Overwintering aggregations are part of Hippodamia undecimnotata's (Coleoptera: Coccinellidae) mating system.

Eline Catherine Susset,J Joe Hull,Jean-Louis Hemptinne,Etienne Danchin,Alexandra Magro

doi:10.1371/journal.pone.0197108

Abstract

Aggregation during diapause is a common phenomenon in arthropods that nevertheless remains poorly understood. The most commonly claimed benefit is that survival is higher in aggregations but animal aggregations could also be driven by sexual selection. In this perspective, we investigated whether aggregations in insects could be part of their mating system. We studied the overwintering aggregations of the ladybird Hippodamia undecimnotata (Schneider), an aphidophagous species from Southern and Eastern Europe as well as Asia. We collected ladybirds at three aggregation sites in Southwest France, during two overwintering periods (2013–2014 and 2014–2015). We checked their reproductive status by counting the viable sperm cells in the sperm storage organs of both males and females, and by assessing the ovarian status of females. We also investigated if mating behaviour occurred in these aggregations. We found that males have a high quantity of viable sperm cells (70–95%) in their reproductive organs throughout the overwintering periods. In contrast, although most females (85–95%) had empty spermatheca at the onset of the aggregations in autumn, the majority (65–91%) had numerous viable sperm in their spermatheca at the time of dispersal from the aggregation in early spring. Furthermore, frequent copulations were observed towards the end of the overwintering period, few weeks before dispersal. These results suggest that finding sexual mates may have been involved in overwintering aggregations in H. undecimnotata.

Highlights

Group living is a common feature in the animal kingdom [1,2,3] and animal aggregations are often temporary and seasonal
The proportion of males with sperm cells in their reproductive organs did not vary in relation to the period of sampling (GLMM: χ23 = 4.50, P > 0.05)
We thoroughly studied the sexual activity of H. undecimnotata during the overwintering aggregation

Summary

Introduction

Group living is a common feature in the animal kingdom [1,2,3] and animal aggregations are often temporary and seasonal. Seasonal aggregations generally consist of monospecific clusters, sometimes of high numbers of individuals for several months, and are located in the same places year after year [e.g. 4–9]. Seasonal aggregations are a fascinating phenomenon, ultimate mechanisms for their formation are still poorly understood [10,11,12]. Aggregations during diapause may generate favourable microclimates [20,21]: they were shown to promote water conservation [22,23,24] or increase temperature inside the group [23]. The most common evolutionary approach underlying the formation of aggregations in insects has focused on survival, there are other major determinants of fitness that have been overlooked [4,25,26]

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Results

Conclusion