COLD-HARDINESS OF OVERWINTERING APHID EGGS

Field observations have suggested that aphid eggs are extremely cold hardy although the level of cold hardiness has not been experimentally determined, Experiments were performed on field acclimated eggs of Rhopalosiphum padi from Britain, and laboratory stored eggs of Acyrthosiphon brevicorne and A, svalbardicum from the sub-Arctic and high Arctic, respectively, Eggs supercool to low sub-zero temperatures and show seasonal changes in supercooling ability, In experiments to determine the LT50 of R. padi eggs there was no significant pre-freeze mortality with short exposures (1 min at the minimum temperature), Mortality from freezing or some other mechanism is evident in all species after extended exposures, being dependent on both temperature and duration of exposure, No eggs from any region survived for more than one month at -30 degrees C although it is possible that field acclimated eggs of the Arctic species would be able to achieve this, The eggs of the Arctic species had a higher level of cold hardiness than the temperate species, Eggs from all regions were found to be sufficiently cold hardy to survive conditions in their respective environments.