Division of labour is a major feature of insect societies. Behavioural differences can be present also during non-colonial stages of the life cycle, when it is difficult to discriminate between distinct behavioural phenotypes and by-products of differences in overall activity levels. We used the social wasp Polistes dominulus to address this issue. In pre-hibernating aggregations some individuals (helpers) perform external tasks by collecting food and providing it to cluster mates. Such helpers have been so far identified using only a descriptive approach, and their behaviour was not disentangled from a possible higher level of overall activity. Here we provide an operational definition of the helper's trait and we then compare behavioural patterns of helpers and non helpers, verifying that helpers actually represent a peculiar behavioural phenotype. Our result expands knowledge on the caste differentiation issue in Polistes wasps and on the assessment of behavioural phenotypes in a non-colonial context.