Voltinism in Chaoborus

This article is to report on a piece of work carried out on Chaoborus crystallinus in order to establish how many generations per year could be produced under semi-natural conditions.
The work was undertaken by Agrochemex Environmental at their facility in Essex, England, on behalf of ADAMA Makhteshim Ltd.
A series of outdoor microcosms was established using plastic tanks, sunk into the ground and provided with sharp sand, lake sediment, borehole water and washed Elodea canadensis. In addition, shredded alder leaves, live Asellus and Gammarus, and zooplankton (rotifers, copepods and Cladocera) were added. Each microcosm was covered with a 1 mm mesh screen to exclude other insects. Finally, one replicate in each series was "seeded" with several hundred 4th instar Chaoborus larvae. These were allowed to complete their development, emerge and lay egg rafts, and these egg rafts were used to seed further replicates.
The process was repeated and up to four sequential generations were recorded in the year (May to September, 2017). The significance of these results are that Chaoborus crystallinus (and other Chaoborus species) have until now been assumed to be univoltine, or possibly bivoltine in hot summers (Berendonk & Spitze, 2006; Verberk et al., 2008). The fact that several size classes of larvae occur together had been explained by wide variations in development time. However it is clear from these results that an alternative explanation is that Chaoborus is multivoltine, and the presence of overlapping generations explains the co-existence of several instars in natural populations.
Chaoborus crystallinus is a frequent subject for ecotoxicology studies, and interpreting experimental observations requires a complete understanding of the life history of the species. A full account of the study will be published in due course, but this note should alert anyone working on Chaoborus to appreciate its potential to be multivoltine.

Robert Cockroft (r.cockroft@agrochemex.com)
Tony Irwin (dr.tony.irwin@gmail.com)

References
Berendonk, T.U., & Spitze, K., 2006. Gene flow between regions: The population genetic structure of the phantom midge Chaoborus crystallinus (Diptera: Chaoboridae). - Limnologica 36: 147-154.
Verberk, W.C.E.P., H. Siepel & H. Esselink, 2008. Life history strategies in freshwater macroinvertebrates. - Freshwater Biology 53: 1772-1738.

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