Swarming promotes <i>Tropilaelaps mercedesae</i> (Mesostigmata: Laelapidae) dispersal in <i>Apis mellifera</i> (Hymenoptera: Apidae)

The ectoparasitic mite Tropilaelaps mercedesae (Delfinado & Baker 1961: Mesostigmata: Laelapidae) poses a major emerging threat to Apis mellifera (Linnaeus 1758: Hymenoptera: Apidae), with its recent detection in Eastern Europe raising concerns over dispersal biology and potential for transcontinental spread. Understanding intra- and inter-colony transmission is crucial for predicting epidemiological impact. While forager-mediated dispersal has been demonstrated under tropical conditions, little is known about transmission in temperate climates. We investigated T. mercedesae dispersal via swarming under temperate conditions in Georgia. In a natural swarming event, 8 female T. mercedesae mites transferred from the source colony. Four died within a week, but the remaining four mites entered and became sealed within the first sealed brood produced by the colony. Two of these mites reproduced, confirming swarm-mediated phoretic mite persistence. To examine the survival of phoretic mites in broodless, queenright colonies, as well as during the building of new colonies, we created two artificial colonies by gathering returning foragers which originated from T. mercedesae infested colonies. Dispersal via foragers was confirmed as the artificial colonies contained 23 and 17 mites, respectively. However, none successfully reproduced, despite colonies producing brood within the same timescale as the natural swarm, and all mites died within 4 to 6 d. Furthermore, we provide the first evidence of T. mercedesae transmission via swarming under temperate conditions, with natural swarms facilitating survival and reproduction. These results highlight swarming as a viable mechanism for mites' dispersal, potentially enabling rapid expansion of T. mercedesae in newly invaded environments.