Influence of experimental warming and shading on host-parasitoid synchrony

As the climate warms, many species are showing altered phenology patterns, potentially disrupting synchrony between interacting species. Recent studies have documented disrupted synchrony in plant-herbivore and predator-prey interactions. However, studies investigating climate-related asynchrony in host-parasitoid interactions and exploring the relative responses of interacting hosts and parasitoids to climate change are lacking. This is an important gap in knowledge given the ubiquity of insect parasitoids and their importance in influencing the abundance and dynamics of their hosts. In the threatened marsh fritillary butterfly Euphydryas aurinia (Lepidoptera: Nymphalidae) and its specialized parasitoid, Cotesia bignellii (Hymenoptera: Braconidae) phenological synchrony (and consequently population fluctuations) are thought to be weather-dependent. To assess the likely influence of climate and microenvironment change on synchrony between E. aurinia and C. bignellii, we experimentally manipulated the exposure of sensitive-stage host larvae and parasitoid pupae to temperature (ambient or elevated) and shading (shaded or unshaded) regimes. We also analysed a 20-year population dynamic dataset from the United Kingdom for E. aurinia to investigate whether population variations could be explained by interannual variations in the thermal and sunshine environment. Development times were affected significantly by the experimental temperature and shading treatments for E. aurinia but not for C. bignellii. However, the contrasting responses were insufficient to significantly affect host availability for parasitoids. In the field, thermal and sunshine conditions did not influence population fluctuations, and population variations across a large (UK-wide) scale were uncorrelated. Changes to the thermal and sunshine environment of the magnitude investigated in our experiment and within the range experienced by wild E. aurinia populations over the last 20-years thus seem unlikely to cause breakdown in host-parasitoid synchrony. We suggest that experiments investigating the mechanistic responses of interacting species to environmental change are needed to support the analysis and interpretation of observational data on species' phenology.