Plants downregulate their defences against insect herbivores upon impending competition for light. This has long been considered a resource trade-off, but recent advances in plant physiology and ecology suggest this mechanism is more complex. Here we propose that to understand why plants regulate and balance growth and defence, the complex dynamics in plant-plant competition and plant-herbivore interactions needs to be considered. Induced growth-defence responses affect plant competition and herbivore colonisation in space and time, which has consequences for the adaptive value of these responses. Assessing these complex interactions strongly benefits from advanced modelling tools that can model multitrophic interactions in space and time. Such an exercise will allow a critical re-evaluation why and how plants integrate defence and competition for light. A strong physiological link exists between light and plant defences. Light signals denoting favourable conditions are positive mediators of plant defences whereas light signals denoting unfavourable conditions, including impending competition, are negative mediators of plant defences.Plant defences mediate dynamics in the composition and distribution of the insect herbivore community over time and space. These dynamics can exert strong selection for or against the inducibility of plant defences, depending on the ecological context of the plant.Plant defences fulfil multiple roles in addition to their defensive function. Secondary metabolites also play a role in primary metabolism, and defence-induced volatiles provide an informative function to other organisms in the ecosystem, including plants and insects of several trophic levels.