a1 Department of Pure and Applied Zoology, University of Leeds, Leeds, LS2 9JT, UK
The cold hardiness of Aphis fabae Scopoli was assessed by determining the ability of individual aphids to supercool. All stages of A. fabae maintained parthenogenetically at 20°C on broad bean (Vicia faba) were capable of extensive supercooling ability to below −20°C; first- and secondinstar nymphs were the most cold hardy individuals. First instars and adult apterae of A. fabae collected from natural summer populations on broad bean and sugarbeet in England showed levels of supercooling similar to that of laboratory-maintained aphids. Compared to aphids on herbaceous hosts, all those associated with the woody host, spindle (Euonymus europaeus), showed a substantially poorer ability to supercool, often to less than −15°C, both in the autumn (oviparae) and spring (fundatrices and fundatrigeniae). All samples of eggs supercooled to below −30°C, becoming fully acclimatized in mid-winter, and lost supercooling potential prior to hatching in spring. The influence of feeding on woody and herbaceous plants on the cold hardiness of A. fabae was confirmed in a series of host-plant transfer experiments in which aphids that were transferred to bean from spindle acquired good levels of supercooling and then lost over 10°C of supercooling potential when transferred back to spindle. The shift from poor (spindle-associated) to good (bean-associated) supercooling was more difficult to achieve and suggested the presence of a nucleator in the spindle sap.
(Received October 16 1985)
p1 Present address: Department of Biology, University of Victoria, Victoria, British Columbia, Canada V8W 2Y2.