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Walnut aphids are major pests of walnut production with few commercially available natural enemies. We conducted laboratory and field experiments to evaluate the potential of <i>Orius sauteri</i> Poppius (Anthocoridae), a predatory bug, as a biological control agent against two walnut aphid species: the dusky-veined aphid (<i>Panaphis juglandis</i> Goeze) and the walnut aphid (<i>Chromaphis juglandicola</i> Kaltenbach). Both species co-occur on walnut trees; <i>P. juglandis</i> is distributed on the upper surface (adaxial) of leaves while <i>C. juglandicola</i> is found on the lower surface (abaxial) of leaves. Based on functional response experiments, <i>O sauteri</i> had a strong capacity for consuming both aphid species. Biocontrol efficacy of <i>O. sauteri</i> for each species in the laboratory and field experiments was high, 77% for <i>P. juglandis</i> and 80% for <i>C. juglandicola</i>, regardless if one or two predators being present. However, biocontrol efficacy declined 15–25% for <i>C. juglandicola</i> and 20–50% for <i>P. juglandis</i> when both aphid species were present on the same leaf. The efficacy of <i>O. sauteri</i> under (semi)-field conditions gave similar findings based on the percentage reduction of aphids and change in population growth rates of aphids. The reduced biocontrol efficacy of the predatory bug against mixed species populations of aphids can be explained by competition between the aphid species and differences in their preferred location on leaves. Our experiments showed that <i>O. sauteri</i> is a promising biocontrol agent, but biocontrol efficacy may decline when both aphid species are present on walnut trees. This should be considered in the commercial release of <i>O. sauteri</i> in walnut orchards to promote economic and environmental benefits of walnuts production.