1. When first instar nymphs and adults of the grain aphid Sitobion avenae (Fabricius) (Hemiptera: Aphidiae) were maintained in long-term cultures (> 6 months) at 20 degrees C and 10 degrees C, the LT50 decreased from -8 and -8.8 degrees C to -16.0 and -13.5 degrees C, respectively. 2. When aphids from the 20 degrees C culture were transferred to 10 degrees C, there was a progressive increase in cold tolerance through three successive generations. Transfer of newly moulted pre-reproductive adults reared at 10 degrees C for three generations back to 20 degrees C resulted in a rapid loss of cold hardiness in their nymphal offspring. 3. In all generations reared at 10 degrees C, first born nymphs were more cold hardy than those born later in the birth sequence. The LT50 of nymphs produced on the first day of reproduction in the first, second and third generations maintained at 10 degrees C were -14.8, -17.0 and -16.6 degrees C, respectively. Thereafter, nymphal cold hardiness decreased over the subsequent 14 days of reproduction in each generation at 10 degrees C with mean LT50 values of -10.3, -12.6 and -14.8 degrees C, respectively. By contrast, the cold tolerance of first born nymphs of aphids reared continuously at 20 degrees C did not differ in comparison with later born siblings. The LT50 of adult aphids was also unaffected by ageing. 4. The ecological relevance of these findings is discussed in relation to the overwintering survival of aphids such as S. avenae.
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