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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
degrees 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
degrees 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.