Emergence timing and voltinism of phantom midges, Chaoborus spp., in the UK

Abstract

20 After introduction of overwintered fourth instar larvae (2027 in total), emergence timing of adult 21 Chaoborus spp. (Diptera: Chaoboridae) was investigated in four outdoor freshwater microcosms in 22 the UK in 2017. Adults started emerging on 13 April and emergence reached a peak on 2 May. The 23 majority of emergence was completed by 3 June. Emergence rates for each microcosm ranged from 24 51.4% to 66.2% with a mean of 60.9%. The great majority of emerged adults were C. obscuripes 25 (99.68%). Males appeared to emerge slightly earlier than females. The results indicated that for 26 overwintered C. obscuripes larvae, the adults emerged en masse in spring (rather than emerging 27 gradually over the course of spring and summer). In a separate experiment at the same location, the 28 number of Chaoborus spp. life-cycles occurring per year was determined using six replicate groups of 29 microcosms, each group containing four microcosms. Each microcosm contained 200 L of water and 30 was enclosed within a 'pop-up' frame covered with 'insect-proof' mesh (1 mm 2 aperture). The first 31 microcosm in each group was 'seeded' with egg rafts (first generation) of Chaoborus spp. Following 32 adult emergence, as soon as the first egg rafts were laid in each microcosm these were removed and 33 transferred to the second microcosm in that group, and so on. The larvae sampled from the second and 34 subsequent generations in the microcosms were all C. crystallinus. C. crystallinus produced up to four 35 discrete generations within the experimental period, and life-cycle times from egg-to-egg ranged from 36 14 days (replicate group 5, first generation) to 56 days (replicate 3, second generation). These two 37 experiments, indicated that i) adult C. obscuripes arising from overwintered larvae emerged en masse 38 in the spring, and ii) up to four generations of C. crystallinus occurred; i.e. C. crystallinus exhibited a 39 multi-voltine life history under the temperate conditions of this UK study. Page 3 of 20 41 Introduction 42 The precise duration of the life-cycle of Chaoborus spp. does not appear to have been clearly defined 43 in published studies. Since the larval stages are known to be extremely sensitive to the effects of 44 insecticides (1) and size-dependent sensitivity can play an important role in the survival and recovery 45 of natural populations, the duration and timing of the life cycle has implications for the interpretation 46 of how natural populations recover from exposure to stressors. 47 48 The Chaoboridae are Diptera with aquatic larvae that live as pelagic predators, feeding on a wide 49 range of prey including copepods, Cladocera, rotifers, chironomids, mosquito larvae and other 50 chaoborids. There are three genera of Chaoboridae in northern and central Europe; Chaoborus, 51 Mochlonyx and Cryophilia, all of which have four aquatic larval instars (2, 3, 4). Chaoborus species 52 are widely distributed throughout Europe and of the six species that are known C. flavicans, C.53 obscuripes and C. crystallinus ar...

Characterization of patterns and variability in the dynamics of outdoor aquatic mesocosms: exploring the capabilities and challenges in data supporting aquatic system models

“…and Cloeon dipterum, this variability might be explained by differences in physical parameters (e.g. water temperature) that not only affect timing of emergence but also length and number of generations per year (Brittain 1982, Ouimet 2008, Cockroft et al 2019. Other factors contributing to variability in macroinvertebrate abundance and abundance dynamics could include differences in food source availability, macrophyte-based habitat structures and presence or absence of overwintering generations (O'Connor 1991, Leszczyńska et al 2017), all of which are likely to differ among ponds of different ages and thus successional stages (Hassall et al 2012, Sferra et al 2017.…”