Bottom-up effects of Brassica napus nutrition on the oviposition preference and larval performance of Ceutorhynchus obstrictus (Coleoptera: Curculionidae)

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is a serious pest of canola, Brassica napus L. and Brassica rapa L., both in Europe and North America. Nitrogen and sulfur levels can be readily manipulated by growers and could potentially be exploited to improve current pest management strategies. In an effort to better understand the relationship between host plant nutrition and C. obstrictus oviposition preference and its larval developmental biology, we exposed gravid females to host plants grown under differing regimes of nitrogen and sulfur. Results indicated that plants grown with a higher supply of nitrogen were preferred as hosts. Plants with a higher sulfur supply were also preferred but only in plants grown at lower nitrogen levels. In contrast, larval development time increased with increasing nitrogen levels although larval dry weights were unaffected. The results were inconsistent with the preference-performance hypothesis. The inconsistencies between oviposition choice and larval growth and development may be due to a conflict between maternal and larval fitness. Observed changes in larval development time may not represent a significant fitness cost for larval C. obstrictus, as no corresponding change in larval weight was observed. Possible causal mechanisms for the increase in development time include differences in oil, protein and glucosinolate contents of the seed. Fertilizer management regimes currently recommended were considered to be optimal for management of C. obstrictus as the yield benefits from higher rates of nitrogen fertilization would more than compensate for increased level of infestation.