Many aphid species exhibit a variation in reproductive mode which is influenced by winter climate regimes,with cyclical parthenogenetic (CP) lines dominating in cold winter areas (because they produce cold-resistant eggs) and obligate parthenogenetic (OP)ones in mild winter regions (because of their parthenogenetic overwintering).Genetic studies on several aphid species have shown that the OP trait can be transmitted during sexual events involving the 2 types of lines.This genetic system could be considered as a local safeguarding mechanism for OP alleles in case severe frost would have killed all parthenogenetically overwintering individuals.However,this strategy would only be efficient in restoring local polymorphism in breeding systems if the newly hatched OP recombinants remain competitive over their CP counterparts.In this study we compared egg hatching sequences of CP and OP F1 clones from several crosses obtained for 2 cereal aphid species,Sitobion avenae (constant 5 ℃,8 h of light) and Rhopalosiphum padi (winter outdoor conditions).For S.avenae,we obtained F1 offspring from 6 crosses,involving 4 clones while in R.padi F1 were obtained from 11 crosses involving 14 clones.We showed that in both species proportions of OP clones were higher in the first half of the progeny relative to the second half.In addition,F1 OP clones hatched in the mean about a week earlier than their CP sibs,which gives them a demographic advantage at the start of the growth season.We then discussed the consequences of this fitness advantage for the maintenance and spread of the OP trait in aphid populations.