Effect of Insect Herbivory on the Architecture and Seed Production of Canola, Brassica napus L.

Several native Lygus spp. (Hemiptera: Miridae) feed on buds, flowers and immature seeds of canola. These plant resources are also exploited by a recent alien pest now established in southern Alberta, Canada: the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae). A caged study was conducted with Brassica napus L. in 2000 and 2002 to determine if various combinations and densities of lygus and seedpod weevils (0, 10 + 10, 20 of either, 20 + 20, 40 + 40, individuals per cage) affected yield and its components such as number of branches, pods and seed weight. Under severely dry conditions, cages with the intermediate (20 + 20) and high (40 + 40) insect combinations had lower grain yield as well as number of racemes and pods per raceme relative to control cages with no insects added. Under more humid conditions in 2002, similar results were observed but the damage by the cabbage seedpod weevil at 20 adults per cage was more apparent than in 2000. These results suggest that in most years the weevil is a more serious pest than lygus bugs but low combined densities do not decrease yield. Furthermore, in southern Alberta at low or moderate densities, these late-season pests do not appear to stimulate plants to overcompensate. There was no indication of competition between these two insects.