Investigating the impact of experimentally-induced phenological mismatch on a solitary bee species (Osmia lignaria) and its parasites

Background/Question/Methods

Numerous ecological studies have documented climate-driven advances in the timing of annual life history events for individual species in both marine and terrestrial environments. These phenological shifts appear to show considerable interspecific variability in magnitude as well as directionality, creating the potential for temporal mismatch between interacting species. As of yet, few studies have investigated the potential impacts of mismatch between pollinators and their parasitoids, despite the fact that springtime emergence of both groups is highly dependent on climatic conditions. This study investigated the impacts of experimentally induced phenological mismatch on parasitism rates and host performance in the orchard mason bee (Osmia lignaria), a commercially important native pollinator in California. Specifically, I experimentally manipulated O. lignaria emergence timing, releasing an “early” cohort of bees prior to the historical peak in activity and a “late” cohort after the peak. Using this experimental set-up, I sought to address whether seasonal timing of bee emergence affected (A) the abundance and species identity of nest parasitoids within O. lignaria brood cells (B) host and parasitoid sex ratios (C) nest construction rate and (D) mortality of bee larvae.

Results/Conclusions

None of the progeny of the treatment bees were parasitized in this study, preventing comparison between the early and late release treatments with regard to differences in parasitism rates. This unusually low incidence of parasitism may reflect seasonal fluctuations in parasitoid population abundance or limitations in parasitoid geographic ranges. It is also possible that the peak in parasitoid activity is synchronized with that of the host bee, leading to reduced risk of parasitism for treatment bees released prior to and after the historic peak in bee activity.

The preliminary results of this study also indicate that the progeny of late release bees were male-biased. Because female progeny are more energetically costly to produce, a male-biased sex ratio potentially signifies that bees provisioning nests late in the season may experience poor environmental conditions, such as a period of low resource availability.