Climate and plant pest dynamics: scales matter

Ben Ari et al. (2011) stated that for “plague and climate, scales matter!” What about plant pest dynamics within canopies? Climate influences the micro-environment and the dispersal of plant pests within plant canopies, as determined by multiscale mass and energy fluxes. Climate influence has been studied following two approaches. First, correlative approaches, extensively used in disease forecasting, statistically link disease and climate variables, e.g. air temperature and humidity. These approaches lack robustness and sensitivity; they cannot satisfyingly explain how an epidemic actually interacts with climate within a canopy and how it would evolve with climate change. Second, mechanistic approaches study the interactions between pests and their physical environment at the individual’s scale and integrate them from organ to canopy scale. Such interactions are described by the ecological concept of reaction norms, which relates performance, plasticity, and evolution (Angilletta et al., 2003). Establishing such reaction norms requires the characterization of phylloclimate (Chelle, 2005), that is the climate actually perceived by individuals (pest, plant organ) involved in the plant-pest interactions. Phylloclimate is highly variable in time and space. This comes from the pest’s energy budget, which non-linearly depends on microclimatic variables, and from the complex transfer of mass and energy from above the canopy to pest, mediated by canopy architecture. Top-canopy microclimate depends it-self on mesoclimate (1km2) through the actions of elements such as hedges, neighboring forests, hills, lakes, roads, etc, which define the landscape architecture. In addition to its downscaling function (from mesoclimate to phylloclimate), canopy architecture is an important component of the integration of many local pest-leaf-phylloclimate relationships, like the processes of infection, latency, lesion growth, and sporulation in fungi. The non-linearity of reaction norms to phylloclimate adds, however, great complexity to this integration. Thus, we will discuss questions raised by such integration, from a spatial and temporal point of view, little being known on the role of phylloclimate on the evolution and plasticity of pests and hosts. Finally, we will list pending issues and show how multiscale interactions between climate and pests could provide innovative levers for pest management, from current climate to future one.