Chironomids (Diptera) of Lake Trasimeno: population status of the pelagic zone

At Lake Trasimeno Chironomids (Diptera) are considered a cause of discomfort for residents and tourists because of their intense swarms attracted by light sources during summer, interfering with human activities. The shallow lake (maximum depth < 6 m), the formation of areas rich in organic material and the lack of an effective predation in the ecosystem determine an ideal environment for the proliferation of these insects. In particular, Chironomus plumosus is the species that causes the main nuisance. Since 2006, to reduce the density of the Chironomid larval populations, the biological larvicide product Bacillus thuringiensis var. israelensis has been used. This treatment is applied only to the littoral zone, where the water is not deeper than 2 m. Since the shallow waters provide a suitable habitat for Chironomid life cycle for all the lake extension, the Chironomid larval populations were investigated also in the pelagic zone. During 2018 and 2019 the macrobenthic community of the pelagic zone of Lake Trasimeno was investigated through 14 monthly samplings (May 2018 - June 2019) at 3 sites, with a depth of about 4-5 m. Samplings were carried out with a compressed air dredge (sampling surface 400 cm2), at each site 5 samples were collected. The macrobenthic community was composed mainly of Chironomids and Oligochaeta. About 2,071 (30%) and 4,660 (70%) Chironomid and Oligochaeta specimens, respectively, were sampled. Negligible numbers of other macroinvertebrate taxa were found (mainly the alien invasive species Dreissena polymorpha). The average densities of the macrobenthic community were about 250 ind./m2, 550 ind./m2, and 0.4 ind./m2, for Chironomids, Oligochaeta and other macroinvertebrate taxa, respectively. The Chironomid community was composed of only 3 taxa: Chironomus plumosus (95%), Tanypus sp. (4%), and Cryptochironomus sp. (1%). From the comparison of the larval densities in the months of July and August 2018 (when the nuisance problem caused by adult emergences is more consistent) between the littoral and pelagic zones, the total Chironomid density was almost equal, but regarding only Chironomus plumosus, the larval density of the pelagic zone was three times higher than the littoral one (about 600 vs 200 ind./m2, respectively). Thus, the pelagic zone (90% of lake surface) represents the main area of C. plumosus adult emergences and therefore the source of the Chironomid-related nuisance problem.