Journal of Insect Physiology 46 (2000) 671–676 www.elsevier.com/locate/jinsphys Overwintering strategies and cold hardiness of two aphid parasitoid species (Hymenoptera: Braconidae: Aphidiinae) Alain Langer * , Thierry Hance Universite ´ Catholique de Louvain, Unite ´ d’Ecologie et de Bioge ´ographie, Place Croix du Sud 4–5, 1348 Louvain-la-Neuve, Belgium Received 4 May 1999; accepted 24 June 1999 Abstract The role of winter diapause in two aphid parasitoid species, Aphidius ervi Haliday and Aphidius rhopalosiphi DeStefani-Peres (Hymenoptera: Braconidae: Aphidiinae), in host synchronization and the induction of cold hardiness was investigated. Parasitoids were reared during three successive generations on Sitobion avenae Fabricius, at 15°C under a photoperiod of 9 h light 15 h dark. Although these conditions are known to be strongly diapause inducing, neither parasitoids showed an incidence of diapause above 65% over the three generations; the rest of the population underwent quiescence. In both parasitoid species, diapausing mummies exhibited greater cold hardiness than non-diapausing mummies, resulting in significantly lower supercooling points (SCP) and in a higher survival rate during long-term exposures at 0 and -10°C. The induction of increased cold hardiness in parasitoids was thus associated with the diapause state. SCPs of third instar larvae of S. avenae were similar to those of non-diapausing mummies of both parasitoid species, but significantly higher than those of diapausing mummies. The effect of winter climate on the stability of the host–parasitoid interaction is discussed.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Aphid; Aphidius; Cold hardiness; Diapause; Parasitoid 1. Introduction Parasitoids have evolved several adaptations in order to synchronize their lifecycle with that of their host (Tauber et al., 1983). In temperate climates, where win- ter is often characterized by the scarcity of biotic resources, winter diapause is an adaptation of particular importance. In Quebec, Brodeur and McNeil (1989, 1994) demonstrated that Aphidius nigripes Ashmead has the capacity to enter winter diapause, and the emergence of this parasitic wasp in spring coincided with the hatch- ing dates of the eggs of its host aphid Macrosiphum euphorbiae Thomas. Bournoville (1973) observed in France a perfect synchronization in spring between the emergence of diapausing Aphidius ervi Haliday and one of its hosts, Acyrthosiphon pisum Harris. It is important that insects overwinter in the develop- mental state that maximises their chance of surviving harsh conditions (Leather et al., 1993). Thus, beside the * Corresponding author. Fax: + 32-10-473490. E-mail address: langer@ecol.ucl.ac.be (A. Langer). 0022-1910/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII:S0022-1910(99)00155-9 role of synchronization, winter diapause may also be an adaptation to deteriorating abiotic conditions by increas- ing cold hardiness. Two main strategies of cold hardiness are recognized in insects that are exposed to tempera- tures below the melting point of their body fluids: freeze- tolerant and freeze-intolerant (avoiding) species (Zachariassen, 1985; Bale, 1989). The majority of insects and other arthropods are killed by freezing and thus belong to the latter group (Somme, 1982). For these species, the avoidance of freezing is essential, and is achieved by supercooling, where the body fluids remain unfrozen at temperatures below the melting point. Cold hardiness is increased by exposure to low tem- peratures. In some diapausing species, increased cold resistance can be linked to the diapausing state itself (Somme, 1982; Pullin, 1996). Researches on the cabbage white butterfly, Pieris brassicae L., have shown that the accumulation of cryoprotectants like sorbitol was influ- enced by diapause rather than temperature (Pullin and Bale, 1989; Pullin, 1992). Kostal and Simek (1995) showed that in the cabbage root fly, Delia radicum L., the induction of cold hardiness was associated with the diapause syndrome rather than with a direct effect of low temperature.