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Keywords:

  • induced plant defence;
  • insect herbivory;
  • leaf trichomes;
  • Salix cinerea;
  • tolerance

Summary

  • • 
    To circumvent the inherent problem of discriminating between the cost of losing photosynthetic tissue and the cost of producing an inducible defence, the growth response of herbivore-damaged plants was compared with plants damaged mechanically to the same extent but without eliciting the defence.
  • • 
    Two experiments were conducted, studying the response of willows (Salix cinerea) to damage by adult leaf beetles (Phratora vulgatissima).
  • • 
    In the first experiment, willows produced new leaves with an enhanced leaf trichome density 10–20 d after damage, coinciding in time with the feeding of beetle offspring. The response was relaxed in foliage produced 30–40 d after damage. In the second experiment, which also included mechanical damage, willows exposed to beetle feeding showed an increase in leaf trichome density of the same magnitude (> 70%) as in the first experiment. The cost of producing the defence was a 20% reduction in shoot length growth and biomass production. Willows exposed to mechanical damage had an 8% reduction in shoot length growth compared with control plants, that is, a cost of leaf area removal.
  • • 
    The results are the first quantitative estimates of the cost of a plant defence induced by natural and low amounts (3.3%) of herbivory.