1. Gregarious behaviours in phytophagous insects are common, but their adaptive value is rarely well understood. In the present study, we document a novel case of cooperative behaviour, the aggregative oviposition of a leaf beetle, Pyrrhalta viburni Paykull, and the realised fitness benefit of overcoming a plant defensive response (wound tissue production). 2. In laboratory choice-tests, females exhibited aggregative oviposition, characterised by (i) a preference for twigs already infested by conspecifics, and (ii) positioning of new egg masses adjacent to existing ones. Field observations supported laboratory results, showing that P. viburni egg masses are most commonly found aggregated in large clusters. 3. In a field experiment using three host plants (Viburnum dentatum L., Viburnum opulus L. and Viburnum Ã— bodnantense Aberc. ex Stearn), mean egg survivorship and twig mortality increased, while twig wound response decreased, with an increasing level of infestation. Egg survivorship was consistently higher on twigs that died than on twigs that remained alive. 4. Overall, these results suggest that, by aggregating their egg masses, P. viburni females overwhelm the twig wound response, often killing the twig in the process. Aggregative oviposition and low defences of V. dentatum to P. viburni could have facilitated the establishment and spread of this beetle in its introduced range (northeastern North America), in areas where V. dentatum is abundant.