Influence of Previous Infestation of Wheat Leaves and Ears by Sitobion avenae on Interaction with Rhopalosiphum padi

Different herbivorous species that share a host plant may interact via competition or facilitation, depending on whether the interaction partners are hindered by or benefit from the interaction. Sap-sucking insects, such as aphids, can influence each other indirectly by altering the composition of the shared phloem sap. Aphid-induced changes in the plant may affect aphid performance and lead to a shift in the balance between different co-occurring aphid species. In this study, we compared the performance of the English grain aphid (<i>Sitobion avenae</i>) and the bird cherry-oat aphid (<i>Rhopalosiphum padi</i>) simultaneously infesting leaves or ears of wheat (<i>Triticum aestivum</i>) plants, which had been either previously infested by <i>S. avenae</i> or kept uninfested. Colonies of <i>S. avenae</i> were larger on ears than on leaves, while the opposite pattern was found for <i>R. padi</i>. Pre-infestation of ears, but not of leaves, by <i>S. avenae</i> led to a higher total aphid number and colony size of <i>S. avenae</i> at some time points. The balance between the two species was only slightly affected by previous infestation at some time points. The findings of this study contribute to the understanding of plant-aphid as well as aphid-aphid interactions in agricultural fields.