Nitrogen (N) is one of the main limiting elements for plant growth and reproduction, and because of its low content in plant tissues, it is also limiting to herbivores. Consequently, the application of N-containing fertilizers in agriculture may increase, unintendedly, the attractiveness of crop plants to herbivores. Decreasing the amount of N supplemented should decrease its negative effects on plant resistance against herbivores, although genotypes with naturally high levels of resistance may benefit less from such a measure. To test these predictions, and to quantify the extent to which fruit production decreases with reduced N addition, we grew four tomato varieties with varying whitefly-resistance levels under three levels of N fertilization. After eight weeks of growth, we collected foliar tissue to assess plant resistance to whiteflies by means of choice assays, and infested half of the plants in each N addition treatment with whiteflies. For each plant, we measured life-time fruit production and biomass allocation to leaves, stems and roots. We also estimated compensatory ability (a proxy of tolerance to herbivory) for plants subjected to whitefly infestation.
Results/Conclusions
Overall, plants fertilized with half of the commercially recommended amount of N had greater resistance against whiteflies, and only a 15%, non-significant decline in fruit production compared to plants fertilized at the full rate of N addition. The magnitude of the gain in resistance gained at lower rates of N addition varied by variety. Compensatory ability did not vary with the level of N addition. Varietal differences in resistance and tolerance showed a clear trade-off between the two modes of defence. Given the high environmental, social and economic costs of agricultural pest control, the use of naturally well-defended crop varieties (through either resistance or tolerance) could be combined with a fertilizer application rate at which the gains brought about by increased N availability are not exceeded by the losses caused by greater attraction of pests.