Do facultative endosymbionts alter stylet-born virus transmission by aphids?

Emerging evidence suggests facultative endosymbiont associations can modify an insect host’s interactions with other trophic levels.  Thus far, endosymbionts have been shown to affect insect herbivores via enhanced parasite defense, heat tolerance, and protection from pathogens; but recent findings suggest they may also be tied to plant usage and facilitating the spread to novel ecological niches. Building on this evidence, we explored whether interactions occur among a vector, two of its facultative endosymbionts, and a stylet-born (nonpersistent) virus transmitted to plants. If endosymbionts modify aphid feeding behavior, transmission and acquisition of plant viruses is likely indirectly altered. Alternatively, it is possible that endosymbiont and virus may interact directly via physiological mechanisms inside a vector. Previous research suggests the cowpea aphid is an important vector of WMV in Midwestern pumpkins (Cucurbita pepo); therefore, to flesh out existing interactions, we assessed feeding behavior on healthy pumpkin plants or plants inoculated with watermelon mosaic virus (WMV)  by the cowpea aphid (Aphis craccivora), with and without endosymbiont association. Four aphid-endosymbiont constructs were used: alfalfa-derived cowpea aphids infected with Hamiltonella; cured, alfalfa-derived cowpea aphids; black-locust-derived cowpea aphids infected with Arsenophonus; and cured, black-locust-derived cowpea aphids. An electrical penetration graph (EPG) measured variations in feeding behaviors associated with virus transmission throughout a 1 h duration, including time to intracellular probe, number of intracellular probes within 15 min (an acceptable virus acquisition period).