Population Dynamics and Seasonal Patterns of Chironomus plumosus (Diptera, Chironomidae) in the Shallow Lake Trasimeno, Central Italy

Field sampling of littoral macrobenthos of the shallow Lake Trasimeno was conducted along 17 years (2005–2021) on 129 different occasions. This long-term field study deepens the knowledge concerning the life cycle of Chironomus plumosus (Diptera), the main responsible for summer chironomid swarms that adversely affect human littoral activities, providing useful information for its management. About 108,000 macrobenthic specimens were collected, belonging to Oligochaeta (Naididae) (62%), Diptera (Chironomidae) (37%), and only 1.5% to other invertebrate taxa. Eighteen chironomid taxa were found. The trend of chironomid density was not affected by C. plumosus, which showed a maximum increase in September. This peak is justified by the presence of large swarms of C. plumosus in late August in which the populations of the central area of Lake Trasimeno consistently participate. The larval density of this species did not increase over the 17 years. A detailed analysis of the sampled larvae and adult biomass catches from 2017 to 2020 reveals that four annual swarmings occurred: in April, July, August, and September–October. The water temperature remains higher than 20 °C during the night hours from the end of May to mid-September, strengthening the hypothesis of the three midge swarming cycles in the summer period until early autumn.