Plant chemical defenses are known to affect herbivores and their natural enemies but exactly how herbivore diet breadth mediates these tritrophic interactions is largely unknown. Here, we characterize the effects of plant chemical defenses on aphids and coccinellids. To do so, we conducted laboratory trials where we measured: 1) plant traits, including secondary metabolites (glucosinolates) in two Brassica napus cultivars with differing glucosinolate concentration; 2) performance characteristics (reproduction, fecundity) of the dietary generalist aphid species, Myzus persicae, and the specialist aphid, Brevicoryne brassicae, as well as aphid resistance and larval performance (voracity, weight gain, developmental duration) of five aphidophagous coccinellid species, and 3) glucosinolate sequestration by the aphids. Our main results show that aphids performed similarly when feeding upon the two canola cultivars but had sequestered different levels of glucosinolates. The specialist aphid, B. brassicae, had 14% higher glucosinolate concentration than the generalist aphid, M. persicae. Subsequently, we found that all five coccinellid species performed worse after consuming the specialist aphid, B. brassicae, which had previously fed upon the canola cultivar with the higher glucosinolate concentration. Low-glucosinolate plants increased coccinellid weight gain by 78% and accelerated development by 14%. When coccinellids consumed generalist aphids that had previously fed upon the low-glucosinolate plants, their weight gain increased only by 11% with no significant changes in developmental duration. Our study reveals the interactive effects of plant chemical defenses and aphid diet breadth on coccinellid performance and highlights the key role of herbivore diet breadth in multitrophic interactions.
