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