Contents - Faperta

Development of Resistance to Transgenic Plants 383
of this gene confer resistance in strains from Hawaii, Pennsylvania, and Philippines. The
biphasic nature of the lepidopteran genetic linkage map has been used to detect this gene
in diamondback moth, with 207 amplifi ed fragment length polymorphism (AFLP) DNA
markers. An AFLP marker has been cloned and sequenced for the chromosome containing
the Bt resistance gene. Resistance in O. nubilalis to Bt appears to be inherited as an incompletely
dominant autosomal gene, indicating that if the fi eld resistance turns out to be
dominant, then the high dose/refuge strategy may be ineffective for management of resistance
to this pest.
Strategies for Resistance Management
To preserve the value of Bt endotoxins in crop protection, it will be necessary to implement
resistance management measures from the very beginning. Individual farmers have
limited incentive to adopt resistance management technologies for Bt endotoxins, and the
greatest incentive lies with the Bt industry (Kennedy and Whalon, 1995). However, the
implementation of a coordinated, industry-wide, Bt resistance management effort is likely
to be constrained by competition among different segments of the Bt industry interested
in different technologies (sprays versus transgenic plants), and among producers of Bt
products using the same technology. Successful implementation of resistance management
for Bt endotoxins will require that the Bt industry prepackage resistance management
techno logies, and that these prepackaged resistance management strategies do not
add signifi cantly to the costs or complexity of pest control by the end user. In view of
criticism of the Bt transgenic crops for their ability to enhance development of resistance,
prevention of development of resistance to Bt is the best policy. The Bt resistance management
tactics have been incorporated in the regulation of Bt transgenic crops in most countries
(Haliday, 2001). Transgenic crops must produce high doses of toxin and there should
be nontransgenic crops or other alternative host crops as refuge for the production of
susceptible insect populations.
To increase the effectiveness of the transgenic plants, it is important to implement the
resistance management strategies from the very beginning. A number of conceptual strategies
have been developed for resistance management (McGaughey and Whalon, 1992;
Tabashnik, 1994; Gould, 1998). A high level of migration or a sensible reduction of the fi tness
associated with the change in the genome may lead to a long-lasting effi cacy of the transgenic
crops (Arpaia, Chiriatti, and Giorio, 1998). Most of the strategies for resistance management
are based on mixtures of toxins to be deployed for insect control, tissue-specifi c
production, and induced toxin production. Two or more insecticidal proteins or different
genes can also be introduced into the same plant to slow down the rate of development of
resistance in the target insects. Plants can also be engineered so that the toxin is produced
only in the tissues where the insect feeds. In induced toxin production, the plants can be
engineered in such a way that the toxin is produced when the insect starts feeding.
The strategies for resistance management would depend on the number and nature of
gene action, insect behavior, and insect-genotype-environment interaction. Spring movement
of emerging adults onto wild hosts may delay the development of resistance if the
movement of adults is far enough from the fi eld in which the pupae over-wintered. Increase
in the summer migration and the distance moved will also delay resistance development.
It has been suggested that Bt sunfl ower might not lead to the development of Bt-resistant