Leaffooted bugs enrich local soil with their horizontally acquired symbiont.

Associations between hosts and their microbial symbionts are considered mutualistic when both partners benefit. While the advantages received by eukaryotic hosts from association with bacterial symbionts are frequently examined, benefits to the bacteria are rarely experimentally tested. Here, we consider whether the bug-Caballeronia symbiosis is truly mutualistic by measuring the effect of a leaffooted bug (Leptoglossus phyllopus) on the abundance of its horizontally acquired symbiont, Caballeronia grimmiae. We predicted that the free-living Caballeronia population would increase over time in the presence of its insect partner. We quantified Caballeronia titer in soil microcosms (i) at different bug densities, and (ii) in the presence and absence of L. phyllopus over time. As bug density increased, the soil Caballeronia population also increased. Insect presence resulted in a marginally higher Caballeronia titer over time. Additionally, Caballeronia titer tended to be higher in wetter soil, though this correlation should be verified by follow-up work. These results suggests that the relationship between Caballeronia and L. phyllopus is likely mutualistic and add to a small but growing body of literature that has quantified the effects of eukaryotic hosts on their bacterial partners.