Supercooling capacity of Eurasian and North American populations of parasitoids of the Russian wheat aphid, Diuraphis noxia

Supercooling points were estimated for seven populations of Aphelinus albipodus, five populations of Aphelinus asychis, and four populations of Diaeretiella rapae to assess whether their supercooling points were sufficiently low to provide the potential for overwintering survival in colder temperate climatic areas. Test individuals from all 16 of the parasitoid populations were collected originally from mummies of the Russian wheat aphid, Diuraphis noxia. Mummies containing parasitoid pupae were maintained for 1 wk under three different temperature conditions (treatments): at room temperature (24.8 +/- 0.2 degreesC), 1 wk at 0 degreesC, and 1 wk -5 degreesC, and the supercooling points across treatments, and within and among species were compared. Statistical differences in supercooling points were found among populations of A. albipodus for each treatment, and for A. asychis when maintained for 1 wk at room temperature. No differences in supercooling points were found among populations of D. rapae mummies maintained under the three temperature treatments. The lowest supercooling points obtained for the three parasitoid species maintained at room temperature were the A. albipodus population from Montana (-31.68 degreesC), the A. asychis population from Greece (-32.04 degreesC), and the D. rapae population from the Caucasus (-33.12 degreesC). Preconditioning the parasitoid mummies to cold had no effect on the supercooling points for A. albipodus, and in some cases unexpectedly increased the supercooling points for A. asychis and D. rapae. In comparing the overall mean supercooling points of the three parasitoid species, no differences were found within species (among temperature treatments), nor among species (within temperature treatments). It was concluded that observed differences in supercooling points of only a few degrees Centigrade among parasitoid populations and species would not be expected to cause differences in their overwintering success, especially given the expected variability in temperatures within and among overwintering sites.