Contents - Faperta

394 Biotechnological Approaches for Pest Management and Ecological Sustainability
dynamics and movement of insects between transgenic and refuge fi elds on evolution of
insecticide resistance have been examined in two different simulation models (Caprio,
2001). The two models were developed to test the hypothesis that increasing the habitat
from fi ne-grained to coarse-grained, and the resultant increase in nonrandom mating,
would increase the rate of local adaptation and hence evolution of resistance. The fi rst, a
stochastic, spatially explicit model, altered habitat by varying adult dispersal rates between
habitat patches. In contrast to the expectation that increasing patch isolation and increasing
the coarseness of the habitats would increase the rate of resistance evolution, intermediate
levels of dispersal actually delayed resistance by as much as fi ve-fold over the range
of dispersal levels observed. A simple deterministic model was also developed to separate
the impact of mating and ovipositional behavior. This model showed qualitatively the
same results. Under similar assumptions, it predicted longer delays in resistance evolution.
In this model, nonrandom mating alone increased the rate at which insects adapted
to transgenic crops, but nonrandom mating in combination with nonrandom oviposition
could signifi cantly delay resistance evolution. Differences between the two models may be
due to population regulation incorporated in a spatially explicit model. The models clearly
suggested that resistance management programs using untreated refuges should not overemphasize
random mating at the cost of making the habitat too fi ne-grained.
Since resistance gene(s) do not appear to be completely recessive, the need for a higher
proportion of refuge vis-à-vis transgenic crop has often been advocated (Gould and
Tabashnik, 1998). The resistant individuals emerging out of Bt cotton should mate randomly
with those from the refuge where the susceptible individuals breed. However, synchrony
of emergence of adults from Bt cotton and refuge will be equally essential. Helicoverpa
armigera, a highly mobile insect, may mate randomly, although closeness of refuge as a
policy also contributes to its success. Soybean and peanut as mixed crops could provide
natural refuge for the second to fourth generations of H. armigera in North China (Wu,
Guo, and Gao, 2002). However, function of refuge appeared to depend on the proportion of
Bt cotton. However, there is a need to protect the natural refuge of noncotton host plants of
H. armigera by not commercializing both Bt cotton and Bt corn in the same area (Zhao
et al., 2000b). In India, small farm holdings may not necessarily follow refuge norms. As
the transgenic cotton area increases, it may lead to almost monocropping without a signifi -
cant contribution of mixed crops or nontrangenic Bt cotton towards the production of
susceptible individuals.
Conclusions
The use of crop protection traits through transgenics will continue to expand in the future,
and gene pyramiding might become very common. This approach of controlling insects
would offer the advantage of allowing some degree of selection for specifi city effects,
so that pests but not benefi cial organisms are targeted. Resistance to Bt is a recessive or
partially recessive mutation in a major autosomal gene, and cross resistance extends to Cry
proteins sharing the same binding site(s). Cry proteins with specifi city to different binding
sites should be considered for gene pyramiding as a component of a resistance management
strategy. In a vast majority of cases, resistance has been found to be quite unstable
because of fi tness costs associated with resistance, although there are a few exceptions. A
detailed understanding of the mechanisms, insect biology, and plant molecular biology