0.79 to 1.11, 3.48 to 4.12, and 4.98 to
6.08 ng cm 2 of leaf, respectively; and of Cry1Ac for the fourth-instars ranged from 12.91
to 21.14 ng cm 2 , and for Cry1Aa and Cry1Ab, the values ranged from 53.0 to 138 ng cm 2 .
Cry1Ab at a concentration corresponding to the upper limit of LC 99 produced 99% mortality,
while in the case of Cry1Ac, further adjustments and validation may be
necessary.
Liu and Tabashnik (1998) devised a procedure to eliminate a recessive allele conferring
resistance to B. thuringiensis in P. xylostella populations composed of resistant and susceptible
individuals. The susceptible homozygous and heterozygous individuals were killed
with a diagnostic concentration of Cry1Ab and Cry1Aa. The LC 50 of Cry1Ab at fi ve days
was sevenfold lower for the susceptible strain than for the heterogeneous strain. Diagnostic
concentrations of Cry1Ab and Cry1Ac for O. nubilalis based on LC 99 and ED 99 (based on
earlier baseline data) did not detect any resistance development in populations collected
from different locations in the United States (Marcon et al., 2000).
Development of Resistance under Laboratory Conditions
The ability of arthropods to develop resistance depends on the genetic variability of insects
used in the selection, presence of resistance alleles, selection pressure, and duration of
selection over the generations. The resistant larvae are able to feed and complete larval
development and produce normal pupae from which fertile adults emerge. Resistance to
B. thuringiensis var kurstaki was fi rst reported in Indian meal moth, Plodia interpunctella
(Hubner) (McGaughey, 1985; Van Rie et al., 1990). A 100-fold resistance was recorded after