Contents - Faperta

376 Biotechnological Approaches for Pest Management and Ecological Sustainability
Studies on the stability of resistance have shown relatively high sustenance of resistance
(about 50- to 100-fold over the susceptible laboratory strain) (Akhurst, James, and Bird,
2000). Resistance to Cry1Ac in H. armigera has been detected after 16 generations of continuous
selection (Akhurst et al., 2003). The resistance ratio (RR) peaked approximately
300-fold at generation 21, after which it declined and oscillated between 57- and 111-fold.
First-instar H. armigera from generation 25 (RR 63) were able to complete larval development
on transgenic cotton expressing cry1Ac and produce fertile adults. There is a fi tness
cost associated with resistance to Bt on cotton and on artifi cial diet. The BX strain is susceptible
to commercial Bt spray formulations Dipel and XenTari (which contain multiple
insecticidal crystal proteins), but is resistant to the MVP formulation, which contains only
Cry1Ac. This strain is also resistant to Cry1Ab, but not to Cry2Aa or Cry2Ab. Liang, Tan,
and Guo (2000) reported about 43-fold resistance in H. armigera to Bt cotton over 16 generations
of selection on the basis of mean concentration for 50% weight loss of larvae. Meng
et al. (2004) selected H. armigera on transgenic cotton using a leaf-feeding method for over
45 generations. Resistance to transgenic Bt cotton was detected after 12 generations. The
survival rates of F 12 neonates feeding on leaves (for 4 days) and boll-opening (fi ve days) of
R19 line were 34 and 72%, respectively, compared to 0% and 40% for the nonselected strain.
Resistance to B. thuringiensis HD-1 (Dipel) in neonates (F 12) was six-fold. After 42 generations,
the strain developed very high levels of resistance to Cry1Ac protoxin, 210 g kg 1
MVPII wettable powder, and 200 g liter 1 MVPII liquid formulation—the resistance ratios
being 540-, 580-, and 510-fold, respectively. Selection of P. gossypiella larvae for resistance to
Cry1Ac resulted in 300-fold resistance, and cross resistance to Cry1Aa and Cry1Ab, low
levels of resistance to Cry1Bb, but no resistance to Cry1Ca, Cry1Da, Cry1Fa, Cry2Aa, and
Cry9Ca. Larvae from the resistant colony showed 40% survival on the Bt transgenic cotton
expressing cry1Ac (Tabashnik et al., 2000a). A resistant colony has also been selected on
artifi cial diet containing leaf powder from the Bt transgenic cotton. A strain of recombinant
Pseudomonas fl uorescens Migula engineered to express the 130 kDa delta-endotoxin
protein of B. thuringiensis subsp. kurstaki strain HD-1 has been used to select a resistant
strain of H. virescens for 14 generations (Sims and Stone, 1991). After selection, the resistant
strain was 12-fold more resistant to the endotoxin than the susceptible strain.
Decreased susceptibility to Dipel has been observed in European corn borer, O. nubilalis,
colonies selected for resistance to this product under laboratory conditions (Huang et al.,
1999). Selection for resistance to Cry1Ac resulted in 160-fold resistance to this toxin.
However, continued selection resulted in a decrease in resistance (Bolin, Hutchinson, and
Andow, 1999). The highest level of resistance in O. nubilalis occurred between generations
7 (14-fold) and 9 (32-fold) for three different strains (Chaufaux et al., 2001). For each strain,
the level of resistance fl uctuated from generation to generation, although there were consistently
signifi cant decreases in toxin susceptibility across generations. These results
suggested that low levels of resistance are common among widely distributed O. nubilalis
populations.
Selection for resistance in beet armyworm, Spodoptera exigua (Hubner), to Cry1Ca resulted
in 850-fold resistance, and cross resistance to Cry1Ab, Cry2Aa, and Cry9Ca (Moar et al.,
1995). The larvae of beet armyworm pupated earliest when fed a Cry1Ac diet (fi rst 20% to
pupate) and produced offspring that developed signifi cantly faster on a Cry1Ac diet than
the parental strain (Sumerford, 2002). After two generations of selection, the selected population
pupated two days faster than the parental population. The selected group also
developed faster on transgenic Bt cotton (NuCOTN 33B) leaves than the parental strain.
Individuals selected on media containing Cry1Ac developed no more rapidly on artifi cial
diet containing Cry2Aa than the parental control colony of S. exigua. Selection for resistance
in S. littorallis to Bt strain expressing Cry1Ca resulted in 500-fold resistance, and