Contents - Faperta

392 Biotechnological Approaches for Pest Management and Ecological Sustainability
moths moving out from crops diminished over a 40 m distance. Dispersing moths penetrated
30 m into new crops to infest the foliage. Direct movement of potato tuberworm and
diamondback moth between crops has been estimated through mark-recapture experiments.
Sweep-net collections of potato tuberworm indicated that 17% of moths moved
between crops. Only 1.2% of the diamondback moths were recaptured by sweep-net outside
the release area, and very few moths were caught in pheromone traps. The results
suggested that suffi cient numbers of potato tuberworm would forage between adjacent
treated and untreated crops to minimize the development of resistance. Use of refuges to
conserve susceptible pest populations has also been recommended for managing resistance
that may arise from future use of Bt-transgenic potatoes or Brassica spp. in New
Zealand. The refuges that are intended to dilute potential resistance of potato tuberworm
to transgenic crops should be placed close to transgenic potato crops.
The Bt-transgenic cotton in China covered over 1.46 million ha in 2001. In Shandong and
Hebei provinces, Bt cotton occupies 80% and 97% of the cotton area, while other provinces
such as Anhui and Jiangsu have less than 30% of the Bt cotton area (Dong et al., 2004). The
high dose and refuge strategy widely accepted in the United States and Australia for
Bt crops is not feasible in China in view of a lack of a consistent high dose of Cry1Ac toxin
in the cotton varieties, and the complexity of small farming by marginal/poor farmers
in a region (Ru, Zhao, and Rui, 2002). Although corn provided refuge to H. armigera larvae
for Bt cotton, nevertheless, resistance management is necessary for regulation of Bt transgenic
crops.
Resistant larvae of P. gossypiella on Bt cotton took longer to develop than susceptible
larvae on non-Bt cotton (Liu et al., 1999). This developmental asynchrony favors nonrandom
mating that could reduce the expected benefi ts of refuge strategy. Consistent with one
of the assumptions of the refuge strategy, P. gossypiella resistance to Bt cotton is recessive.
Survival of the hybrid F 1 progeny (2%) was not higher than the survival of the susceptible
strain (6%), and both were markedly lower than the survival of the resistant strain (37%).
Resistant larvae on Bt cotton required an average of 5.7 days longer to develop than susceptible
larvae on non-Bt cotton. Field data suggested that median longevity of male
P. gossypiella is less than one week, and 80% of moths mate within three days of emergence.
This developmental asynchrony, therefore, favors assortative mating among resistant
moths from Bt plants. In the fi eld, the extent of developmental asynchrony and assortative
mating would be affected by variation in toxin expression, weather, and overlap between
generations. Deliberate inclusion of a refuge may also reduce the proportion of marketable
produce, and may affect use of this resistance management strategy in both Bt sprays and
transgenic crops expressing Bt toxins (Perez, Shelton, and Roush, 1997).
Removal of Alternate Hosts and Destruction of Carryover Population
Removal of alternate hosts in case the alternate hosts play an important role in pest population
buildup will also be effective in delaying the development of resistance to Bt toxins
through reduction in carryover of the post population from one season to another. Efforts
should be made to remove the alternate hosts of the pests from the vicinity of the crop.
This will help in reducing the density of the pests, and low to moderate levels of insect
abundance can be effectively controlled through transgenic plants. However, this approach
may reduce the number of susceptible individuals, which otherwise will act as a source of
refuge to dilute the frequency of resistance alleles.
Destruction of the carryover population, for example diapausing larvae or pupae, that
have been exposed to transgenic crops in the previous generation, is another important