Ecological, Agricultural, Genetic, and Commercial Considerations in the Deployment of Insect-resistant Germplasm

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Abstract
We make an attempt to identify the agricultural and ecological factors of greatest importance in assessing the applicability of different modalities (antibiosis, antixenosis, tolerance) and levels of insect resistance to the pest management requirements of different crops and cropping systems. Emphasis is placed on the importance of matching the type and level of resistance to the pest's biology (e.g., feeding habits, development on the crop, alternate hosts, patterns of invasion of the crop, number of generations passed in the crop) and the production requirements of the crop. We illustrate how, depending on context, the use of a particular modality and level of resistance may simplify pest management, reduce crop losses without simplifying pest management, or by changing a pest whose occurrence in damaging populations is highly predictable to one whose occurrence is irregular and unpredictable, complicate pest management. The HELSIM Heliothis zea population dynamics model is used to illustrate how simulation models can be used to explore the consequences of deploying particular modalities and levels of insect resistance. In exploring genetic considerations in the use of insect-resistant germplasm, we focus on the problem of maximizing the durability of insect resistance by minimizing selection for virulent biotypes. We examine whether some modalities of resistance are inherently more stable than others regarding selection of virulent biotypes, and argue that in judging the inherent durability of a particular resistance, exclusive focus on the genetic nature of the plant resistance is inadequate. Knowledge of the genetic variability of the target pest vis à vis the plant resistance and an understanding of the direct biological effects of the resistance on the insect are also essential. The selection pressure for virulent insect biotypes exerted by resistant crop cultivars is shown to be dependent upon the modality of resistance as well as the agricultural and ecological context in which it is deployed. Simulation models are used to illustrate how different deployment strategies can affect the durability of resistance.

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