Detecting spatial interactions in the ragweed (Ambrosia artemissifolia L.) and the ragweed beetle (Ophraella communa LeSage) populations

We observed a weed (Ambrosia artemissifolia) - beetle herbivore (Ophraella communa) system for three years in a spatially continuous field (ca 200 ha). We analyzed our field data in the light of two contrasting theories, "resource-concentration hypothesis" and "reaction-diffusion theory". For the resource-concentration hypothesis, we calculated the correlation coefficients between weed and beetle abundances for every season in each year. Though we found weak support for resource-concentration in some seasons, we could not find any clear relationships in other seasons. We discuss a dispersal-based mechanism to explain the differences observed among seasons in lieu of the resource-concentration hypothesis. For the reaction-diffusion theory, we estimated the non-parametric spatial covariance functions (NCF) for the spatial autocorrelation of weeds and beetles. Though we could not find any strong spatial structure for the individual species, we found evidence of spatial interactions between weeds and beetles using time lagged cross-correlation functions (SCCF). Weed abundance enhanced local beetle abundance. Through time, there was an evidence of beetle spillover to adjacent locations at roughly the one beetle-generation time scale. Sites with large number of beetles did not seem to reduce subsequent weed abundance.