Undamaged cotton plants yield more if their neighbour is damaged: implications for pest management.

Understanding the compensatory responses of crops to pest damage is important in developing pest thresholds. Compensation for pest damage in crops can occur at the plant level, where the architecture, growth dynamics and allocation patterns of damaged plants are altered, allowing them to recover or, at the crop level, where differential damage between plants may alter plant-to-plant interactions. We investigated growth and yield of cotton (Gossypium hirsutum L.) following non-uniform manual defoliation of seedlings. This partially replicates real pest damage and is valuable in understanding crop-level responses to damage because it can be inflicted precisely. Damage distributions included damaging 0, 25, 50, 75 or 100% of the plants. Damage intensity for the damaged plants was varied by removing 100 or 75% of each true leaf when plants had two, four and six true leaves. At the crop level, yield loss increased as the proportion of plants damaged and intensity of damage per damaged plant increased. Neighbour interactions occurred; undamaged plants with damaged neighbours grew larger and yielded better than undamaged plants with undamaged neighbours, while the converse applied for damaged plants with undamaged neighbours. Neighbour interactions were influenced by the intensity of damage and were stronger when 100% of the leaf area was removed than when 75% was removed. At the crop level, when compared with yield estimates based on yield of plants from uniformly damaged or undamaged plots, these interactions resulted in higher yield than expected (+8%). This suggests that damage distribution may have to be considered in studies where artificial or real pest damage is inflicted uniformly on plants.