Contents - Faperta

12
Development of Resistance to Transgenic Plants
and Strategies for Resistance Management
Introduction
Biopesticides, particularly those based on Bacillus thuringiensis (Bt) Berliner, are valuable
alternatives to synthetic pesticides. Because of limited stability under fi eld conditions and
narrow activity spectrum, their use in pest management represents only a small fraction
of the total pesticide usage in agriculture. Deployment of Bt Cry proteins in transgenic
plants has overcome some of these drawbacks, and several cultivars of cotton, maize,
potato, tomato, and tobacco have been released for cultivation. Use of Bt transgenic crops
is expected to reduce pesticide use and provide a capability to control some of the insect
pests that have developed high levels of resistance to conventional pesticides. However,
concerns have been raised that deployment of insect-resistant transgenic plants will lead
to development of resistance in insect populations to the transgene, and to herbicide and
antibiotic genes used as selective markers. While some of these concerns may be real, others
seem to be highly exaggerated. Therefore, careful thought should be given to the production
and release of transgenic crops in different agroecosystems. Insect pest populations
have a remarkable capacity to develop resistance, and over 500 species of insects have
shown resistance to synthetic insecticides (Georghiou, 1990; Moberg, 1990; Rajmohan,
1998). Therefore, there is a need to take a critical view of resistance to Bt and other novel
genes that are being deployed for pest management.
The spores and toxins of Bt are degraded quickly, and have a half-life of one week to a
few months. Under UV light, the toxins are degraded within a few hours to a few days, while
the spores need soil for germination. Bt toxins can persist in the soil for a longer period
when absorbed on to humic acids. In such cases, the insecticidal activity is retained, and
the toxins are more resistant to biodegradation. Toxin accumulation increases with an
increase in the amount of humic acids (Crecchio and Stotzky, 1998). In comparison, transgenic
plants produce toxin proteins throughout the crop cycle in all parts of the plant.
Transgenic plants express semiactivated proteins, which are ingested by the insects.