Late is not great: fitness implications of delayed symbiont acquisition

Acquisition of beneficial microbes is often vital to the success of animals. In terrestrial arthropods, obligate symbionts are often acquired from mothers. However, the leaffooted bug, Leptoglossus zonatus, acquires its soil-dwelling bacterial symbiont, Caballeronia, from the environment in each generation starting in the 2nd instar. Bugs must locate their symbionts in a complex and heterogeneous environment, and location and acquisition may take time. We asked if there are costs to acquisition delay and if such costs accrue over longer intervals. A cohort of L. zonatus nymphs was split among eight treatments: seven different intervals for symbiont acquisition after bugs reached the 2nd instar (0-24 days) and a negative control in which bugs were never fed Caballeronia. We assessed the impacts on host performance by measuring acquisition success, juvenile survivorship, development time, adult mass, and development of the midgut symbiotic organ compared with the 0-day treatment. All measures were reduced by delayed acquisition of symbionts, beginning 8 days after nymphs reached the 2nd instar. As the delay increased, host survivorship and acquisition success were further reduced. Surviving hosts developed more slowly, were smaller, and their midgut symbiotic organs were smaller. Our results suggest that symbiont acquisition is most beneficial to hosts within a narrow window, within ~1 week of reaching the 2nd instar. Our results point to an important potential cost to this type of symbiosis. Nymphs must not only find the symbiont, but find it before it is too late to provide a benefit.