The presence of APV does not impact symbiont defensive phenotypes
Little is known about the performance of defensive symbionts when challenged with simultaneous threats (Hrček et al. 2016, Smith et al. 2021). Here, we found aphids carrying H. defensa were similarly susceptible to parasitism by A. ervi regardless of APV infection and APV had no effect on R. insecticola conferred protection against P. neoaphidis (Fig 2). While protection levels conferred by defensive symbionts are known to vary depending on abiotic factors (Guay et al. 2009, Doremus et al. 2018, Higashi et al. 2020), host genotypes (Vorburger and Gouskov 2011, Łukasik et al. 2013, Parker et al. 2017, Weldon et al. 2020), symbiont strain (Cayetano et al. 2015, McLean et al. 2018, Oliver and Higashi 2019, McLean et al. 2020), or co-occurrence with other symbionts (Weldon et al. 2020), our results indicate that APV does not alter defensive phenotypes. In contrast, APV had variable impacts on endogenous defenses against these specialized enemies. Aphid lines free of facultative symbionts were equally susceptible to parasitism by A. ervi with and without APV, but those challenged with the fungus P. neoaphidis performed significantly worse when APV was present (Fig. 2, Fig. S1). The latter suggests that the aphid immune system may not be able to effectively respond to simultaneous pathogen challenges. This result also indicates that the anti-fungal benefits of carrying R. insecticola were greater when APV was present. Aphids carrying R. insecticola are likely to benefit from both enhanced anti-fungal benefits and tolerance to APV, spreading at the expense of symbiont-free aphids or those with