Geographical and diapause-related cold tolerance in the blow fly, Calliphora vicina

Three geographical strains of the blow fly, Calliphora vicina, were tested for cold tolerance at 0 degrees, -4 degrees and -8 degrees C. Survival to eclosion after 1 to 18 days of cold exposure was greater for diapause-destined larvae than for nondiapause-destined larvae of the two northern strains (Nallikari, Finland 65 degrees N and Edinburgh, Scotland 55 degrees N) but not for the southernmost strain (Barga, Italy 44 degrees N) where no clear differences were apparent. Diapause-destined larvae of the Edinburgh strain were more cold tolerant than those from Nallikari, at both -4 degrees and -8 degrees C, a difference possibly attributable to the long-lasting snow cover in the more northern locality, which might insulate the overwintering soil microclimate. At 0 degrees C, however, Nallikari larvae were more cold tolerant than Edinburgh or Barga. This was also the case for nondiapause-destined larvae, indicating that cold tolerance may occur, in part, independently of the diapause programme. In all three strains diapausing larvae were more cold tolerant than same-age (nondiapausing) pupae. For Nallikari, but not Barga, wandering larvae from short-day exposed flies, therefore initially programmed for diapause, but diverted from the diapause pathway by larval breeding at 19 degrees C, were significantly more cold tolerant than nondiapause larvae from long-day parents, indicating some maternal regulation of larval cold tolerance. There was, however, no evidence for an additional cold hardiness in larvae acclimatised to cold by a gradual reduction of temperature.