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Three new grayanane diterpenoids, pierisoids C‒E (1–3), as well as 10 known ones (4–13), were evaluated from the flowers of Pieris formosa, which is used as an insecticide in rural areas of China. Their structures were elucidated on the basis of extensive 1D and 2D NMR spectroscopic data analyses. Significant antifeedant activity of 1, 3 and 10 against the beet armyworm (Spodoptera exigua) was found, indicating that these diterpenoids might also be involved in the plant defense against insect herbivores.