Facultative symbionts did not influence APV transmission
We found that vertical transmission rates of APV were about 35% and not impacted by H. defensa (Table 2A). This rate is similar with prior reports for APV (Lu et al. 2020) and other aphid viruses (Laubscher and Von Wechmar 1992, van Munster et al. 2003). It was also previously reported that feeding by aphids resulted in the horizontal transfer of APV to plants, with the virus persisting up to 7 days without replication in plant tissues (Lu et al. 2020). Here, we confirmed lateral transmission of APV through food plants by showing that that APV-free aphids readily acquired the virus through phloem feeding on plants previously fed on by APV+ aphids (Table 2B). We further showed that APV acquisition rates were not affected by the presence of H. defensa or R. insecticola . Thus, despite reducing APV abundance and improving tolerance to infection as described above, R. insecticola did not impede APV acquisition. We also found that parasitoids can transfer APV via oviposition, a previously undescribed route of transmission for pathogenic viruses. Rates of wasp-mediated transfer of APV were low and only occurred when oviposition occurred immediately following contact with a virus-infected aphid. Parasitoids have been shown to move H. defensahorizontally among black bean aphids (Gehrer and Vorburger 2012) suggesting microbial transfer via the contaminated ovipositors of wasps may be an underappreciated route of microbe exchange among multicellular eukaryotes.
Together these results show moderate vertical transmission and horizontal transfer through food plants are the major routes of APV spread. That APV can promote aphid feeding and colonization by modulating plant defenses (Lu et al. 2020) suggests this virus may employ tactics that facilitate its spread as seen for plant viruses vectored by sap-feeding insects (Roossinck 2015).