Abstract
Adults of some coccinellid species overwinter in aggregations consisting of many individuals. They may clump because adults are attracted to particular environmental stimuli and/or of an innate tendency to join conspecific individuals. Aggregation behaviour was studied in Coccinella septempunctata L., which forms small clumps, and Ceratomegilla undecimnotata (Schneider) and Hippodamia variegata (Goeze), which form large clumps. Adults were recorded at five hilltop hibernacula (400-1500 m altitude) in central Europe (50-51°N, 14-16°E) for periods up to 27 years. The hibernacula occur in areas sparsely covered with isolated grass tussocks or completely with stones. Numbers of adults recorded under each of 300-900 stones or among the stems of the grass tussocks were counted every year at each hibernaculum. The degree of aggregation was determined using Taylor's power law. The coccinellid distributions became more aggregated and the size of the aggregations increased as their abundance increased, less in C. septempunctata than in the other two species. Aggregations formed even in the absence of prominent structures, which may attract immigrants, and even when unoccupied stones or tussocks suitable for overwintering were available. Aggregations may be established and their size limited by the strength of the intrinsic preference to join conspecifics. The supposed advantage of aggregated overwintering must be greater than the risk associated with the easy spread of diseases.
Highlights
In central Europe the life histories of aphidophagous lady-beetles (Coleoptera: Coccinellidae) are similar
The size of the clumps and the frequency of large clumps increased with overall abundance. This was apparent in C. septempunctata (Fig. 1) as well as in H. variegata and C. undecimnotata, for which there was less data
For C. septempunctata (Fig. 3) the proportion of occupied tussocks/stones increased with abundance (R2 = 0.940, P < 0.0001) but all were only occupied at abundances of 670 individuals per 100 tussocks/stones
Summary
In central Europe the life histories of aphidophagous lady-beetles (Coleoptera: Coccinellidae) are similar. Young adults forage in temporary habitats, where they feed on both insect and vegetable food but do not reproduce. They search for places to overwinter (hibernacula), usually small areas often well delimited against the surroundings, with a typical ground surface structure, vegetation cover and microclimate (Hodek, 1996). Some individuals of some species fly to prominent hilltops, often several kilometres away from where they bred At these sites diapausing or quiescent adults stay from mid-summer (July–August) until the following spring (April–May), shelter in various structures and largely refrain from feeding (Hagen, 1962; Hodek, 1996). They spend a considerable amount of time in hibernation and have physiological and behavioural adaptations that minimize mortality during this period
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