HOPKINS HOST SELECTION PRINCIPLE, ANOTHER NAIL IN ITS COFFIN

Adult female parasitoids (Aphidius rhopalosiphi De Stef.) (Hymenoptera : Aphidiidae) reared on the aphid Metopolophium dirhodum (Walk.) (Homo ptera: Aphididae) changed their responses to odours of two wheat culti vars ('Maris Huntsman' and 'Rapier') when the parasitized aphids had b een moved from the former to the latter cultivar at various times afte r parasitization. If the aphids were moved after less than 6 days, the female parasitoids emerging from mummies by then formed on 'Rapier' r esponded positively to that cultivar, whereas parasitoids emerging fro m mummies transferred as still living aphids after 8 days on 'Maris Hu ntsman', responded positively to that cultivar rather than to 'Rapier' on which they had emerged. This appeared to be evidence for Hopkins' 'host selection principle', which states that chemical experience acqu ired by the larva of an endopterygote insect can be transferred throug h the pupal stage to the adult. However, discrimination in favour of e ither cultivar disappeared when the parasitoids were reared in aphids on both cultivars, and the emerging females were tested following exci sion of the pupae from the aphid mummies. The previous discrimination shown by adult parasitoids emerging from aphid mummies must therefore be determined by the chemicals contacted by a parasitoid on the skin o f the mummy or while biting its way out of the dead aphid.