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Conventional signals are maintained via social costs and commonly used in the animal kingdom to assess conspecifics' agonistic ability during disputes over resources. In the last decade, some experimental studies reported the existence of visual conventional signals in several social wasp species, being good rank predictors in different social contexts. Females of the social wasp Polistes gallicus do not cooperate to start nests but they often try to usurp conspecific nests. Here, we showed that the reproductive females of this species have variable facial colour patterns that function as conventional signals. Wasps with larger black spots on their clypeus are more likely to successfully overwinter, are larger, and are better at fighting and at holding a nest. Furthermore, in field experiments, resident foundresses rely on facial pattern to assess usurpers' fighting abilities, modulating their defence reaction accordingly, so that rivals with larger black spot receive more aggression than rivals with smaller or no black spots on the clypeus. Our study reveals that visual recognition abilities are widespread among paper wasps that, regardless of their social biology, face similar selective pressures within competitive contexts. [ABSTRACT FROM AUTHOR]