The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), is an alien invasive pest of canola, Brassica napus L., in North America. Field populations of both adults and larvae are known to be aggregated in their distributions, but the causal mechanism for this clustering is not understood. We investigated the hypothesis that spatial distributions of C. obstrictus adults, larvae, and their parasitoids were directly or indirectly related to host plant quality as indicated by leaf tissue nutrient contents. During the years of 2005 and 2007, these insect populations were sampled in a grid pattern in three commercial fields of B. napus located near Lethbridge, Alberta, Canada. Leaf samples were collected from plants in each grid plot and analyzed for nutrient contents. Spatial patterns and associations among the collected variables were analyzed using Spatial Analysis by Distance IndicEs software. We observed significant spatial associations among C. obstrictus adults, larvae, and parasitoids. The distributions of C. obstrictus adults and larvae were correlated with several plant nutrients, most notably nitrogen and sulfur. Evidence was found for a possible nitrogen-sulfur interaction in which ovipositioning C. obstrictus females appeared to prefer plants with high levels of sulfur and low levels of nitrogen. Spatial tracking of C. obstrictus larvae by its parasitoids was evident in only one field, and this appeared related to comparatively low parasitoid populations in two of the three fields. The associations between C. obstrictus and plant nutrients may have possible applications in fertility management, trap cropping, and precision insecticide application technology.
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