Summary1. Changes in plant physiological features after herbivory may alleviate the adverse effects of herbivores. We examined the ability of Ilex aquifolium L. (European Holly) plants to compensate for scale insect herbivory through increased photosynthesis. Based on assumptions of the source-sink hypothesis, we predicted that scale insects, by inducing supplementary sinks for photosynthates, would increase photosynthesis. 2. Photosynthetic performance was evaluated by measuring chlorophyll fluorescence parameters in plants growing under natural light and temperature at the field station of the University of Santiago (north-west Spain). We measured photosynthesis on infested leaves of infested trees; scale-free leaves of infested trees; and scale-free leaves of scale-free trees. 3. Insect infestation significantly increased photosynthetic efficiency. Effects of scale insects on photosynthesis were particularly enhanced by high temperature and light. Scale insects also altered the photosynthesis of leaves not directly affected by the insects. 4. Our study showed that changes in the balance between source and sink tissues, induced by scale insects, may partly explain host plant photosynthesis. We suggest that increased feeding rates of scale insects promoted by high temperature and light conditions would increase the carbon demand of infested plants, resulting in greater compensatory photosynthesis.

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