Winter climate determines the success of the two main reproductive strategies employed by aphids. Permanent parthenogens survive as parthenogenetic females in mild winters, but are regularly eliminated by low temperatures; while cyclical parthenogens, which switch to sexual reproduction by the end of summer, produce every year fertilised diapausing eggs resistant to frost.
We have studied the variation in sexual morph production of several clones of the cereal aphid Rhopalosiphum padi (L.) showing both strategies. Twenty clones of this species differing by their geographic origin and their mode of reproduction were placed in two laboratory environments mimicking the changes of photoperiod and thermoperiod occurring naturally from the end of summer and during the autumn in oceanic and continental conditions. The analysis of clonal responses in both climatic conditions showed (i) a wide variation in investment of clones in sexual reproduction with, in particular, evidence for a mixed strategy employed by clones producing both sexuals without ceasing parthenogenetic reproduction, (ii) no geographic adaptation among clones belonging to cyclical parthenogenetic populations, (iii) an earlier production of sexuals in continental conditions and a higher production of males in oceanic conditions.
Furthermore, we have compared the dates of first appearance of sexuals in our experiments with those occurring in the field based on a suction trap database and found that sexuals were caught in nature at least four weeks earlier than in the lab. These results underline the need for a better understanding of the influence of the whole array of environmental factors inducing the transition from parthenogenetic to sexual reproduction in aphids., Maurice Hullé, Damien Maurice, Claude Rispe, Jean-Christophe Simon, and Lit
Cold hardiness of larvae of the summer fruit tortrix moth, Adoxophyes orana (Fischer von Rosslerstamm) (Lepidoptera: Tortricidae) was examined in the laboratory. Supercooling point of field collected larvae increased significantly from a mean value of -23.9°C in February 1998 to -16.9°C in June 1998. Mean supercooling points for laboratory diapause and non-diapause larvae were -20.7°C and -17.2°C respectively. Short period of acclimation (10 days at 0°C) significantly decreased supercooling point to -24.7°C for laboratory diapause larvae. Acclimation for 12 days at 5°C decreased supercooling point to -19.4°C for non-diapause larvae. Pre-freeze mortality for diapause and non-diapause larvae was also studied. Constant exposure of diapause larvae at -5°C resulted in high mortality (63.1%) after a period of 30 days. in contrast, only 6 days at -5°C were sufficient to cause 100% mortality of non-diapause larvae. Mortality of non-diapause larvae reached 100% after 12 and 18 days at 0 and 5°C respectively. The importance of these findings for the overwintering strategy of A. orana is discussed., Panagiotis G. Milonas, Mathilde Savopoulou-Soultani, and Lit
First stadium juveniles of P. angustus were reared under controlled seasonal conditions to maturity, reproduction and death. Individuals born in any one breeding season either had a 1-year or a 2-year life cycle (cohort-splitting). The life cycle was annual for individuals born in the first part of the breeding season (May-August), but became biennial for those born later (August-October). Two phenomena were involved: (1) Only individuals reaching the penultimate stadium (stadium VII) before a critical period at the end of spring could become adult in the breeding season following that of their birth. After this time, stadium VII individuals entered into aestivation and only became adult in the second autumn after their birth. (2) Females becoming adult in autumn entered reproductive dormancy and only laid eggs in the following spring. Overall, individuals born at the start of the breeding season easily reached stadium VII before the critical period and were able to breed at I year, whereas individuals born at the end of the breeding season reached stadium VII after the critical period, then had two consecutive periods of dormancy and only bred at 2 years age. Individuals from the same nest born in the middle of the breeding season (August) could have either annual or biennial life cycles, depending on whether they reached stadium VII before or during aestivation. The environmental factors capable of triggering aestivation in subadults and reproductive dormancy in autumn-maturing females are discussed., Jean-Francois David, Marie-Louise Celerier, Jean-Jacques Geoffroy, and Lit