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Social wasp swarm emigration: males stay behind

Many species of tropical social wasps initiate colonies by means of a swarm of queens and workers. Swarms either bud from mature colonies (reproductive swarms) or form from the entire adult population when it evacuates following destruction of the nest by predation or accident (absconding swarms). Colonies giving rise to founding swarms of either type may have males in their populations. Swarms emigrate along a chemical trail to a site selected by scouts, where the workers begin building a new nest. Published analyses of newly-founded colonies indicate that such colonies occasionally contain males. This raises the question of whether these are alien males who joined the new colony shortly after its founding, or are siblings of the females, having emigrated with them from the natal nest site. We used a double censusing technique to determine whether males of the Neotropical social wasp, Polybia occidentalis, emigrate with females in absconding swarms. Nine absconding swarms in Costa Rica were censused and sexed directly before they emigrated and were shown to contain males as well as females. When the swarms departed for the new site, males remained behind, and when the swarms were censused again soon after reaching their new sites, none contained males. Thus males of this species cluster temporarily with absconding females, but fail to emigrate with them. We suggest that the males’ failure to join pre-emergence colonies is in the best interest of both sexes.