Many arthropod communities are undergoing declines, with agricultural activity being a likely cause. However, we know little about what determines arthropod community composition in seasonal agroecosystems. Here I quantify how variation in climate, agricultural intensity, and local habitat along with spatial and temporal covariates impacts arthropod community composition in Southern Ontario, Canada. Local habitat factors, particularly canopy openness and plant community composition, had the strongest effect. Climatic variables followed closely, driven primarily by seasonal variation in temperature and humidity. Agricultural intensity had the smallest effect. Direct effects of spatial and temporal distances also occurred besides differences in environment. I discuss the possible roles of niche and neutral mechanisms in generating these patterns. My findings show that habitat restoration should be a priority and that seasonality plays a strong but underappreciated role in structuring arthropod communities.