Dragonflies in mosquito larvae control: a laboratory experience on predation

The aim of the study was to evaluate the suitability of dragonflies larvae for the biological control of mosquitoes, in particular in lentic, temporary water bodies in which fish predators are lacking. Odonata larve have been collected in the field and chosen among those of the last three ages; mosquito larvae of the species Culex pipiens have been reared in the laboratory and used for the experiments once they reached the 3th and the 4th stages. 24 hours before the experiments Odonata were transferred to mesocosms with no food;. Experiments have been carried on in 8 litres aquariums, with a metal net inside, simulating a macrophyte substratum. In each aquarium one odonate larva and 20 mosquito larvae were put. After 24 hours the number of remaining mosquitoes was counted and, at the same time, the presence of mosquito pupae or dead mosquitoes were recorded, For each odonate larvae the total body length (except caudal lamellae for the Zygoptera) was recorded, together with the moult or moult signs. About 260 specimens of nine species of Odonata (Chalcolestes viridis, Lestes barbarus, L. virens, Coenagrion puella, Ischnura elegans, Erithromma lindeni, E. viridulum e Sympetrum sanguineum, Sympetrum striolatum) have been used for experiments. Each odonate individual was used only once. We considered as measure of the predation rate in 24 hours the ratio (in %) between the number of consumed mosquito individuals and the available one, being the available mosquitoes the difference between 20 and the dead and pupate individuals. Results have shown average consumptions ranging from 27% to 40% in the case of Coenagrionidae and from 33% to 40 % in the case of Lestidae; average consumptions of Sympetrum were of 57% and 75% for S. sanguineum and S. striolatum respectively The expected positive correlation between consumption and size was significant in two cases only, while the effect of the moult was not univocal The study was financed by a grant of Latium region administration, (research project LILAZ, dragonflies in biological control of mosquitoes) to G. Carchini