Journal of Insect Physiology 46 (2000) 99–106 www.elsevier.com/locate/jinsphys Cold hardiness of the fly pupal parasitoid Nasonia vitripennis is enhanced by its host Sarcophaga crassipalpis David B. Rivers a,* , Richard E. Lee Jr b , David L. Denlinger c a Department of Biology, Loyola College in Maryland, 4501 North Charles Street, Baltimore, MD 21210, USA b Department of Zoology, Miami University, Oxford, OH, USA c Department of Entomology, The Ohio State University, Columbus, OH, USA Received 2 March 1999; accepted 9 April 1999 Abstract Supercooling points (SCPs) and low temperature survival were determined for diapausing and nondiapausing larvae of the ectopar- asitoid Nasonia vitripennis. Neither nondiapausing nor diapausing larvae could survive tissue freezing. The SCP profiles were nearly identical for nondiapause-destined (-27°C) and diapausing larvae (-25°C), but these values were not indicative of the lower limits of tolerance in either type of larvae: larvae were killed by chilling at temperatures well above the SCP. Diapausing larvae could withstand low temperature exposures 3–8 times longer than their nondiapausing counterparts. Low temperature survival was enhanced in diapausing and nondiapausing larvae by their encasement within the puparium of the host flesh fly, Sarcophaga crassi- palpis: the LT 50 s determined for nondiapausing and diapausing larvae enclosed by fly puparia were 2–3 times higher than values calculated for larvae removed from the puparia. Additional low temperature protection was gained through acquisition of host cryoprotectants during larval feeding: nondiapausing parasitoid larvae that fed on diapausing flesh fly pupae with high levels of glycerol were able to survive exposure to a subzero temperature 4–9 times longer than wasps reared on nondiapausing fly pupae that contained lower quantities of glycerol. Alanine may also contribute to the cold hardiness of N. vitripennis, as evidenced by the fact that larvae feeding on diapausing fly pupae both contained higher concentrations of alanine and exhibited greater cold hardiness. The results thus demonstrate that several critical features of cold hardiness in the wasp are derived from biochemical and physical attributes of the host.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Cold tolerance; Cold shock injury; Supercooling points; Glycerol; Larval diapause; Ectoparasitoid 1. Introduction Though cold tolerance has been studied in many insects, little is known about the effects of low tempera- tures on insect parasitoids. Many endoparasitic wasps depend on the physiological status of the host insect to regulate their own diapause (Schoonhoven, 1962; Brown et al., 1990; Polgar et al., 1991), and possibly the cold hardiness of these wasps is also host dependent. Parasitic wasps that feed externally (ectoparasitoids) are also influenced by the nutritional and biochemical conditions of the host (Vinson and Iwantsch, 1980; Rivers and Denlinger 1994a, 1995), but such factors seem to have * Corresponding author. Tel.: + 1-410-617-2057; fax: + 1-410-617- 2646. E-mail address: rivers@vax.loyola.edu (D.B. Rivers) 0022-1910/99/$ - see front matter.  1999 Elsevier Science Ltd. All rights reserved. PII:S0022-1910(99)00106-7 a significantly reduced role in the development of these insects compared to their endoparasitic counterparts. Consequently, physiological features of ectoparasitoids, such as diapause, appear to be independent of host com- position (Rivers and Denlinger, 1995). Consistent with this prediction is the apparent absence of diapause in at least some ectoparasitic pteromalids (Petersen and Meyer, 1983), suggesting that the development of these species is not synchronized with the diapause program of the host and therefore the capacity to survive adverse environmental conditions is host independent as well. In this study, the pupal ectoparasitic wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) was used to examine cold tolerance characteristics in larvae, the developmental stage that normally overwinters. Fac- ultative larval diapause in N. vitripennis is of maternal origin (Schneiderman and Horwitz, 1958) and regulated by photoperiod (Saunders, 1965), temperature