An alternative hibernation strategy involving sun-exposed 'hotspots', dispersal by flight, and host plant finding by olfaction in an alpine leaf beetle

<i>Oreina cacaliae</i> (Schrank) (Coleoptera: Chrysomelidae) has a 2-year life cycle that it has to complete within the short warm seasons of the harsh alpine environment. Three years of field observations and experiments revealed that not all beetles overwintered in the soil next to their principal host <i>Adenostyles alliariae</i> (Asteraceae), as was previously assumed, but that many <i>O. cacaliae</i> left their host in autumn and flew to overwintering sites that were extensively sun-exposed. In spring, these individuals became active 2 months earlier than their conspecifics that had remained in the soil close to the host plant. These early beetles flew from their hibernation sites against the direction of the prevailing wind. After a random landing in snow, they walked to the spring host <i>Petasites paradoxus</i> (Asteraceae) and fed on its floral stalks, the only plant parts present at that time. A few weeks later, they took flight again to locate newly emerging <i>A. alliariae</i> on which they would feed and deposit larvae as did individuals that had overwintered close to <i>A. alliariae</i>. Leaves of <i>A. alliariae</i> contain pyrrolizidine alkaloids (PAs), which the beetles sequester for their own defence. The dominating PA (seneciphylline) was also found to be present in the floral stalks of <i>P. paradoxus</i>. With additional behavioural assays in the field and laboratory, we demonstrated the importance of plant odours in the short-range host location process. This study reveals a unique hibernation behaviour in which part of the beetle population uses exceptionally warm locations from which they emerge in spring, long before all the snow has melted. This early, but risky emergence allows them to exploit a second, highly suitable host plant, which they locate first by wind-guided flight and then by odour-guided walking. The well-fed beetles then use odour again to move to their principal host plant, on which they reproduce.