Overwintering ecology of Philonthus rotundicollis (Coleoptera, Staphylinidae) in the nest of the carpenter ant Camponotus herculeanus on the coast of the Sea of Okhotsk

Abstract

A widespread rove beetle species, Philonthus rotundicollis, whose distribution range stretches across different climatic zones, including the coldest regions of the Asian northeast, was discovered as an inquiline within the nests of the carpenter ant Camponotus herculeanus on the coast of the Sea of Okhotsk in winter. It remained unclear if the beetles had significant cold-hardiness and whether they overwintered deep in the soil or were confined to particularly warm habitats. To clarify these aspects, the following metrics of cold-hardiness were measured: supercooling point (SCP), freezing point (FP), supercooling capacity (SCP-FP), and temperature minima at the beetles’ overwintering sites. In Ph. rotundicollis, mean SCP was -11.1 ± 0.7°C (ranging from - 7.9 to -18.8°C, n + 15), which was insufficient for successful overwintering even on the coast, since temperature minima in leaf litter during a snow-deficient winter fell to -14°C at the depth of 5 cm and -12°C at 20 cm. The beetles could not burrow deep into stiff soil and made use of crevices in dry peat-like soil layers as well as tunnels of soil- and rootdwelling animals, including carpenter ants. The presence of this rove beetle species in the ant nest was probably due to feeding on ant larvae because, at near-zero temperatures, the activity threshold of the beetles was lower than that of the ants that guarded the larvae.