1. Environmentally induced variation in plant defences may partially account for variation in herbivory rates among plants. 2. Some herbivores alter the microenvironment within a host to promote favourable changes in plant tissues. Caterpillars from the families Pyralidae and Ctenuchidae form a roll of leaves around an expanding bud which reduces light availability by 95%. 3. In the neotropical shrub, Psychotria horizontalis, shading by the roll of leaves decreased the toughness of the expanding leaves by 31% and leaf tannin concentration by 15%, while having no effect on two measures of nutritional quality, nitrogen concentration and water content. Moreover, herbivores preferred leaves that had grown within a leaf roll over control leaves (P<0.001). 4. This behavioural mechanism may allow the herbivore to disable an otherwise well-defended plant.