Frass happens: Conspecific frass as a source for an environmental symbiont

Successful symbiont acquisition may be critical to host success, especially when a symbiotic partner is required for normal development and reproduction. In many systems acquisition is nearly assured, where symbionts are transmitted vertically either intracellularly or horizontally through tropholaxis with great reliability. However, when a microbial partner is environmentally acquired, acquisition is not guaranteed. The nutritional symbiont of the arboreal Leptoglossus zonatus (Hemiptera: Coreidae), Caballeronia, is a free-living soil-bacterium and must be located in each generation during the 2^nd instar. We hypothesized first that an alternate route for transmission exists through frass, and secondly that olfaction may facilitate long range detection of frass for aposymbiotic nymphs. Leptoglossus zonatus nymphs were fed Caballeronia and frass was collected from bugs at each lifestage following the experiment to determine if symbiont was excreted, which suggests that Caballeronia is released largely by adults, and rarely excreted by nymphs. Next a rearing experiment was performed in which aposymbiotic nymphs were exposed to Caballeronia-laden adult frass. The results showed high rates of acquisition success, though slight, although significant reductions in adult weight and survivorship relative to bugs acquiring cultured bacteria. Lastly, aposymbiotic nymphs were presented with binary choices using a Y-tube olfactometer to identify whether taxis toward adult frass occurs. The results suggest that adult frass is likely detected through olfaction. Together, these data suggest an alternate and viable route for transmission, potentially eliminating the meters-long journey to the soil during a vulnerable life stage.