Contents - Faperta

Transgenic Resistance to Insects: Nature of Risk and Risk Management 433
Assessment of Risk to Agriculture
The importance of assessing the invasiveness of genetically modifi ed crops has been the
subject of much debate (Fitter, Perrins, and Williamson, 1990). While it is important to be
cautious when predicting weediness based on plant attributes, an assessment of such
characteristics can be made with much greater confi dence when assessing whether wellestablished
crops can invade agricultural or natural ecosystems. Modern crop cultivars no
longer possess the ability to become weeds, as this ability has been severely reduced in the
absence of gene introgression from wild relatives. Common distinctive attributes of weeds,
such as seed dormancy, phenotypic plasticity, indeterminate growth, continuous fl owering
and seed production, and seed dispersal have been bred out of the crop plants over
thousands of generations (Baker, 1965). Such changes appeared early in the domestication
of crop plants as a consequence of repeated sowing and harvesting cycles (Harlan, 1992).
These characters are not candidates for gene transfer back into crops, whether by genetic
modifi cation or traditional breeding, because they would severely reduce the agronomic
performance of a crop for modern farming practices. Furthermore, these attributes do not
arise from a single or few gene transfers. Therefore, genetically modifi ed crops are no
more likely to become weeds outside farming situations than the crop cultivars developed
through traditional breeding in the past.
Selection for Weedy Traits
Weeds generally exhibit a preference for habitats disturbed by human activities, such as
cultivated fi elds, fi eld margins, gardens, roadsides, soil dumps, and waste sites (Harlan
and de Wet, 1965). One feature that all weeds tend to share is high phenotypic plasticity
that allows continuous adaptation to changing environments. It is suspected that genetic
modifi cation for adaptation to drought or heat stress may change some of these characteristics
of the wild and weedy relatives of crops through gene fl ow. Closely related species
can sometimes be highly successful as weeds as they possess different combinations of
weedy characteristics (Williamson, 1993). Whether a genetically modifi ed plant species
becomes a serious weed in a new environment may relate more to its ability to grow in the
new environment coupled with the absence of effective natural enemies such as insect
herbivores and diseases (Williamson, 1994). There has been a close association between
the evolution of weeds and the domestication of crop plants (de Wet and Harlan, 1975;
Harlan, 1992), and some weeds mimic crop characteristics such as growth form and
maturity cycle (Barrett, 1983). Selection away from cultivated attributes and gene introgression
between genetically modifi ed crops and wild relatives may lead to evolution of
weeds with better adaptation to the environment, and this aspect needs to be carefully
assessed for each transgene and crop combination.
Invasiveness
An appropriate measure of invasiveness of a plant is its fi nite rate of increase (Crawley,
1986), the constant at which a population increases over time, assuming a stable age distribution,
and the absence of density-dependent constraints. Risk assessment encompasses
all demographic processes regulating population growth of a species, such as rate of
growth and maturity, fecundity, seed survival, seed germination, and seedling survival to