Selenium protects plants from phloem-feeding aphids due to both deterrence and toxicity.

•  Certain plant species hyperaccumulate selenium (Se) to 1000 mg kg-1 d. wt, even from low-Se soils. It is not known whether Se hyperaccumulation offers these plants any advantage. In this study the hypothesis was tested that Se can protect plants from phloem-feeding herbivores. •  Indian mustard (Brassica juncea) grown with or without Se was subjected to colonization by green peach aphids (Myzus persicae). •  In choice feeding experiments the aphids clearly avoided Se-containing plant material, and were able to detect and avoid Se-containing leaves with levels as low as 10 mg Se kg-1 d. wt. In nonchoice feeding experiments aphid population growth was inversely correlated with leaf Se concentration. The leaf Se concentration leading to a 50% reduction in aphid population growth was 1.5 mg kg-1 d. wt, and ≥ 10 mg Se kg-1 d. wt was lethal. •  In summary, Se can protect plants from feeding by aphids at leaf levels two orders of magnitude lower than those found in hyperaccumulators in the field. These results shed light on the possible functional significance of Se hyperaccumulation.