The interspecific variation of plant traits in brassicas engenders stronger aphid suppression than the intraspecific variation of single plant trait

Increasing either inter- or intraspecific plant diversity has shown the potential to engender bottom-up and top-down effects upon herbivorous pests. Nonetheless, because interspecific plant diversity is likely to pose logistical challenges to farmers, the use of intraspecific plant diversity has been gaining more interest. Regardless, studies in this area still show mixed results, including neutral, positive or negative effects of plant diversity upon herbivorous pests. To improve such studies, various researchers advocate for the use of a functional trait-based approach to investigate the role of plant diversity in pest suppression. In this context, we examined how intraspecific and interspecific variation of plant traits (i.e., brassica morphology and flowering time/availability) could influence lepidopteran and aphid suppression. To do so, four treatments consisting of either brassica monoculture (collards alone) or brassica polyculture (collards + broccolis + cabbages), fully crossed with presence or absence of alyssum plants (Lobularia maritima L.), were tested during summer and winter in Brazil. A lower number of cumulative aphid-days was noticed in treatments associated with alyssum flowers, especially during the winter. Likewise, natural enemy abundance and collard productivity were higher in treatments associated with alyssum flowers. The early availability of alyssum flowers was a trait that seemed to benefit more specifically aphid parasitoids. Additionally, alyssum plants showed a potential to be a trap crop for Ascia monuste orseis (Godart) (Lepidoptera: Pieridae) by allowing a slower development during its immature phase. Taken together, our results suggest that promoting interspecific variation of multiple plant traits may be more important than the intraspecific variation of single plant traits in regard to engendering pest suppression.