EFFECTS OF SUBLETHAL COLD STRESS ON THE APHID RHOPALOSIPHUM-PADI

1. An anholocyclic clone of the aphid Rhopalosiphum padi was reared on leaves of barley seedlings at 20-degrees-C in a 16:8 LD photoperiod. First instar nymphs and newly moulted, pre-reproductive adults were co oled at 1-degrees-C min-1 to -5 and -7-5-degrees-C, respectively, for 1 and 6h to represent increasing levels of sublethal cold stress. Surv iving aphids were returned to 20-degrees-C and a 16:8 LD photoperiod, after which observations were made on the rate of nymphal development, adult reproduction and longevity, and survival of progeny. 2. Mean de velopment time from birth to adult increased with increasing cold stre ss, ranging from 6.6 +/- 0.14 days in the control to 10.3 +/- 0.14 day s in nymphs exposed to -7.5-degrees-C for 6h. 3. When cooled as first instar nymphs to -7.5-degrees-C for 6h, surviving aphids that develope d to the adult stage produced a mean of 4.22 +/- 0.26 nymphs per day w ith an average reproductive life of 7.91 +/- 0.89 days and a mean tota l fecundity of 32.30 +/- 3.51 nymphs per adult in an adult life span o f 8.04 +/- 0.91 days. Aphids cooled to -7.5-degrees-C for 6h as newly moulted, pre-reproductive adults produced 5.87 +/- 0.19 nymphs per day over 5.58 +/- 0.45 days with a mean total fecundity of 31.88 +/- 2.40 nymphs per adult in a life span of 5.58 +/- 0.45 days. In general, me an daily fecundity, length of reproductive life, total fecundity and a dult longevity all increased with less stressful exposures. A control sample produced 7.58 +/- 0.14 nymphs per day over 11.90 +/- 0.30 days with a mean total fecundity of 89.36 +/- 1.28 nymphs per adult in a li fe span of 17.44 +/- 1.14 days. Approximately 50% of nymphs born on th e first day of the reproductive life of aphids cooled as first instars or newly moulted adults to -7.5-degrees-C for 6h died within 3 days o f birth. 4. These results provide a clear demonstration of the subleth al effects of cold stress arising from a single exposure to subzero te mperatures affecting key processes (development, reproduction and long evity) that exert a dominant influence on aphid population dynamics. F urthermore, these deleterious effects also act prenatally, predisposin g some nymphs to die shortly after birth.