Supercooling ability and cold hardiness of the pollen beetle Meligethes aeneus

Abstract

Supercooling point (SCP) and cold-hardiness of the pollen beetle Meligethes aeneus (Fabricius) (Coleoptera: Nitidulidae) were investigated. Mature eggs from the oviduct were supercooled on average to −28.0 °C and from oilseed rape buds to −24.4 °C; first instars were supercooled to −21.0 °C and second instars to −16.8 °C. Despite their high supercooling ability, none of the eggs survived 24 h exposure to −2.5 °C. The supercooling ability of adults varied significantly among feeding and non-feeding beetles: high SCPs prevailed during the whole warm period, being about −12 °C; low values of SCP of −20 °C dominated in non-feeding beetles. In spring and autumn, beetles displayed the same acclimation efficiency: after 1 week of exposure at 2.0 °C with no access to food their SCPs were depressed equally by about 3 °C. Meligethes aeneus beetles have a different response to low temperatures depending on the season. The lowest tolerance was found in reproductively active beetles after emergence from overwintering sites; the time needed to kill 50% of individuals (Ltime50) was 56.2 h at −7 °C and the lower lethal temperature needed to kill 50% (Ltemp50) after 24 h exposure was −8.6 °C. Cold hardiness increased from midsummer to midwinter; Ltime50 was 80 h in August, 182.8 h in September, and 418.1 h in January. Lethal temperature after 24 h exposure was −9.1 °C in August and −9.8 °C in September. In February, after diapause, the beetles started to loose their cold tolerance, and Ltemp50 was slightly increased to −9.5 °C. Hibernating beetles tolerated long exposure at −7 °C well, but mortality was high after short exposure if the temperature dropped below −9 °C for 24 h. Despite the season, the beetles died at temperatures well above their mean SCP; consequently, SCP is not a suitable index for cold hardiness of M. aeneus.