VARIATION IN THE EFFECTS OF HOST PLANT ON THE PEA APHID, ACYRTHOSIPHON PISUM (HARRIS) (HOMOPTERA: APHIDIDAE)

Field and laboratory studies were carried out on the variation in host plant responses of A. pisum. The relationship between aphid response on previous host plant and responses to new host plants was examined. Field transfers were made among plots of alfalfa, sainfoin, trefoil, sweet clover, peas and fababeans. Parameters estimated were mean survival and mean fecundity. Detailed studies were carried out in the laboratory with field collected clones from different hosts. In this case, nymphal survival and adult dry weight were measured in addition to the parameters mentioned above.;Sample of clones from older, geographically separated populations showed some differences. This may be attributed to spacial separation, the high selection pressures exerted by insecticides on one of the populations or to the different varieties of the host. Relative status of this variation of clones available from widely different geographic areas, where anholo-cycle prevails, remains to be established.;Clones exhibiting variable characteristics are not mutually exclusive. Therefore, the system of nomenclature currently being employed seems inappropriate. It appears that the sympatric biotypes of A. pisum prevailing in any population represent fairly simple genetic variants. Therefore, identifying and naming or otherwise labelling of clones could lead to confusion as it may give a false impression of the biological status of the insect.;Among aphids in any single field, great variation in response was observed. This variation was as great as that reported between biotypes having diverse geographic origin. Moreover, characteristics of aphid populations on different hosts were not distinct and persistent. Clones present on one particular host show some short term selection when reproduction is parthenogenetic. This is followed by genetic recombination during sexual reproduction. There is no relationship between responses of clones to different host plants in one year and their responses to the same host plants in another year. Therefore, biotypes are annual phenomena.