All plants respond to herbivore-inflicted damage with the enhanced emission of volatile organic compounds (VOCs), and plants in numerous taxa also respond with the secretion of extrafloral nectar (EFN) . Both VOCs and EFN attract adult parasitoids and predators (hereinafter collectively termed ‘carnivores’), an effect that can significantly reduce herbivore pressure on wild plants . Nevertheless, relatively few attempts have made conscious use of VOCs or EFN for biological pest control and, to the best of our knowledge, classical breeding has never aimed to improve anti-herbivore defense via VOCs or EFN 1 for the first attempt to genetically engineer wheat (Triticum aestivum) . How and to what degree these traits can be optimized to allow better biological control of pests and crop diseases?
With consideration that I have not practically work on this topic but I think there is a general negative correlation between defense system of plant and yield quantity or quality (commercially valued). For example silica aggregation in leaf or stems of wheat causes some problems for pests feeding them but this trait probably contrast with grain yield according to energy-matter balance rule. We should remember that yield is the trait that controlled by many complex genes and breeding for yield is the way that separated from gene manipulation for surviving in nature. optimization point is based on our attitude to eco-production (economic or ecologic).
There is no hard and fast rule. However factors associated with the control programmes (invasive region, native region, plant growth form, target longevity, control agent guild, taxonomy and study duration) have varying patterns in control success. On average, biocontrol agents significantly reduced plant size (28 ± 4%), plant mass (37 ± 4%), flower and seed production (35 ± 13% and 42 ± 9%, respectively) and target plant density (56 ± 7%). Beetles in the Chrysomelidae and Curculionidae families were more effective at reducing plant size than other groups.
Source: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2664.2012.02209.x/abstract
As the above comments imply with respect to bioengineered varieties, every adaptation costs in terms of energy, But two questions remain:
1. Is the yield loss even going to be noticeable?
2. Is the yield loss going to exceed the gain through reduced loss to herbivores, to other pest control expenses, and to quality loss. Such questions are probably best answered in terms of actual monetary costs and returns.
Peripheral to this question is that classical plant breeders tend to use insecticide to protect their precious cultivars and accessions while breeding for such things as yield. An approach which would have some of the effect you seek is to have unsprayed plots identical to the sprayed breeding plots. Then the breeder can evaluate herbivore damage while preserving breeding stock. However, the breeder must be certain to evaluate in terms of the effect of herbivore caused reduction in yield rather than apparent damage. The latter can be quite misleading in some crops (for instance soybeans).
Trap crops which can attract the oviposition but higher mortality in larvae.
I agree with Lewis, he has appropriately mentioned an approach which would test unsprayed plots identical to the sprayed breeding plots. Further herbivore damage can be worked out.
True much of the breeding work is usually based on increased yield output, not developing plant cultivars to support predators and parasitoids which suppress pest densities. Once the focus of promoting such plant is incorporated into breeding programs then the target agro ecosystems will be richer in biodiversity of such bio agents. The efficiency of the cultivar technology will be depended upon host specificity level of the bio agent and its survival on the plant diet in the absence of the prey.
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