Toona ciliata that suffer frequent height-reducing herbivore damage by a shoot-boring moth (Hypsipyla robusta) are taller

Herbivore damage is expected to reduce plant growth. The plant vigour hypothesis suggests, however, that many herbivores are attracted to fast growing plants. If these two opposing forces occur, what is the expected relationship between plant size and herbivore damage? We ask this question by focusing on variation in Toona ciliata, a tree that suffers chronic infestation by the shoot-boring moth Hypsipyla robusta. Considering trees between 6 and 36 months old, the dominant pattern was a positive relationship between tree height and the frequency of damage received. This pattern in cumulative damage is explained by the incidence of fresh damage recorded in each survey, which was positively associated with tree height. By controlling for this attractive effect of plant height it was also possible to detect a transient negative impact of damage on plant growth: trees with current H. robusta damage experienced smaller increases in height than those without damage. When plant vigour and attractiveness to key herbivores are strongly linked evolution of plant defences will be inextricably entwined with whole plant architecture and growth strategies.