| Summary: | The barley yellow dwarf virus (BYDV) of cereals is thought to substantially increase the high-temperature tolerance of its aphid vector, <i>Rhopalosiphum padi</i>, which may enhance its transmission efficiency. This is based on experiments with North American strains of BYDV and <i>R. padi</i>. Here, we independently test these by measuring the temperature tolerance, via Critical Thermal Maximum (CTmax) and knockdown time, of Australian <i>R. padi</i> infected with a local BYDV isolate. We further consider the interaction between BYDV transmission, the primary endosymbiont of <i>R. padi</i> (<i>Buchnera aphidicola</i>), and a transinfected secondary endosymbiont (<i>Rickettsiella viridis)</i> which reduces the thermotolerance of other aphid species. We failed to find an increase in tolerance to high temperatures in BYDV-infected aphids or an impact of <i>Rickettsiella</i> on thermotolerance. However, BYDV interacted with <i>R. padi</i> endosymbionts in unexpected ways, suppressing the density of <i>Buchnera</i> and <i>Rickettsiella</i>. BYDV density was also fourfold higher in <i>Rickettsiella</i>-infected aphids. Our findings indicate that BYDV does not necessarily increase the temperature tolerance of the aphid transmission vector to increase its transmission potential, at least for the genotype combinations tested here. The interactions between BYDV and <i>Rickettsiella</i> suggest new ways in which aphid endosymbionts may influence how BYDV spreads, which needs further testing in a field context.
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