Influence of plant architecture on tritrophic interactions between canola, aphids and Hippodamia convergens

Myzus persicae, (green peach aphid, GPA), and Brevicoryne brassicae (cabbage aphid, CA) can cause up to 33% yield loss to winter canola. Reproductive potential of these aphids is affected by within-plant distribution of aphids, were populations grow faster on reproductive compared to vegetative canola plant structures. Even though higher concentrations of glucosinolates (GLS) are also found in reproductive tissues, these aphids evolved different mechanisms to overcome GLS toxicity through sequestration (CA) or excretion (GPA) of toxic compounds. In a no-choice laboratory experiment we evaluated if differential expression of GLS within the host plant influences toxicity of prey by examining immature Hippodamia convergens life histories. Newly hatched H. convergens were assigned to a diet and placed in individual wells. Diet treatments included: CA or GPA restricted to feeding on reproductive or vegetative canola structures and two GLS-free controls (aphids not exposed to GLS or a non-aphid diet). All larvae were reared on the same diet for the entire study and fed ad libitum. Consumption rates were greater for larvae on SA diet followed by aphids that either secrete or sequester GLS, respectively (P < 0.0001). Greater amounts of partially consumed aphids were observed when lady beetles were exposed to CA previously located on reproductive plant tissue (P < 0.0001). GLS-free diets yielded greater pupal weights overall compared to larvae that fed on CA or GPA (P < 0.0001). These results suggest that within-plant distribution of aphids not only affects aphid reproductive potential but also has implications on predator communities.