LOCAL VARIABILITY IN THE LIFE-CYCLE OF THE BIRD CHERRY-OAT APHID, RHOPALOSIPHUM-PADI (HOMOPTERA, APHIDIDAE) IN WESTERN FRANCE

To study life cycle variability in Rhopalosiphum padi (Linnaeus), the responses to short days (L:D 10:14) of 70 clones were tested at two temperatures (10-degrees-C and 15-degrees-C) in the laboratory. Three clones originating from spring populations on the primary host, bird cherry (Prunus padus), were holocyclic, producing gynoparae and then males in the second generation under experimental conditions. Of the other clones, all originating from Gramineae, 78.5% were anholocyclic and 21.5% androcyclic (male-producing) at 15-degrees-C, but at 10-degrees-C only 12.7% were anholocyclic, 82.6% were androcyclic, and 4.7% showed an 'intermediate' response (i.e., some of the alate females produced both sexual and parthenogenetic offspring). There was considerable interclonal and inter-individual variation in the numbers of males produced by androcyclic clones. These results are discussed in relation to field studies on R. padi in Britanny, which indicate that 1. primary hosts are rather rare, but nevertheless commonly develop spring infestations; 2. anholocyclic genotypes are regularly decimated every few years by severe winters. It appears that the preponderance of androcyclic clones is important for the maintenance of life cycle variability.