Contents - Faperta

380 Biotechnological Approaches for Pest Management and Ecological Sustainability
only 2.3-fold resistance to Cry2Aa, suggesting that Cry1Ac resistance extended to Cry1Ab,
but not to Cry2Aa. Helicoverpa armigera resistant to Cry1Ac did not possess cross resistance
to Cry1C and Cry2Aa, but was resistant to Bt corn expressing Cry1A (Zhao et al., 2000b).
Chandrashekar et al. (2005) also observed 31.4-fold resistance to Cry1Ac, 25.4-fold resistance
to Cry1Ab, and only 8.4-fold resistance to Cry1Aa in strains of H. armigera selected
for resistance to these Bt toxins.
Resistance to the Bt formulation, Javelin increased 1.9 to 4.4 times under fi eld conditions,
but was signifi cant only at an application rate of 1.12 kg ha 1 , irrespective of the presence or
absence of a refuge (Perez, Shelton, and Roush, 1997). Selection with Javelin at 0.3 kg ha 1
or Xentari did not cause a signifi cant increase in resistance to Btk nor did P. xylostella
selected with Xentari resistance show resistance to B. thuringiensis subsp. aizawai. Two strains
of the diamondback moth, P. xylostella, selected using Cry1C protoxin and transgenic broccoli
plants expressing cry1C, were resistant to Cry1C, but had different cross-resistance
patterns (Zhao et al., 2001). In a study involving 12 protoxins (Cry1Aa, Cry1Ab, Cry1Ac,
Cry1Bb, Cry1C, Cry1D, Cry1E, Cry1F, Cry1J, Cry2Ab, Cry9Aa, and Cry9C), the resistance
ratio (RR) of one strain (BCS-Cry1C-1) to the Cry1C protoxin was 1,090-fold with a high
level of cross-resistance to Cry1Aa, Cry1Ab, Cry1Ac, Cry1F, and Cry1J (RR 390-fold). The
cross resistance to Cry1A, Cry1F, and Cry1J in this strain was probably related to Cry1A
resistance gene(s) that came from the initial fi eld population as a result of intensive sprayings
of Bt products containing Cry1A protoxins. The neonates of this strain survived on
transgenic broccoli plants expressing either cry1Ac or cry1C toxins. The other strain
(BCS-Cry1C-2) resistant to Cry1C did not show cross resistance to other Bt protoxins. The
neonates of this strain survived on transgenic broccoli expressing cry1C, but not cry1Ac.
Genetics of Resistance
Frequency of Resistance Alleles
Frequency of resistance allele between generations in the insect population is based on the
selection intensity, fi tness cost, percentage of refugia, mortality in refugia due to insecticide
sprays, initial frequency of the resistance allele, and the rate of insect movement
(Cerda and Wright, 2004). A simulation model for resistance development based on four
different spatial patterns of refugia inside the fi eld (border, central, equidistant, and random),
four crop rotation patterns, fi ve different sizes of refuge (5% to 50%), and two levels
of non-Bt insecticide plus an untreated control in the refugia indicted that:
• The greater the size of refugia, the lower the rate of increase in the frequency of
resistance alleles.
• Positioning patches of refugia at random in the fi eld resulted in a higher rate of
increase in the frequency of resistance alleles compared with nonrandom
positions.
• Positioning refugia along the border resulted in a lower rate of increase in the
frequency of resistance alleles compared with equidistant and central patterns.
• The frequency of resistance allele increased markedly when the refugia was less
than 10%.