The cost and benefit tradeoff of mutualistic association: Does the symbiont benefit?

Mutualistic association between two organisms consists of reciprocal benefit to the fitness of both partners. Via environmental acquisition, eastern leaf-footed bugs (Leptoglossus phyllopus) establish a beneficial symbiotic association with the free-living Caballeronia bacteria. In the presence of the symbiont, the insects exhibit faster development time, higher fecundity, and lower mortality rate than aposymbiotic insects. However, very little is known about the impact of the host on symbiont fitness. It is generally assumed that the benefits a microbial symbiont receives from associating with a host include nutrient acquisition, a competition-free environment, and transmission to a new environment. However, these assumptions are rarely tested and have many caveats. We used qPCR to quantify the effect of Leptoglossus on the fitness of its symbiotic bacteria by measuring the abundance of Caballeronia in soil microcosms in the presence and absence of the host insect. Our initial findings indicate that Caballeronia abundance increases in soil when the host insect is present. These results suggest that Leptoglossus enriches its local soil with Caballeronia. This has important implications for agriculture because it may enhance the acquisition of symbionts from the soil by future insect generations, potentially leading to a larger and more vigorous pest population over time.